Fuel-line restriction problems
December 2009 87
Joe Wagner The Engine Shop | [email protected]
Also included in this column:
• Copycatting fuel orifice size
• After-run oiling for a four-stroke
• Spark-ignition quirks
Trouble-free fuel supply to this Cox Medallion .15 required a spraybar adapter to permit
use of medium-size fuel tubing. Notice the gently curved tubing path.
This Norvel .25 has a large-diameter inlet passage for its size. But muffler pressure to its
fuel tank guarantees dependable running in any flight attitude.
The 1948 Ohlsson .23’s greatly oversized
inlet reduced fuel suction too much and
required the addition of a restrictor around
its spraybar for the engine to run.
A FEW READERS wrote to me about
fuel-feed difficulties. One was Ross
McMullen of Wendell, North Carolina—a
longtime modeler and a former MA editor.
He was having trouble obtaining rich
settings on one of his Cox Medallions. He
could unscrew its needle almost all the way
without affecting the engine’s operation.
Ross solved the problem himself,
tracking it down to an accumulation of
minor restrictions in fuel flow. For one
thing, the tubing was too small. For another,
it was bent too sharply between the tank and
the spraybar.
His experience in locating the cause of
his trouble provided yet another
demonstration that suction-feed fuel systems
in our models need great care in minimizing
flow restriction in their “plumbing.”
Suction at a model engine’s spraybar
fuel orifice is never great. At best, it’s a
mere 3 or 4 inches of H2O, as measured
with a water manometer. And many power
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:09 AM Page 87
plants have
oversized intake
passageways, to
maximize power
output. An internalcombustion
engine’s power
depends on how
much air it can
“inhale.”
Oversized inlets
tend to minimize
suction at the
spraybar, because
they reduce the inlet
air velocity. That’s
why mufflerpressurized
setups
can be so helpful in
achieving reliable
fuel delivery into
our engines.
But even with
muffler pressure, I
recommend using
the largest-size fuel
tubing that will fit,
making certain that there are no kinks in the brass tubing in the tank
and eliminating long, complicated tubing paths between the tank
and the engine. When the tubing installation in your airplane is
complete, blow through it to make sure that the flow is as free as
you can manage.
The preceding topic reminds me of something from my Veco days.
I was working with Mel Anderson (who designed the Super
Cyclone, Anderson Spitfire, Baby Spitfire, etc.) on the Veco .35.
While discussing desirable features of the new Veco engine, I
mentioned to Mel that every model power plant spraybar I’d ever
checked had the same-size fuel orifice: .040 inch in diameter, for a
#60 drill.
Mel told me that while he was working out production design in
1935 for the Baby Cyclone .36 (first of the West Coast model
engines), he decided, for no particular reason, to use the smallestsize
drill bit he had for the fuel orifice. Evidently all of the other
engine makers either copied the Baby Cyclone’s spraybar setup or
chose a #60 drill for the same reason Mel had.
He and I then experimented and found that a larger fuel orifice
worked better in Veco glow engines. No doubt, that was because the
all-castor-oil-lubed fuel everyone used back then in glow engines
was more viscous than the three-to-one white gas and 70-weight-oil
fuel blend that had long been standard for spark-ignition engines.
I’ve received several inquiries from readers about four-stroke
engine lubrication, such as how oil can get into the crankcase during
running, the purpose of the crankcase breather, and how to go about
injecting after-run oil into the case, to prevent rust in the bearings.
Good questions!
The main purpose of a four-stroke’s crankcase breather port is to
permit lubrication of the shaft, bearings, gears, cam, and pushrods.
In a two-stroke, the fuel-oil-air mixture passes through the
crankcase before entering the combustion chamber; thus its oil
content lubricates all of the “lower-end” moving parts directly
before the fresh mixture flows through the bypass and into the
combustion chamber.
However, a four-stroke doesn’t work that way. The fuel-oil-air
mixture goes straight into the head’s combustion chamber. That
makes direct lubrication of its “lower end” impossible. But
combustion-chamber pressure during the power stroke forces oil
through the tiny clearance space between the piston OD and the
cylinder bore, into the crankcase interior.
That is, it does that provided there’s a vent in the case to permit
that flow. Without a vent, no throughflow would be possible.
Therefore, although
it might be tempting
to seal off a fourstroke’s
crankcase
vent and eliminate
the messy dripping
from that, don’t do
it.
Connecting a
short length of fuel
tubing to the vent
nipple and leading
that “overboard” is
fine. But don’t
block or obstruct
the vent.
A reader
suggested
connecting a fourstroke’s
vent to its
intake somehow, so
that the oil
emerging from the
vent could be
burned instead of
wasted. That’s a
mistaken notion.
For one thing, the oil from a four-stroke’s case vent isn’t exactly
“wasted.” It has already done its intended job of lubricating the
shaft, bearings, gears, cam, etc., but it needs to exit the case, to make
room for fresh oil being delivered from the power strokes.
For another thing, oil used in model engines should never burn.
It’s there to lubricate, not to provide extra fuel.
As for after-run oil (ARO) treatment of a four-stroke, I’d never
been happy with haphazardly squirting ARO freehand into one of
those engine’s case vents. That’s both messy and inefficient. I’ve
88 MODEL AVIATION
This 1954 K&B Greenhead .35 is one of
many model engines, spark and glow,
using .040-inch-diameter fuel orifices.
Notice the variety of spraybars with the
same-size hole.
A neat and effective way to inject ARO into a four-stroke is
shown. This method supplies plenty of oil and is then used to
drain away the surplus.
Testing spark coils with a multimeter,
makes sure they’re usable. Notice the
low-voltage winding’s 1-ohm resistance.
Never use more than 4 VDC with a
model spark coil!
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:09 AM Page 88
come up with a better and neater
procedure.
I sliced off the plastic-bottle nozzle of a
Tower Hobbies ARO a bit at a time, until
the hole in its end fit firmly over a 4-inch
length of medium-size silicone fuel tubing.
With the engine upright, I pushed the end
of the ARO tubing onto the vent nipple.
With the ARO bottle upside down, gently
rotating the engine’s propeller draws in
quite a lot of ARO on the piston’s
upstroke, even without squeezing the
bottle.
Shaking and twisting the engine to
distribute the ARO throughout the case
interior ensures that the oil flows into all of
the moving parts. Turning the four-stroke
and the ARO bottle back to an upright
position and briskly flipping the propeller
will force the surplus ARO back into its
bottle. But you’ll need to loosen the
bottle’s top to avoid “inflation.”
Some readers have inquired about oldtime
spark-ignition operation and its
components. Two queries involved spark
coils: how to tell whether or not old ones
are still good and how today’s model
engine coils stack up against Smith,
AeroSpark, and OK coils from the Good
Old Days.
Modern coils—such as those available
from Larry Davidson—work as well as the
larger and heavier coils of the past,
although their “winding ratios” are far
different. And today’s transistorized sparkignition
modules (also available from
Larry) plus Ni-Cd batteries eliminate 99%
of the problems we had to suffer with the
original Kettering spark circuitry, with its
insufferably unreliable condensers and
pencell batteries.
However, coils—old or new—do fail.
The main reason is overheating.
A typical model spark coil’s primary
windings have a resistance of only 1 ohm.
That means that with a 3-volt input, 3
amperes pass through the coil. And since
watts equals volts multiplied by amperes,
the coil must dissipate 9 watts of heat
energy whenever the engine’s points are
closed.
Old and new quality coils can withstand
that, but some modelers, in the quest for a
hotter spark, will use three- or even fourcell
battery packs in their spark circuits.
The excess heat from that much current can
melt the wax with which most old-time
coils were impregnated. And the excess
voltage generated in the coil’s “hightension”
windings by too high of an input
voltage can burn through insulation, or
even the hair-thin wire itself.
However, Larry’s new coils were
designed to use three Ni-Cd cells in series.
Even when fully charged, that battery pack
won’t harm one of his coils.
These types of failures can be checked
for with a digital multimeter. Using the
“ohms” function, check the primary (lowvoltage)
winding. That should show a
resistance of roughly 1 ohm.
Then measure the high-tension
circuit—between one of the primary
terminals and the one to which the spark
plug lead connects. That should measure
somewhere near 1,600 ohms for a “Larry”
coil; 6,500 for a large AeroSpark; 4,500 for
an Arden, small AeroSpark, or “tall”
Smith; and 3,500 for a Smith “competitor.”
An “infinite” reading means a burnedout
wire. Any reading that is noticeably
lower than those I cited indicates a
shorted-out section of the high-tension
windings. But the coil may still work; try
it to see. MA
Sources:
Cox Engines
(250) 398-2600
www.coxengines.ca
Saito Engines
(800) 338-4639
www.saitoengines.com
Larry Davidson Model Airplane Supplies
(540) 721-4563
www.modelflight.com/larrydavidson.html
Model Engine Collectors Association
(817) 295-8209
www.modelenginecollectors.org
Edition: Model Aviation - 2009/12
Page Numbers: 87,88,90
Edition: Model Aviation - 2009/12
Page Numbers: 87,88,90
Fuel-line restriction problems
December 2009 87
Joe Wagner The Engine Shop | [email protected]
Also included in this column:
• Copycatting fuel orifice size
• After-run oiling for a four-stroke
• Spark-ignition quirks
Trouble-free fuel supply to this Cox Medallion .15 required a spraybar adapter to permit
use of medium-size fuel tubing. Notice the gently curved tubing path.
This Norvel .25 has a large-diameter inlet passage for its size. But muffler pressure to its
fuel tank guarantees dependable running in any flight attitude.
The 1948 Ohlsson .23’s greatly oversized
inlet reduced fuel suction too much and
required the addition of a restrictor around
its spraybar for the engine to run.
A FEW READERS wrote to me about
fuel-feed difficulties. One was Ross
McMullen of Wendell, North Carolina—a
longtime modeler and a former MA editor.
He was having trouble obtaining rich
settings on one of his Cox Medallions. He
could unscrew its needle almost all the way
without affecting the engine’s operation.
Ross solved the problem himself,
tracking it down to an accumulation of
minor restrictions in fuel flow. For one
thing, the tubing was too small. For another,
it was bent too sharply between the tank and
the spraybar.
His experience in locating the cause of
his trouble provided yet another
demonstration that suction-feed fuel systems
in our models need great care in minimizing
flow restriction in their “plumbing.”
Suction at a model engine’s spraybar
fuel orifice is never great. At best, it’s a
mere 3 or 4 inches of H2O, as measured
with a water manometer. And many power
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:09 AM Page 87
plants have
oversized intake
passageways, to
maximize power
output. An internalcombustion
engine’s power
depends on how
much air it can
“inhale.”
Oversized inlets
tend to minimize
suction at the
spraybar, because
they reduce the inlet
air velocity. That’s
why mufflerpressurized
setups
can be so helpful in
achieving reliable
fuel delivery into
our engines.
But even with
muffler pressure, I
recommend using
the largest-size fuel
tubing that will fit,
making certain that there are no kinks in the brass tubing in the tank
and eliminating long, complicated tubing paths between the tank
and the engine. When the tubing installation in your airplane is
complete, blow through it to make sure that the flow is as free as
you can manage.
The preceding topic reminds me of something from my Veco days.
I was working with Mel Anderson (who designed the Super
Cyclone, Anderson Spitfire, Baby Spitfire, etc.) on the Veco .35.
While discussing desirable features of the new Veco engine, I
mentioned to Mel that every model power plant spraybar I’d ever
checked had the same-size fuel orifice: .040 inch in diameter, for a
#60 drill.
Mel told me that while he was working out production design in
1935 for the Baby Cyclone .36 (first of the West Coast model
engines), he decided, for no particular reason, to use the smallestsize
drill bit he had for the fuel orifice. Evidently all of the other
engine makers either copied the Baby Cyclone’s spraybar setup or
chose a #60 drill for the same reason Mel had.
He and I then experimented and found that a larger fuel orifice
worked better in Veco glow engines. No doubt, that was because the
all-castor-oil-lubed fuel everyone used back then in glow engines
was more viscous than the three-to-one white gas and 70-weight-oil
fuel blend that had long been standard for spark-ignition engines.
I’ve received several inquiries from readers about four-stroke
engine lubrication, such as how oil can get into the crankcase during
running, the purpose of the crankcase breather, and how to go about
injecting after-run oil into the case, to prevent rust in the bearings.
Good questions!
The main purpose of a four-stroke’s crankcase breather port is to
permit lubrication of the shaft, bearings, gears, cam, and pushrods.
In a two-stroke, the fuel-oil-air mixture passes through the
crankcase before entering the combustion chamber; thus its oil
content lubricates all of the “lower-end” moving parts directly
before the fresh mixture flows through the bypass and into the
combustion chamber.
However, a four-stroke doesn’t work that way. The fuel-oil-air
mixture goes straight into the head’s combustion chamber. That
makes direct lubrication of its “lower end” impossible. But
combustion-chamber pressure during the power stroke forces oil
through the tiny clearance space between the piston OD and the
cylinder bore, into the crankcase interior.
That is, it does that provided there’s a vent in the case to permit
that flow. Without a vent, no throughflow would be possible.
Therefore, although
it might be tempting
to seal off a fourstroke’s
crankcase
vent and eliminate
the messy dripping
from that, don’t do
it.
Connecting a
short length of fuel
tubing to the vent
nipple and leading
that “overboard” is
fine. But don’t
block or obstruct
the vent.
A reader
suggested
connecting a fourstroke’s
vent to its
intake somehow, so
that the oil
emerging from the
vent could be
burned instead of
wasted. That’s a
mistaken notion.
For one thing, the oil from a four-stroke’s case vent isn’t exactly
“wasted.” It has already done its intended job of lubricating the
shaft, bearings, gears, cam, etc., but it needs to exit the case, to make
room for fresh oil being delivered from the power strokes.
For another thing, oil used in model engines should never burn.
It’s there to lubricate, not to provide extra fuel.
As for after-run oil (ARO) treatment of a four-stroke, I’d never
been happy with haphazardly squirting ARO freehand into one of
those engine’s case vents. That’s both messy and inefficient. I’ve
88 MODEL AVIATION
This 1954 K&B Greenhead .35 is one of
many model engines, spark and glow,
using .040-inch-diameter fuel orifices.
Notice the variety of spraybars with the
same-size hole.
A neat and effective way to inject ARO into a four-stroke is
shown. This method supplies plenty of oil and is then used to
drain away the surplus.
Testing spark coils with a multimeter,
makes sure they’re usable. Notice the
low-voltage winding’s 1-ohm resistance.
Never use more than 4 VDC with a
model spark coil!
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:09 AM Page 88
come up with a better and neater
procedure.
I sliced off the plastic-bottle nozzle of a
Tower Hobbies ARO a bit at a time, until
the hole in its end fit firmly over a 4-inch
length of medium-size silicone fuel tubing.
With the engine upright, I pushed the end
of the ARO tubing onto the vent nipple.
With the ARO bottle upside down, gently
rotating the engine’s propeller draws in
quite a lot of ARO on the piston’s
upstroke, even without squeezing the
bottle.
Shaking and twisting the engine to
distribute the ARO throughout the case
interior ensures that the oil flows into all of
the moving parts. Turning the four-stroke
and the ARO bottle back to an upright
position and briskly flipping the propeller
will force the surplus ARO back into its
bottle. But you’ll need to loosen the
bottle’s top to avoid “inflation.”
Some readers have inquired about oldtime
spark-ignition operation and its
components. Two queries involved spark
coils: how to tell whether or not old ones
are still good and how today’s model
engine coils stack up against Smith,
AeroSpark, and OK coils from the Good
Old Days.
Modern coils—such as those available
from Larry Davidson—work as well as the
larger and heavier coils of the past,
although their “winding ratios” are far
different. And today’s transistorized sparkignition
modules (also available from
Larry) plus Ni-Cd batteries eliminate 99%
of the problems we had to suffer with the
original Kettering spark circuitry, with its
insufferably unreliable condensers and
pencell batteries.
However, coils—old or new—do fail.
The main reason is overheating.
A typical model spark coil’s primary
windings have a resistance of only 1 ohm.
That means that with a 3-volt input, 3
amperes pass through the coil. And since
watts equals volts multiplied by amperes,
the coil must dissipate 9 watts of heat
energy whenever the engine’s points are
closed.
Old and new quality coils can withstand
that, but some modelers, in the quest for a
hotter spark, will use three- or even fourcell
battery packs in their spark circuits.
The excess heat from that much current can
melt the wax with which most old-time
coils were impregnated. And the excess
voltage generated in the coil’s “hightension”
windings by too high of an input
voltage can burn through insulation, or
even the hair-thin wire itself.
However, Larry’s new coils were
designed to use three Ni-Cd cells in series.
Even when fully charged, that battery pack
won’t harm one of his coils.
These types of failures can be checked
for with a digital multimeter. Using the
“ohms” function, check the primary (lowvoltage)
winding. That should show a
resistance of roughly 1 ohm.
Then measure the high-tension
circuit—between one of the primary
terminals and the one to which the spark
plug lead connects. That should measure
somewhere near 1,600 ohms for a “Larry”
coil; 6,500 for a large AeroSpark; 4,500 for
an Arden, small AeroSpark, or “tall”
Smith; and 3,500 for a Smith “competitor.”
An “infinite” reading means a burnedout
wire. Any reading that is noticeably
lower than those I cited indicates a
shorted-out section of the high-tension
windings. But the coil may still work; try
it to see. MA
Sources:
Cox Engines
(250) 398-2600
www.coxengines.ca
Saito Engines
(800) 338-4639
www.saitoengines.com
Larry Davidson Model Airplane Supplies
(540) 721-4563
www.modelflight.com/larrydavidson.html
Model Engine Collectors Association
(817) 295-8209
www.modelenginecollectors.org
Edition: Model Aviation - 2009/12
Page Numbers: 87,88,90
Fuel-line restriction problems
December 2009 87
Joe Wagner The Engine Shop | [email protected]
Also included in this column:
• Copycatting fuel orifice size
• After-run oiling for a four-stroke
• Spark-ignition quirks
Trouble-free fuel supply to this Cox Medallion .15 required a spraybar adapter to permit
use of medium-size fuel tubing. Notice the gently curved tubing path.
This Norvel .25 has a large-diameter inlet passage for its size. But muffler pressure to its
fuel tank guarantees dependable running in any flight attitude.
The 1948 Ohlsson .23’s greatly oversized
inlet reduced fuel suction too much and
required the addition of a restrictor around
its spraybar for the engine to run.
A FEW READERS wrote to me about
fuel-feed difficulties. One was Ross
McMullen of Wendell, North Carolina—a
longtime modeler and a former MA editor.
He was having trouble obtaining rich
settings on one of his Cox Medallions. He
could unscrew its needle almost all the way
without affecting the engine’s operation.
Ross solved the problem himself,
tracking it down to an accumulation of
minor restrictions in fuel flow. For one
thing, the tubing was too small. For another,
it was bent too sharply between the tank and
the spraybar.
His experience in locating the cause of
his trouble provided yet another
demonstration that suction-feed fuel systems
in our models need great care in minimizing
flow restriction in their “plumbing.”
Suction at a model engine’s spraybar
fuel orifice is never great. At best, it’s a
mere 3 or 4 inches of H2O, as measured
with a water manometer. And many power
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:09 AM Page 87
plants have
oversized intake
passageways, to
maximize power
output. An internalcombustion
engine’s power
depends on how
much air it can
“inhale.”
Oversized inlets
tend to minimize
suction at the
spraybar, because
they reduce the inlet
air velocity. That’s
why mufflerpressurized
setups
can be so helpful in
achieving reliable
fuel delivery into
our engines.
But even with
muffler pressure, I
recommend using
the largest-size fuel
tubing that will fit,
making certain that there are no kinks in the brass tubing in the tank
and eliminating long, complicated tubing paths between the tank
and the engine. When the tubing installation in your airplane is
complete, blow through it to make sure that the flow is as free as
you can manage.
The preceding topic reminds me of something from my Veco days.
I was working with Mel Anderson (who designed the Super
Cyclone, Anderson Spitfire, Baby Spitfire, etc.) on the Veco .35.
While discussing desirable features of the new Veco engine, I
mentioned to Mel that every model power plant spraybar I’d ever
checked had the same-size fuel orifice: .040 inch in diameter, for a
#60 drill.
Mel told me that while he was working out production design in
1935 for the Baby Cyclone .36 (first of the West Coast model
engines), he decided, for no particular reason, to use the smallestsize
drill bit he had for the fuel orifice. Evidently all of the other
engine makers either copied the Baby Cyclone’s spraybar setup or
chose a #60 drill for the same reason Mel had.
He and I then experimented and found that a larger fuel orifice
worked better in Veco glow engines. No doubt, that was because the
all-castor-oil-lubed fuel everyone used back then in glow engines
was more viscous than the three-to-one white gas and 70-weight-oil
fuel blend that had long been standard for spark-ignition engines.
I’ve received several inquiries from readers about four-stroke
engine lubrication, such as how oil can get into the crankcase during
running, the purpose of the crankcase breather, and how to go about
injecting after-run oil into the case, to prevent rust in the bearings.
Good questions!
The main purpose of a four-stroke’s crankcase breather port is to
permit lubrication of the shaft, bearings, gears, cam, and pushrods.
In a two-stroke, the fuel-oil-air mixture passes through the
crankcase before entering the combustion chamber; thus its oil
content lubricates all of the “lower-end” moving parts directly
before the fresh mixture flows through the bypass and into the
combustion chamber.
However, a four-stroke doesn’t work that way. The fuel-oil-air
mixture goes straight into the head’s combustion chamber. That
makes direct lubrication of its “lower end” impossible. But
combustion-chamber pressure during the power stroke forces oil
through the tiny clearance space between the piston OD and the
cylinder bore, into the crankcase interior.
That is, it does that provided there’s a vent in the case to permit
that flow. Without a vent, no throughflow would be possible.
Therefore, although
it might be tempting
to seal off a fourstroke’s
crankcase
vent and eliminate
the messy dripping
from that, don’t do
it.
Connecting a
short length of fuel
tubing to the vent
nipple and leading
that “overboard” is
fine. But don’t
block or obstruct
the vent.
A reader
suggested
connecting a fourstroke’s
vent to its
intake somehow, so
that the oil
emerging from the
vent could be
burned instead of
wasted. That’s a
mistaken notion.
For one thing, the oil from a four-stroke’s case vent isn’t exactly
“wasted.” It has already done its intended job of lubricating the
shaft, bearings, gears, cam, etc., but it needs to exit the case, to make
room for fresh oil being delivered from the power strokes.
For another thing, oil used in model engines should never burn.
It’s there to lubricate, not to provide extra fuel.
As for after-run oil (ARO) treatment of a four-stroke, I’d never
been happy with haphazardly squirting ARO freehand into one of
those engine’s case vents. That’s both messy and inefficient. I’ve
88 MODEL AVIATION
This 1954 K&B Greenhead .35 is one of
many model engines, spark and glow,
using .040-inch-diameter fuel orifices.
Notice the variety of spraybars with the
same-size hole.
A neat and effective way to inject ARO into a four-stroke is
shown. This method supplies plenty of oil and is then used to
drain away the surplus.
Testing spark coils with a multimeter,
makes sure they’re usable. Notice the
low-voltage winding’s 1-ohm resistance.
Never use more than 4 VDC with a
model spark coil!
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:09 AM Page 88
come up with a better and neater
procedure.
I sliced off the plastic-bottle nozzle of a
Tower Hobbies ARO a bit at a time, until
the hole in its end fit firmly over a 4-inch
length of medium-size silicone fuel tubing.
With the engine upright, I pushed the end
of the ARO tubing onto the vent nipple.
With the ARO bottle upside down, gently
rotating the engine’s propeller draws in
quite a lot of ARO on the piston’s
upstroke, even without squeezing the
bottle.
Shaking and twisting the engine to
distribute the ARO throughout the case
interior ensures that the oil flows into all of
the moving parts. Turning the four-stroke
and the ARO bottle back to an upright
position and briskly flipping the propeller
will force the surplus ARO back into its
bottle. But you’ll need to loosen the
bottle’s top to avoid “inflation.”
Some readers have inquired about oldtime
spark-ignition operation and its
components. Two queries involved spark
coils: how to tell whether or not old ones
are still good and how today’s model
engine coils stack up against Smith,
AeroSpark, and OK coils from the Good
Old Days.
Modern coils—such as those available
from Larry Davidson—work as well as the
larger and heavier coils of the past,
although their “winding ratios” are far
different. And today’s transistorized sparkignition
modules (also available from
Larry) plus Ni-Cd batteries eliminate 99%
of the problems we had to suffer with the
original Kettering spark circuitry, with its
insufferably unreliable condensers and
pencell batteries.
However, coils—old or new—do fail.
The main reason is overheating.
A typical model spark coil’s primary
windings have a resistance of only 1 ohm.
That means that with a 3-volt input, 3
amperes pass through the coil. And since
watts equals volts multiplied by amperes,
the coil must dissipate 9 watts of heat
energy whenever the engine’s points are
closed.
Old and new quality coils can withstand
that, but some modelers, in the quest for a
hotter spark, will use three- or even fourcell
battery packs in their spark circuits.
The excess heat from that much current can
melt the wax with which most old-time
coils were impregnated. And the excess
voltage generated in the coil’s “hightension”
windings by too high of an input
voltage can burn through insulation, or
even the hair-thin wire itself.
However, Larry’s new coils were
designed to use three Ni-Cd cells in series.
Even when fully charged, that battery pack
won’t harm one of his coils.
These types of failures can be checked
for with a digital multimeter. Using the
“ohms” function, check the primary (lowvoltage)
winding. That should show a
resistance of roughly 1 ohm.
Then measure the high-tension
circuit—between one of the primary
terminals and the one to which the spark
plug lead connects. That should measure
somewhere near 1,600 ohms for a “Larry”
coil; 6,500 for a large AeroSpark; 4,500 for
an Arden, small AeroSpark, or “tall”
Smith; and 3,500 for a Smith “competitor.”
An “infinite” reading means a burnedout
wire. Any reading that is noticeably
lower than those I cited indicates a
shorted-out section of the high-tension
windings. But the coil may still work; try
it to see. MA
Sources:
Cox Engines
(250) 398-2600
www.coxengines.ca
Saito Engines
(800) 338-4639
www.saitoengines.com
Larry Davidson Model Airplane Supplies
(540) 721-4563
www.modelflight.com/larrydavidson.html
Model Engine Collectors Association
(817) 295-8209
www.modelenginecollectors.org
