How to Buy a Used Gas Engine
by Don Apostolico
Learn what to look for before you buy
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 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 them. By noon today I had received two calls on our tech line from modelers complaining about their rough-running 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 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 rpm 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 may have statically balanced but dynamically unbalanced the propeller because of different airfoil shapes resulting from the removed material.
A spinning propeller like that 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-the-headlight 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: has the propeller 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. 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 also result from 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.
Ask if the engine has had any propeller strikes. Many engine designs have split crankshafts, which are 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, 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 pop-off 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 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. Install and tighten the propeller, then use a plastic or wooden mallet to rap the end of the crankshaft. This will snap the bearing into place. Retighten the propeller and tap 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 helps prevent 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.
Sources: Don’s Hobby Shop (785) 827-3222 www.donshobbyshop.com
Transcribed from original scans by AI. Minor OCR errors may remain.
