by Don Apostolico
exception. Unreliable, vibration-prone
engines often are blamed for being
troublesome when the problems were
unintentionally induced by the previous
owner.
You have probably heard some
modelers say, “Boy, that XXXX brand
engine is a shaker.” The most common
reason why engines and equipment shake
is that the engines are not set up correctly.
The shaking or reliability problems are
symptoms, not problems.
The following topics are setup issues
that cause engines to operate unreliably or
vibrate.
Maladjusted Needle Valve Settings:
Don’t discount the importance of needles
being set correctly. I’ve helped too many
modelers throughout the years who have
complained about troublesome engines,
only to find out that the entire problem
was maladjusted needles, although they
all said they had “adjusted” the needle
valves.
Failure to set needles properly is a red
flag that can lead to damaged engines. I
wish I had a dollar for every time I’ve
been told that every engine expert at the
field has adjusted the needles and it still
doesn’t run correctly.
Turning the needles doesn’t always
correctly adjust. By noon today I had
received two calls on our tech line from
modelers complaining about their roughrunning
gas engines. Neither modeler
could quantify his needle setting. Neither
knew the baffling, nor could they tell me
the inlet exit-area ratios or the cylinder
head temperatures of their setup. These
critical setup items can damage the engine
and cause unreliable operation.
When determining if an engine was
rich or lean, quantify the settings. What is
February 2012 35
Dave Johnson’s model has a 150cc Desert Aircraft
engine with KS 1090 quiet pipes. This setup is reliable
and has been on multiple aircraft since the 1990s.
SOME FIND OUT the hard way when
buying a used gas engine that a good deal
sometimes turns out to be the worst deal,
and the cheapest sometimes turns out to be
the most expensive.
Avoid purchasing troublesome used
engines by first determining if the current
owner misused the engine. One thing is for
sure: a problematic vibrating or erratic
running engine is no fun, is a disappointing
experience, and is often the most
expensive way to go.
The following information will help
modelers determine if the engine is a true
bargain or a boat anchor. A clunker may
run, but have numerous problems if it has
not been set up or correctly operated by the
seller.
The good news is that a used engine
can be a good deal if the buyer knows
what to look for and what to avoid.
Most modelers are honest and don’t
knowingly sell their damaged engines to
unsuspecting buyers, but clunkers get sold
all the time. I get calls on Don’s Hobby
Shop’s tech line frequently. The seller and
buyer sometimes do not know the cause
and effect of improper setups, so the
clunker gets sold and another buyer gets a
paint-shaker.
Sometimes the shaker is considered the
norm, when it should always be the
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:09 PM Page 3536 MODEL AVIATION
This spinning propeller will shake and
cause vibration issues. If that is the case, the
bearings are probably shot. Pass on this deal.
Ask the seller if he or she has vertically
balanced the propeller in the 12 and 6
o’clock position. If you get the deer-in-theheadlight
look, find another engine to buy.
Vibrating propellers can quickly damage
bearings and cause other issues. Many
modelers view replacing a bearing as minor
work, but few know how to do it correctly.
To achieve vertical balance, permanently
attach Velcro to the light side of the hub and
add CA to balance the propeller.
There is another key question to ask the
seller. Find out if the propeller has been
tracked. Is the hub square? If you get a blank
stare or are asked to explain what propeller
tracking is, don’t buy the engine.
I just finished speaking with a modeler
who didn’t track or vertically balance the
propeller, and had an unbalanced spinner.
A tachometer,
along with the
decades-old pinch
test, is used to set
the high- and lowspeed
needles. Set
a 100 rpm rise for
gas while the fuel
line is pinched.
Proper vertical and horizontal balance
of a propeller and spinner is important.
Use a high-quality balancer to eliminate
vibration damage to your airplane and
engine.
To track a propeller, rotate it from 12 to
6 o’clock. The propeller tip should be the
same distance from the reference point
on both tips.
Photos by the author
the point of chasing other issues that can
cause problems if the needles are not
correctly set?
To ascertain the modeler’s technical
skills, I ask the simple test question: How
many rpm, on the rich side of peak, are the
needles set? Failure to quantify this
important piece of information tells me the
modeler is guessing the correct needle
setting. I don’t guess; I measure. It’s either
correct or incorrect.
Modelers damage engines by running
them rich, which can cause ring-fouling,
stuck rings, and other problems. Running
them lean will cause overheating, scored
cylinders, etc.
Assuming the cowl inlet and outlet ratios
are correct and the engine is properly
baffled, a good starting point for needle
settings on a gas engine is to use the pinch
test. This has been the standard test used on
gas- and glow fuel-powered engines for
decades.
The correct answer is to set gas engine
needles at 100 rpm on the rich side of peak
for both high- and low-speed needles, and
200 for glow engines. Use a tachometer and
there’s no guesswork, no opinion, and no
estimate.
If a modeler can’t quantify where the
needles are set, then the only thing one
knows for sure is that the needles were
turned. That is not good enough.
Assuming other setup issues are correct,
this 100-rpm rise is a safe starting point for
your test flight. The readings must be taken
with the cowl on because the pressure is
different with the cowl off.
Needle adjustments may be required after
the test flight, but the changes will be minor
if the cooling ratios and airflow baffling are
set up correctly. Never adjust needles with
the engine running. If you have ever seen a
screwdriver slip into a big, gas engine
propeller and go flying across the flightline
like a bullet, you will not consider this trick.
It’s dangerous—don’t do it.
Propeller Not Balanced or Tracked: When
buying a used engine, ask the seller if the
propellers have been balanced. Most sellers
will say yes. Many balance their propellers
in the 3-9 o’clock position. That is the first
step of balancing.
Unbalanced propellers cause vibration
that affects engine bearings, hinges, radio
gear, and structural integrity. Note: if the
seller has horizontally balanced the propeller
by removing material from the heavy blade,
beware. This person just statically balanced,
and dynamically unbalanced, the propeller
because of different airfoil shapes resulting
from the removed material.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 36February 2012 37
His engine was shaking and he couldn’t keep
the engine screws tight. Untracked propellers
and unbalanced spinners and propellers
cause vibration, which quickly destroys
bearings and causes other problems.
Check to see if the engine has run hot.
Overheated engines may be light pink.
Check for scored pistons and cylinders.
These can have metal shavings in the
bearings and require expensive replacement
parts.
Overheating can result from improper
needle setting or improper or no cowl
baffling. (Proper baffling can decrease
temperatures by roughly 150°.) Overheating
can cause improper inlet exit-area ratios,
(three times the exit area of the inlet for gas,
and two times for glow, are the minimum
ratios; four times for gas and three times for
glow are recommended), improper tank
plumbing, fuel foaming, improper needle
settings, and lubrication issues. Normal
operating cylinder head temperatures for gas
engines are 180°-220°. If there is evidence
of overheating, don’t buy this engine, either.
Ask if the engine has had any propeller
strikes. Many engine designs have split
crankshafts, which is a shaft within a shaft.
A propeller strike can change how the inner
and outer shaft align, displacing the
counterweight. The displaced
counterweights create vibration that can
damage your engine, servos, switches,
Below: Correct baffling is essential to keep engines
from overheating. Proper baffling can drop cylinder
temperatures by more than 100°.
Modelers can bend this little fulcrum while working
on the carburetor. This unit controls the internal
pumping pressure and if bent, the carburetor will be
out of tolerance.
Left: The author uses a popoff
pressure gauge to check
the internal calibration of the
internal needle and spring
pressure.
Right: Fromeco’s TNC
tachometer is used to
measure, set, and adjust
needle valve settings.
Every modeler can be a
“local expert” if he or she
sets up an engine by the
numbers.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 3738 MODEL AVIATION
hinges, and glue joints.
Propeller strikes can also bend
crankshafts, causing the engine to run rough
and vibrate. Use a dial indicator to check
crank runout. Propeller impact can bend
shafts, creating landing problems.
Crankshafts are expensive to replace, so save
your money for another engine.
The engine should also have been
operated with a balanced spinner. An
unbalanced spinner creates vibration,
damaging the engine and aircraft.
Determine if the seller has taken the
carburetor apart. There are critical
dimensions of the internal needle and
pumping mechanism within the carburetor.
If damaged or changed, the result will be a
cantankerous-running engine, an engine that
won’t run at all, or one where fuel may be
draining out of the carburetor.
The little, internal fork-like device, if
sprung, will affect your pumping pressure
and is often damaged when modelers
improperly remove the black pump
diaphragm. The damage will affect the popoff
pressure (internal metering settings),
creating operational problems.
Does the engine have any signs of dings,
dents, or contact damage? Dings and dents
don’t come from flying—they come from
crashing. Crash damage indicates that an
engine suddenly stopped, and there could be
a bent crank or displaced counterweight.
Look at the Spark Plug: The spark plug is a
diary of how rich or lean an engine has been
operated. If the electrode is white, the engine
has run lean (indicated by scored cylinder
walls, scored piston, and ring scratches).
If the electrode is black, the engine has
run rich (resulting in carbon buildup, stuck
rings, loss of compression, uneven ring
wear, and scratched cylinder walls). If the
needles are set correctly, the color of the
Spark plugs are a diary of your engine’s operation. The white electrode plug has been run lean, the tan electrode shows the
needles are correctly set, and the black electrode denotes an engine that has been run rich.
spark plug electrode will be a light tan.
It’s important to ask the seller if the front
or rear bearings have been replaced. If so,
then ask if the bearing preload was removed
during reassembly. Bearing preload is the
pressure exerted on the bearing and race,
due to the misalignment that occurs when
the bearings are pulled or pressed into place.
If the seller has replaced the bearings on gas
or glow engines and doesn’t know what a
bearing preload is, or how to remove it, the
engine is already damaged and not worth
buying.
If a modeler fails to perform this
important step properly, the engine will
easily lose hundreds or thousands of rpm,
depending on how much preload is present.
The engine will run hot, causing premature
bearing failure
To remove the bearing preload, heat the
case a few hundred degrees with a torch,
near the bearing area. The propeller is
installed and tightened and a plastic or
wooden mallet is used to rap the end of the
crankshaft. This will snap the bearing into
place. Retighten the propeller and hit it
again until you feel no difference. It may
take several taps and propeller tightening to
remove the preload.
Summary of Key Items to Check When
Buying a Used Engine:
• Check to see if the engine has been
operated with a propeller that wasn’t
vertically and horizontally balanced or
tracked.
• Check the plug color to see if the engine
has run with misadjusted needle valves.
Lean or rich settings can cause damage.
• Has the engine been operated with no or
improper baffling, causing it to run hot?
• Ensure that the engine has been operated
with proper inlet exit-area ratios, which
prevents overheating.
• Has the engine had a propeller strike? This
can bend the crank or slip the counterweight,
which ruins bearings and causes rough
running. Check it with a dial indicator.
• Has the carburetor been “overhauled” by
someone who is not familiar with procedural
issues? If so, delicate adjustments may be
damaged.
• Verify that the modeler properly removed
the bearing preload if the front or rear
bearing was replaced.
These issues create problematic engines.
Modelers who know which questions to ask
and inspections to perform, can avoid the
clunkers that result in unreliable operation,
engines that vibrate, and hard-to-fire
engines.
Armed with this knowledge, modelers
can now do their homework before buying
used engines and avoid those that have
suffered the aforementioned abuse. Buying
an engine without inspecting it is like rolling
the dice. If you don’t mind spending the
time, effort, or money to get it fixed, then go
for it. Many problems are created by the
modeler. Be knowledgeable and buy once.
The cheapest is not always the bargain,
the most expensive is not always necessary,
and an abused engine usually is problematic.
If you ask the proper questions and perform
the inspections noted and get unsatisfactory
answers, skip the bargain engine.
If a buyer knows what to look for and
asks the right questions, he or she has a
lower chance of purchasing a problematic
used engine. MA
Sources:
Don’s Hobby Shop
(785) 827-3222
www.donshobbyshop.com
Too Lean Correct Too Rich
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 38
Edition: Model Aviation - 2012/02
Page Numbers: 35,36,37,38
Edition: Model Aviation - 2012/02
Page Numbers: 35,36,37,38
by Don Apostolico
exception. Unreliable, vibration-prone
engines often are blamed for being
troublesome when the problems were
unintentionally induced by the previous
owner.
You have probably heard some
modelers say, “Boy, that XXXX brand
engine is a shaker.” The most common
reason why engines and equipment shake
is that the engines are not set up correctly.
The shaking or reliability problems are
symptoms, not problems.
The following topics are setup issues
that cause engines to operate unreliably or
vibrate.
Maladjusted Needle Valve Settings:
Don’t discount the importance of needles
being set correctly. I’ve helped too many
modelers throughout the years who have
complained about troublesome engines,
only to find out that the entire problem
was maladjusted needles, although they
all said they had “adjusted” the needle
valves.
Failure to set needles properly is a red
flag that can lead to damaged engines. I
wish I had a dollar for every time I’ve
been told that every engine expert at the
field has adjusted the needles and it still
doesn’t run correctly.
Turning the needles doesn’t always
correctly adjust. By noon today I had
received two calls on our tech line from
modelers complaining about their roughrunning
gas engines. Neither modeler
could quantify his needle setting. Neither
knew the baffling, nor could they tell me
the inlet exit-area ratios or the cylinder
head temperatures of their setup. These
critical setup items can damage the engine
and cause unreliable operation.
When determining if an engine was
rich or lean, quantify the settings. What is
February 2012 35
Dave Johnson’s model has a 150cc Desert Aircraft
engine with KS 1090 quiet pipes. This setup is reliable
and has been on multiple aircraft since the 1990s.
SOME FIND OUT the hard way when
buying a used gas engine that a good deal
sometimes turns out to be the worst deal,
and the cheapest sometimes turns out to be
the most expensive.
Avoid purchasing troublesome used
engines by first determining if the current
owner misused the engine. One thing is for
sure: a problematic vibrating or erratic
running engine is no fun, is a disappointing
experience, and is often the most
expensive way to go.
The following information will help
modelers determine if the engine is a true
bargain or a boat anchor. A clunker may
run, but have numerous problems if it has
not been set up or correctly operated by the
seller.
The good news is that a used engine
can be a good deal if the buyer knows
what to look for and what to avoid.
Most modelers are honest and don’t
knowingly sell their damaged engines to
unsuspecting buyers, but clunkers get sold
all the time. I get calls on Don’s Hobby
Shop’s tech line frequently. The seller and
buyer sometimes do not know the cause
and effect of improper setups, so the
clunker gets sold and another buyer gets a
paint-shaker.
Sometimes the shaker is considered the
norm, when it should always be the
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:09 PM Page 3536 MODEL AVIATION
This spinning propeller will shake and
cause vibration issues. If that is the case, the
bearings are probably shot. Pass on this deal.
Ask the seller if he or she has vertically
balanced the propeller in the 12 and 6
o’clock position. If you get the deer-in-theheadlight
look, find another engine to buy.
Vibrating propellers can quickly damage
bearings and cause other issues. Many
modelers view replacing a bearing as minor
work, but few know how to do it correctly.
To achieve vertical balance, permanently
attach Velcro to the light side of the hub and
add CA to balance the propeller.
There is another key question to ask the
seller. Find out if the propeller has been
tracked. Is the hub square? If you get a blank
stare or are asked to explain what propeller
tracking is, don’t buy the engine.
I just finished speaking with a modeler
who didn’t track or vertically balance the
propeller, and had an unbalanced spinner.
A tachometer,
along with the
decades-old pinch
test, is used to set
the high- and lowspeed
needles. Set
a 100 rpm rise for
gas while the fuel
line is pinched.
Proper vertical and horizontal balance
of a propeller and spinner is important.
Use a high-quality balancer to eliminate
vibration damage to your airplane and
engine.
To track a propeller, rotate it from 12 to
6 o’clock. The propeller tip should be the
same distance from the reference point
on both tips.
Photos by the author
the point of chasing other issues that can
cause problems if the needles are not
correctly set?
To ascertain the modeler’s technical
skills, I ask the simple test question: How
many rpm, on the rich side of peak, are the
needles set? Failure to quantify this
important piece of information tells me the
modeler is guessing the correct needle
setting. I don’t guess; I measure. It’s either
correct or incorrect.
Modelers damage engines by running
them rich, which can cause ring-fouling,
stuck rings, and other problems. Running
them lean will cause overheating, scored
cylinders, etc.
Assuming the cowl inlet and outlet ratios
are correct and the engine is properly
baffled, a good starting point for needle
settings on a gas engine is to use the pinch
test. This has been the standard test used on
gas- and glow fuel-powered engines for
decades.
The correct answer is to set gas engine
needles at 100 rpm on the rich side of peak
for both high- and low-speed needles, and
200 for glow engines. Use a tachometer and
there’s no guesswork, no opinion, and no
estimate.
If a modeler can’t quantify where the
needles are set, then the only thing one
knows for sure is that the needles were
turned. That is not good enough.
Assuming other setup issues are correct,
this 100-rpm rise is a safe starting point for
your test flight. The readings must be taken
with the cowl on because the pressure is
different with the cowl off.
Needle adjustments may be required after
the test flight, but the changes will be minor
if the cooling ratios and airflow baffling are
set up correctly. Never adjust needles with
the engine running. If you have ever seen a
screwdriver slip into a big, gas engine
propeller and go flying across the flightline
like a bullet, you will not consider this trick.
It’s dangerous—don’t do it.
Propeller Not Balanced or Tracked: When
buying a used engine, ask the seller if the
propellers have been balanced. Most sellers
will say yes. Many balance their propellers
in the 3-9 o’clock position. That is the first
step of balancing.
Unbalanced propellers cause vibration
that affects engine bearings, hinges, radio
gear, and structural integrity. Note: if the
seller has horizontally balanced the propeller
by removing material from the heavy blade,
beware. This person just statically balanced,
and dynamically unbalanced, the propeller
because of different airfoil shapes resulting
from the removed material.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 36February 2012 37
His engine was shaking and he couldn’t keep
the engine screws tight. Untracked propellers
and unbalanced spinners and propellers
cause vibration, which quickly destroys
bearings and causes other problems.
Check to see if the engine has run hot.
Overheated engines may be light pink.
Check for scored pistons and cylinders.
These can have metal shavings in the
bearings and require expensive replacement
parts.
Overheating can result from improper
needle setting or improper or no cowl
baffling. (Proper baffling can decrease
temperatures by roughly 150°.) Overheating
can cause improper inlet exit-area ratios,
(three times the exit area of the inlet for gas,
and two times for glow, are the minimum
ratios; four times for gas and three times for
glow are recommended), improper tank
plumbing, fuel foaming, improper needle
settings, and lubrication issues. Normal
operating cylinder head temperatures for gas
engines are 180°-220°. If there is evidence
of overheating, don’t buy this engine, either.
Ask if the engine has had any propeller
strikes. Many engine designs have split
crankshafts, which is a shaft within a shaft.
A propeller strike can change how the inner
and outer shaft align, displacing the
counterweight. The displaced
counterweights create vibration that can
damage your engine, servos, switches,
Below: Correct baffling is essential to keep engines
from overheating. Proper baffling can drop cylinder
temperatures by more than 100°.
Modelers can bend this little fulcrum while working
on the carburetor. This unit controls the internal
pumping pressure and if bent, the carburetor will be
out of tolerance.
Left: The author uses a popoff
pressure gauge to check
the internal calibration of the
internal needle and spring
pressure.
Right: Fromeco’s TNC
tachometer is used to
measure, set, and adjust
needle valve settings.
Every modeler can be a
“local expert” if he or she
sets up an engine by the
numbers.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 3738 MODEL AVIATION
hinges, and glue joints.
Propeller strikes can also bend
crankshafts, causing the engine to run rough
and vibrate. Use a dial indicator to check
crank runout. Propeller impact can bend
shafts, creating landing problems.
Crankshafts are expensive to replace, so save
your money for another engine.
The engine should also have been
operated with a balanced spinner. An
unbalanced spinner creates vibration,
damaging the engine and aircraft.
Determine if the seller has taken the
carburetor apart. There are critical
dimensions of the internal needle and
pumping mechanism within the carburetor.
If damaged or changed, the result will be a
cantankerous-running engine, an engine that
won’t run at all, or one where fuel may be
draining out of the carburetor.
The little, internal fork-like device, if
sprung, will affect your pumping pressure
and is often damaged when modelers
improperly remove the black pump
diaphragm. The damage will affect the popoff
pressure (internal metering settings),
creating operational problems.
Does the engine have any signs of dings,
dents, or contact damage? Dings and dents
don’t come from flying—they come from
crashing. Crash damage indicates that an
engine suddenly stopped, and there could be
a bent crank or displaced counterweight.
Look at the Spark Plug: The spark plug is a
diary of how rich or lean an engine has been
operated. If the electrode is white, the engine
has run lean (indicated by scored cylinder
walls, scored piston, and ring scratches).
If the electrode is black, the engine has
run rich (resulting in carbon buildup, stuck
rings, loss of compression, uneven ring
wear, and scratched cylinder walls). If the
needles are set correctly, the color of the
Spark plugs are a diary of your engine’s operation. The white electrode plug has been run lean, the tan electrode shows the
needles are correctly set, and the black electrode denotes an engine that has been run rich.
spark plug electrode will be a light tan.
It’s important to ask the seller if the front
or rear bearings have been replaced. If so,
then ask if the bearing preload was removed
during reassembly. Bearing preload is the
pressure exerted on the bearing and race,
due to the misalignment that occurs when
the bearings are pulled or pressed into place.
If the seller has replaced the bearings on gas
or glow engines and doesn’t know what a
bearing preload is, or how to remove it, the
engine is already damaged and not worth
buying.
If a modeler fails to perform this
important step properly, the engine will
easily lose hundreds or thousands of rpm,
depending on how much preload is present.
The engine will run hot, causing premature
bearing failure
To remove the bearing preload, heat the
case a few hundred degrees with a torch,
near the bearing area. The propeller is
installed and tightened and a plastic or
wooden mallet is used to rap the end of the
crankshaft. This will snap the bearing into
place. Retighten the propeller and hit it
again until you feel no difference. It may
take several taps and propeller tightening to
remove the preload.
Summary of Key Items to Check When
Buying a Used Engine:
• Check to see if the engine has been
operated with a propeller that wasn’t
vertically and horizontally balanced or
tracked.
• Check the plug color to see if the engine
has run with misadjusted needle valves.
Lean or rich settings can cause damage.
• Has the engine been operated with no or
improper baffling, causing it to run hot?
• Ensure that the engine has been operated
with proper inlet exit-area ratios, which
prevents overheating.
• Has the engine had a propeller strike? This
can bend the crank or slip the counterweight,
which ruins bearings and causes rough
running. Check it with a dial indicator.
• Has the carburetor been “overhauled” by
someone who is not familiar with procedural
issues? If so, delicate adjustments may be
damaged.
• Verify that the modeler properly removed
the bearing preload if the front or rear
bearing was replaced.
These issues create problematic engines.
Modelers who know which questions to ask
and inspections to perform, can avoid the
clunkers that result in unreliable operation,
engines that vibrate, and hard-to-fire
engines.
Armed with this knowledge, modelers
can now do their homework before buying
used engines and avoid those that have
suffered the aforementioned abuse. Buying
an engine without inspecting it is like rolling
the dice. If you don’t mind spending the
time, effort, or money to get it fixed, then go
for it. Many problems are created by the
modeler. Be knowledgeable and buy once.
The cheapest is not always the bargain,
the most expensive is not always necessary,
and an abused engine usually is problematic.
If you ask the proper questions and perform
the inspections noted and get unsatisfactory
answers, skip the bargain engine.
If a buyer knows what to look for and
asks the right questions, he or she has a
lower chance of purchasing a problematic
used engine. MA
Sources:
Don’s Hobby Shop
(785) 827-3222
www.donshobbyshop.com
Too Lean Correct Too Rich
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 38
Edition: Model Aviation - 2012/02
Page Numbers: 35,36,37,38
by Don Apostolico
exception. Unreliable, vibration-prone
engines often are blamed for being
troublesome when the problems were
unintentionally induced by the previous
owner.
You have probably heard some
modelers say, “Boy, that XXXX brand
engine is a shaker.” The most common
reason why engines and equipment shake
is that the engines are not set up correctly.
The shaking or reliability problems are
symptoms, not problems.
The following topics are setup issues
that cause engines to operate unreliably or
vibrate.
Maladjusted Needle Valve Settings:
Don’t discount the importance of needles
being set correctly. I’ve helped too many
modelers throughout the years who have
complained about troublesome engines,
only to find out that the entire problem
was maladjusted needles, although they
all said they had “adjusted” the needle
valves.
Failure to set needles properly is a red
flag that can lead to damaged engines. I
wish I had a dollar for every time I’ve
been told that every engine expert at the
field has adjusted the needles and it still
doesn’t run correctly.
Turning the needles doesn’t always
correctly adjust. By noon today I had
received two calls on our tech line from
modelers complaining about their roughrunning
gas engines. Neither modeler
could quantify his needle setting. Neither
knew the baffling, nor could they tell me
the inlet exit-area ratios or the cylinder
head temperatures of their setup. These
critical setup items can damage the engine
and cause unreliable operation.
When determining if an engine was
rich or lean, quantify the settings. What is
February 2012 35
Dave Johnson’s model has a 150cc Desert Aircraft
engine with KS 1090 quiet pipes. This setup is reliable
and has been on multiple aircraft since the 1990s.
SOME FIND OUT the hard way when
buying a used gas engine that a good deal
sometimes turns out to be the worst deal,
and the cheapest sometimes turns out to be
the most expensive.
Avoid purchasing troublesome used
engines by first determining if the current
owner misused the engine. One thing is for
sure: a problematic vibrating or erratic
running engine is no fun, is a disappointing
experience, and is often the most
expensive way to go.
The following information will help
modelers determine if the engine is a true
bargain or a boat anchor. A clunker may
run, but have numerous problems if it has
not been set up or correctly operated by the
seller.
The good news is that a used engine
can be a good deal if the buyer knows
what to look for and what to avoid.
Most modelers are honest and don’t
knowingly sell their damaged engines to
unsuspecting buyers, but clunkers get sold
all the time. I get calls on Don’s Hobby
Shop’s tech line frequently. The seller and
buyer sometimes do not know the cause
and effect of improper setups, so the
clunker gets sold and another buyer gets a
paint-shaker.
Sometimes the shaker is considered the
norm, when it should always be the
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:09 PM Page 3536 MODEL AVIATION
This spinning propeller will shake and
cause vibration issues. If that is the case, the
bearings are probably shot. Pass on this deal.
Ask the seller if he or she has vertically
balanced the propeller in the 12 and 6
o’clock position. If you get the deer-in-theheadlight
look, find another engine to buy.
Vibrating propellers can quickly damage
bearings and cause other issues. Many
modelers view replacing a bearing as minor
work, but few know how to do it correctly.
To achieve vertical balance, permanently
attach Velcro to the light side of the hub and
add CA to balance the propeller.
There is another key question to ask the
seller. Find out if the propeller has been
tracked. Is the hub square? If you get a blank
stare or are asked to explain what propeller
tracking is, don’t buy the engine.
I just finished speaking with a modeler
who didn’t track or vertically balance the
propeller, and had an unbalanced spinner.
A tachometer,
along with the
decades-old pinch
test, is used to set
the high- and lowspeed
needles. Set
a 100 rpm rise for
gas while the fuel
line is pinched.
Proper vertical and horizontal balance
of a propeller and spinner is important.
Use a high-quality balancer to eliminate
vibration damage to your airplane and
engine.
To track a propeller, rotate it from 12 to
6 o’clock. The propeller tip should be the
same distance from the reference point
on both tips.
Photos by the author
the point of chasing other issues that can
cause problems if the needles are not
correctly set?
To ascertain the modeler’s technical
skills, I ask the simple test question: How
many rpm, on the rich side of peak, are the
needles set? Failure to quantify this
important piece of information tells me the
modeler is guessing the correct needle
setting. I don’t guess; I measure. It’s either
correct or incorrect.
Modelers damage engines by running
them rich, which can cause ring-fouling,
stuck rings, and other problems. Running
them lean will cause overheating, scored
cylinders, etc.
Assuming the cowl inlet and outlet ratios
are correct and the engine is properly
baffled, a good starting point for needle
settings on a gas engine is to use the pinch
test. This has been the standard test used on
gas- and glow fuel-powered engines for
decades.
The correct answer is to set gas engine
needles at 100 rpm on the rich side of peak
for both high- and low-speed needles, and
200 for glow engines. Use a tachometer and
there’s no guesswork, no opinion, and no
estimate.
If a modeler can’t quantify where the
needles are set, then the only thing one
knows for sure is that the needles were
turned. That is not good enough.
Assuming other setup issues are correct,
this 100-rpm rise is a safe starting point for
your test flight. The readings must be taken
with the cowl on because the pressure is
different with the cowl off.
Needle adjustments may be required after
the test flight, but the changes will be minor
if the cooling ratios and airflow baffling are
set up correctly. Never adjust needles with
the engine running. If you have ever seen a
screwdriver slip into a big, gas engine
propeller and go flying across the flightline
like a bullet, you will not consider this trick.
It’s dangerous—don’t do it.
Propeller Not Balanced or Tracked: When
buying a used engine, ask the seller if the
propellers have been balanced. Most sellers
will say yes. Many balance their propellers
in the 3-9 o’clock position. That is the first
step of balancing.
Unbalanced propellers cause vibration
that affects engine bearings, hinges, radio
gear, and structural integrity. Note: if the
seller has horizontally balanced the propeller
by removing material from the heavy blade,
beware. This person just statically balanced,
and dynamically unbalanced, the propeller
because of different airfoil shapes resulting
from the removed material.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 36February 2012 37
His engine was shaking and he couldn’t keep
the engine screws tight. Untracked propellers
and unbalanced spinners and propellers
cause vibration, which quickly destroys
bearings and causes other problems.
Check to see if the engine has run hot.
Overheated engines may be light pink.
Check for scored pistons and cylinders.
These can have metal shavings in the
bearings and require expensive replacement
parts.
Overheating can result from improper
needle setting or improper or no cowl
baffling. (Proper baffling can decrease
temperatures by roughly 150°.) Overheating
can cause improper inlet exit-area ratios,
(three times the exit area of the inlet for gas,
and two times for glow, are the minimum
ratios; four times for gas and three times for
glow are recommended), improper tank
plumbing, fuel foaming, improper needle
settings, and lubrication issues. Normal
operating cylinder head temperatures for gas
engines are 180°-220°. If there is evidence
of overheating, don’t buy this engine, either.
Ask if the engine has had any propeller
strikes. Many engine designs have split
crankshafts, which is a shaft within a shaft.
A propeller strike can change how the inner
and outer shaft align, displacing the
counterweight. The displaced
counterweights create vibration that can
damage your engine, servos, switches,
Below: Correct baffling is essential to keep engines
from overheating. Proper baffling can drop cylinder
temperatures by more than 100°.
Modelers can bend this little fulcrum while working
on the carburetor. This unit controls the internal
pumping pressure and if bent, the carburetor will be
out of tolerance.
Left: The author uses a popoff
pressure gauge to check
the internal calibration of the
internal needle and spring
pressure.
Right: Fromeco’s TNC
tachometer is used to
measure, set, and adjust
needle valve settings.
Every modeler can be a
“local expert” if he or she
sets up an engine by the
numbers.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 3738 MODEL AVIATION
hinges, and glue joints.
Propeller strikes can also bend
crankshafts, causing the engine to run rough
and vibrate. Use a dial indicator to check
crank runout. Propeller impact can bend
shafts, creating landing problems.
Crankshafts are expensive to replace, so save
your money for another engine.
The engine should also have been
operated with a balanced spinner. An
unbalanced spinner creates vibration,
damaging the engine and aircraft.
Determine if the seller has taken the
carburetor apart. There are critical
dimensions of the internal needle and
pumping mechanism within the carburetor.
If damaged or changed, the result will be a
cantankerous-running engine, an engine that
won’t run at all, or one where fuel may be
draining out of the carburetor.
The little, internal fork-like device, if
sprung, will affect your pumping pressure
and is often damaged when modelers
improperly remove the black pump
diaphragm. The damage will affect the popoff
pressure (internal metering settings),
creating operational problems.
Does the engine have any signs of dings,
dents, or contact damage? Dings and dents
don’t come from flying—they come from
crashing. Crash damage indicates that an
engine suddenly stopped, and there could be
a bent crank or displaced counterweight.
Look at the Spark Plug: The spark plug is a
diary of how rich or lean an engine has been
operated. If the electrode is white, the engine
has run lean (indicated by scored cylinder
walls, scored piston, and ring scratches).
If the electrode is black, the engine has
run rich (resulting in carbon buildup, stuck
rings, loss of compression, uneven ring
wear, and scratched cylinder walls). If the
needles are set correctly, the color of the
Spark plugs are a diary of your engine’s operation. The white electrode plug has been run lean, the tan electrode shows the
needles are correctly set, and the black electrode denotes an engine that has been run rich.
spark plug electrode will be a light tan.
It’s important to ask the seller if the front
or rear bearings have been replaced. If so,
then ask if the bearing preload was removed
during reassembly. Bearing preload is the
pressure exerted on the bearing and race,
due to the misalignment that occurs when
the bearings are pulled or pressed into place.
If the seller has replaced the bearings on gas
or glow engines and doesn’t know what a
bearing preload is, or how to remove it, the
engine is already damaged and not worth
buying.
If a modeler fails to perform this
important step properly, the engine will
easily lose hundreds or thousands of rpm,
depending on how much preload is present.
The engine will run hot, causing premature
bearing failure
To remove the bearing preload, heat the
case a few hundred degrees with a torch,
near the bearing area. The propeller is
installed and tightened and a plastic or
wooden mallet is used to rap the end of the
crankshaft. This will snap the bearing into
place. Retighten the propeller and hit it
again until you feel no difference. It may
take several taps and propeller tightening to
remove the preload.
Summary of Key Items to Check When
Buying a Used Engine:
• Check to see if the engine has been
operated with a propeller that wasn’t
vertically and horizontally balanced or
tracked.
• Check the plug color to see if the engine
has run with misadjusted needle valves.
Lean or rich settings can cause damage.
• Has the engine been operated with no or
improper baffling, causing it to run hot?
• Ensure that the engine has been operated
with proper inlet exit-area ratios, which
prevents overheating.
• Has the engine had a propeller strike? This
can bend the crank or slip the counterweight,
which ruins bearings and causes rough
running. Check it with a dial indicator.
• Has the carburetor been “overhauled” by
someone who is not familiar with procedural
issues? If so, delicate adjustments may be
damaged.
• Verify that the modeler properly removed
the bearing preload if the front or rear
bearing was replaced.
These issues create problematic engines.
Modelers who know which questions to ask
and inspections to perform, can avoid the
clunkers that result in unreliable operation,
engines that vibrate, and hard-to-fire
engines.
Armed with this knowledge, modelers
can now do their homework before buying
used engines and avoid those that have
suffered the aforementioned abuse. Buying
an engine without inspecting it is like rolling
the dice. If you don’t mind spending the
time, effort, or money to get it fixed, then go
for it. Many problems are created by the
modeler. Be knowledgeable and buy once.
The cheapest is not always the bargain,
the most expensive is not always necessary,
and an abused engine usually is problematic.
If you ask the proper questions and perform
the inspections noted and get unsatisfactory
answers, skip the bargain engine.
If a buyer knows what to look for and
asks the right questions, he or she has a
lower chance of purchasing a problematic
used engine. MA
Sources:
Don’s Hobby Shop
(785) 827-3222
www.donshobbyshop.com
Too Lean Correct Too Rich
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 38
Edition: Model Aviation - 2012/02
Page Numbers: 35,36,37,38
by Don Apostolico
exception. Unreliable, vibration-prone
engines often are blamed for being
troublesome when the problems were
unintentionally induced by the previous
owner.
You have probably heard some
modelers say, “Boy, that XXXX brand
engine is a shaker.” The most common
reason why engines and equipment shake
is that the engines are not set up correctly.
The shaking or reliability problems are
symptoms, not problems.
The following topics are setup issues
that cause engines to operate unreliably or
vibrate.
Maladjusted Needle Valve Settings:
Don’t discount the importance of needles
being set correctly. I’ve helped too many
modelers throughout the years who have
complained about troublesome engines,
only to find out that the entire problem
was maladjusted needles, although they
all said they had “adjusted” the needle
valves.
Failure to set needles properly is a red
flag that can lead to damaged engines. I
wish I had a dollar for every time I’ve
been told that every engine expert at the
field has adjusted the needles and it still
doesn’t run correctly.
Turning the needles doesn’t always
correctly adjust. By noon today I had
received two calls on our tech line from
modelers complaining about their roughrunning
gas engines. Neither modeler
could quantify his needle setting. Neither
knew the baffling, nor could they tell me
the inlet exit-area ratios or the cylinder
head temperatures of their setup. These
critical setup items can damage the engine
and cause unreliable operation.
When determining if an engine was
rich or lean, quantify the settings. What is
February 2012 35
Dave Johnson’s model has a 150cc Desert Aircraft
engine with KS 1090 quiet pipes. This setup is reliable
and has been on multiple aircraft since the 1990s.
SOME FIND OUT the hard way when
buying a used gas engine that a good deal
sometimes turns out to be the worst deal,
and the cheapest sometimes turns out to be
the most expensive.
Avoid purchasing troublesome used
engines by first determining if the current
owner misused the engine. One thing is for
sure: a problematic vibrating or erratic
running engine is no fun, is a disappointing
experience, and is often the most
expensive way to go.
The following information will help
modelers determine if the engine is a true
bargain or a boat anchor. A clunker may
run, but have numerous problems if it has
not been set up or correctly operated by the
seller.
The good news is that a used engine
can be a good deal if the buyer knows
what to look for and what to avoid.
Most modelers are honest and don’t
knowingly sell their damaged engines to
unsuspecting buyers, but clunkers get sold
all the time. I get calls on Don’s Hobby
Shop’s tech line frequently. The seller and
buyer sometimes do not know the cause
and effect of improper setups, so the
clunker gets sold and another buyer gets a
paint-shaker.
Sometimes the shaker is considered the
norm, when it should always be the
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:09 PM Page 3536 MODEL AVIATION
This spinning propeller will shake and
cause vibration issues. If that is the case, the
bearings are probably shot. Pass on this deal.
Ask the seller if he or she has vertically
balanced the propeller in the 12 and 6
o’clock position. If you get the deer-in-theheadlight
look, find another engine to buy.
Vibrating propellers can quickly damage
bearings and cause other issues. Many
modelers view replacing a bearing as minor
work, but few know how to do it correctly.
To achieve vertical balance, permanently
attach Velcro to the light side of the hub and
add CA to balance the propeller.
There is another key question to ask the
seller. Find out if the propeller has been
tracked. Is the hub square? If you get a blank
stare or are asked to explain what propeller
tracking is, don’t buy the engine.
I just finished speaking with a modeler
who didn’t track or vertically balance the
propeller, and had an unbalanced spinner.
A tachometer,
along with the
decades-old pinch
test, is used to set
the high- and lowspeed
needles. Set
a 100 rpm rise for
gas while the fuel
line is pinched.
Proper vertical and horizontal balance
of a propeller and spinner is important.
Use a high-quality balancer to eliminate
vibration damage to your airplane and
engine.
To track a propeller, rotate it from 12 to
6 o’clock. The propeller tip should be the
same distance from the reference point
on both tips.
Photos by the author
the point of chasing other issues that can
cause problems if the needles are not
correctly set?
To ascertain the modeler’s technical
skills, I ask the simple test question: How
many rpm, on the rich side of peak, are the
needles set? Failure to quantify this
important piece of information tells me the
modeler is guessing the correct needle
setting. I don’t guess; I measure. It’s either
correct or incorrect.
Modelers damage engines by running
them rich, which can cause ring-fouling,
stuck rings, and other problems. Running
them lean will cause overheating, scored
cylinders, etc.
Assuming the cowl inlet and outlet ratios
are correct and the engine is properly
baffled, a good starting point for needle
settings on a gas engine is to use the pinch
test. This has been the standard test used on
gas- and glow fuel-powered engines for
decades.
The correct answer is to set gas engine
needles at 100 rpm on the rich side of peak
for both high- and low-speed needles, and
200 for glow engines. Use a tachometer and
there’s no guesswork, no opinion, and no
estimate.
If a modeler can’t quantify where the
needles are set, then the only thing one
knows for sure is that the needles were
turned. That is not good enough.
Assuming other setup issues are correct,
this 100-rpm rise is a safe starting point for
your test flight. The readings must be taken
with the cowl on because the pressure is
different with the cowl off.
Needle adjustments may be required after
the test flight, but the changes will be minor
if the cooling ratios and airflow baffling are
set up correctly. Never adjust needles with
the engine running. If you have ever seen a
screwdriver slip into a big, gas engine
propeller and go flying across the flightline
like a bullet, you will not consider this trick.
It’s dangerous—don’t do it.
Propeller Not Balanced or Tracked: When
buying a used engine, ask the seller if the
propellers have been balanced. Most sellers
will say yes. Many balance their propellers
in the 3-9 o’clock position. That is the first
step of balancing.
Unbalanced propellers cause vibration
that affects engine bearings, hinges, radio
gear, and structural integrity. Note: if the
seller has horizontally balanced the propeller
by removing material from the heavy blade,
beware. This person just statically balanced,
and dynamically unbalanced, the propeller
because of different airfoil shapes resulting
from the removed material.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 36February 2012 37
His engine was shaking and he couldn’t keep
the engine screws tight. Untracked propellers
and unbalanced spinners and propellers
cause vibration, which quickly destroys
bearings and causes other problems.
Check to see if the engine has run hot.
Overheated engines may be light pink.
Check for scored pistons and cylinders.
These can have metal shavings in the
bearings and require expensive replacement
parts.
Overheating can result from improper
needle setting or improper or no cowl
baffling. (Proper baffling can decrease
temperatures by roughly 150°.) Overheating
can cause improper inlet exit-area ratios,
(three times the exit area of the inlet for gas,
and two times for glow, are the minimum
ratios; four times for gas and three times for
glow are recommended), improper tank
plumbing, fuel foaming, improper needle
settings, and lubrication issues. Normal
operating cylinder head temperatures for gas
engines are 180°-220°. If there is evidence
of overheating, don’t buy this engine, either.
Ask if the engine has had any propeller
strikes. Many engine designs have split
crankshafts, which is a shaft within a shaft.
A propeller strike can change how the inner
and outer shaft align, displacing the
counterweight. The displaced
counterweights create vibration that can
damage your engine, servos, switches,
Below: Correct baffling is essential to keep engines
from overheating. Proper baffling can drop cylinder
temperatures by more than 100°.
Modelers can bend this little fulcrum while working
on the carburetor. This unit controls the internal
pumping pressure and if bent, the carburetor will be
out of tolerance.
Left: The author uses a popoff
pressure gauge to check
the internal calibration of the
internal needle and spring
pressure.
Right: Fromeco’s TNC
tachometer is used to
measure, set, and adjust
needle valve settings.
Every modeler can be a
“local expert” if he or she
sets up an engine by the
numbers.
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 3738 MODEL AVIATION
hinges, and glue joints.
Propeller strikes can also bend
crankshafts, causing the engine to run rough
and vibrate. Use a dial indicator to check
crank runout. Propeller impact can bend
shafts, creating landing problems.
Crankshafts are expensive to replace, so save
your money for another engine.
The engine should also have been
operated with a balanced spinner. An
unbalanced spinner creates vibration,
damaging the engine and aircraft.
Determine if the seller has taken the
carburetor apart. There are critical
dimensions of the internal needle and
pumping mechanism within the carburetor.
If damaged or changed, the result will be a
cantankerous-running engine, an engine that
won’t run at all, or one where fuel may be
draining out of the carburetor.
The little, internal fork-like device, if
sprung, will affect your pumping pressure
and is often damaged when modelers
improperly remove the black pump
diaphragm. The damage will affect the popoff
pressure (internal metering settings),
creating operational problems.
Does the engine have any signs of dings,
dents, or contact damage? Dings and dents
don’t come from flying—they come from
crashing. Crash damage indicates that an
engine suddenly stopped, and there could be
a bent crank or displaced counterweight.
Look at the Spark Plug: The spark plug is a
diary of how rich or lean an engine has been
operated. If the electrode is white, the engine
has run lean (indicated by scored cylinder
walls, scored piston, and ring scratches).
If the electrode is black, the engine has
run rich (resulting in carbon buildup, stuck
rings, loss of compression, uneven ring
wear, and scratched cylinder walls). If the
needles are set correctly, the color of the
Spark plugs are a diary of your engine’s operation. The white electrode plug has been run lean, the tan electrode shows the
needles are correctly set, and the black electrode denotes an engine that has been run rich.
spark plug electrode will be a light tan.
It’s important to ask the seller if the front
or rear bearings have been replaced. If so,
then ask if the bearing preload was removed
during reassembly. Bearing preload is the
pressure exerted on the bearing and race,
due to the misalignment that occurs when
the bearings are pulled or pressed into place.
If the seller has replaced the bearings on gas
or glow engines and doesn’t know what a
bearing preload is, or how to remove it, the
engine is already damaged and not worth
buying.
If a modeler fails to perform this
important step properly, the engine will
easily lose hundreds or thousands of rpm,
depending on how much preload is present.
The engine will run hot, causing premature
bearing failure
To remove the bearing preload, heat the
case a few hundred degrees with a torch,
near the bearing area. The propeller is
installed and tightened and a plastic or
wooden mallet is used to rap the end of the
crankshaft. This will snap the bearing into
place. Retighten the propeller and hit it
again until you feel no difference. It may
take several taps and propeller tightening to
remove the preload.
Summary of Key Items to Check When
Buying a Used Engine:
• Check to see if the engine has been
operated with a propeller that wasn’t
vertically and horizontally balanced or
tracked.
• Check the plug color to see if the engine
has run with misadjusted needle valves.
Lean or rich settings can cause damage.
• Has the engine been operated with no or
improper baffling, causing it to run hot?
• Ensure that the engine has been operated
with proper inlet exit-area ratios, which
prevents overheating.
• Has the engine had a propeller strike? This
can bend the crank or slip the counterweight,
which ruins bearings and causes rough
running. Check it with a dial indicator.
• Has the carburetor been “overhauled” by
someone who is not familiar with procedural
issues? If so, delicate adjustments may be
damaged.
• Verify that the modeler properly removed
the bearing preload if the front or rear
bearing was replaced.
These issues create problematic engines.
Modelers who know which questions to ask
and inspections to perform, can avoid the
clunkers that result in unreliable operation,
engines that vibrate, and hard-to-fire
engines.
Armed with this knowledge, modelers
can now do their homework before buying
used engines and avoid those that have
suffered the aforementioned abuse. Buying
an engine without inspecting it is like rolling
the dice. If you don’t mind spending the
time, effort, or money to get it fixed, then go
for it. Many problems are created by the
modeler. Be knowledgeable and buy once.
The cheapest is not always the bargain,
the most expensive is not always necessary,
and an abused engine usually is problematic.
If you ask the proper questions and perform
the inspections noted and get unsatisfactory
answers, skip the bargain engine.
If a buyer knows what to look for and
asks the right questions, he or she has a
lower chance of purchasing a problematic
used engine. MA
Sources:
Don’s Hobby Shop
(785) 827-3222
www.donshobbyshop.com
Too Lean Correct Too Rich
02sig2.QXD_00MSTRPG.QXD 12/16/11 12:10 PM Page 38
