Engine Troubleshooting, Part V
[Editor’s Note: this information was contributed by Rick Scott and was obtained through an Internet search. Thanks to Tim Mihalski of First State RC, the source has been identified as Aircraft Proving Grounds located at
www.geistware.com/rcmodeling/engine_troubleshooting.htm. This is the final segment of this article]
Overheating, Over revving
If your engine over heats it'll randomly stop for apparently no reason or freeze up.
Using too small or too low of a pitch in a propeller will cause the engine to over rev. this not only wears out the engine quickly it also overheats it. Most engines have a recommended propeller in the instructions. You may have to use one a little bigger or smaller then the one recommended according to the aircraft it is on, but you should not go too wrong if you stay in that area. If you have a tachometer you can easily see if the engine is in the manufactures suggested RPM range.
Nitro
Using too high a nitromethane content can also cause overheating do to over revving and because the fuel burns at a higher temperature.
Exhaust restrictions.
If something is plugging up your muffler the engine will run a lot hotter then it should do to back pressure. The use of rubber extender tubes restricts the exhaust flow and will cause overheating, low power output and poor idle on two cycle engines. Four cycles will tolerate extenders usually without problems.
Air flow
On aircraft with cowlings around the engine you must have air holes to allow air to cool the engine. Just because you have a hole for air to get in does not mean anything if you do not have a hole for air to get out.
Too Lean
The leaner the fuel setting the faster the engine runs, but it also makes the engine hotter. Make sure you have some smoke coming out of the muffler at all times.
Timing
On four cycle engines if the valve timing is out so that the valves are open a tiny bit all the time the engine will overheat. When setting the timing, error on the too wide side there must be some clearance however small. On two cycles too hot a plug can cause the engine to work against itself making it run hotter.
Engine Preventive Maintenance.
The best thing you can do to prevent engine problems is to keep your engine clean. Dirt causes a lot of problems that are not always obvious as to cause. Most dirt enters through the carburetor, but can also enter through contaminated fuel or loose muffler connections. One of the most risky times is after a crash or landing in high weeds, dirt and plant material may enter the engine at this time. Never turn over an engine after a crash until you make sure all dirt has been removed from the carburetor. If dirt may have entered the carburetor you should remove the carburetor from the engine and wash in clean fuel. Check the input hole under the carburetor for dirt before replacing the carburetor.
When fuel burns it chemically changes, the exhaust gases and waste oil become acidic and can quickly corrode ball bearings and other parts in your engine. The best way to prevent this is to use after run oil. You can buy bottles of after run oil at most hobby outlets, but a lower cost alternative is to buy air tool oil. Most large hardware stores have several brands. The reason you have to use air tool oil instead of motor oil is that it is not petroleum based. Petroleum based oils can attack the silicone o-rings in a glow engine. If you have a gasoline engine use any good 2 cycle motor oil, do not use air tool oil, as it will attack your neoprene rubber fittings. Make sure you check the manufacturer's instructions before using any after run oil, some glow engines with pumps can be damaged with after run oil.
With 2 cycles engines simply drip some after run oil into the carburetor while turning over the engine by hand until you have put about one half ounce into the engine.
With four cycle engines you will have to connect a fuel hose to the crankcase breather port and inject a half-ounce of oil with a syringe while turning over the engine by hand. This is best done with the airplane held tail up so the oil is more likely to run into the bearings. Next inject another half ounce into the fuel input port with the throttle open this time make sure the barrel of the carburetor is pointing up or the oil will just run out.. This is messy and oil will be dripping out of your engine until you go flying again, but it is better then having your engine freeze up.
Next time you go flying make sure that you turn the engine over a few times by hand before starting to make sure the engine is not full of oil and liquid locked.
Four cycles require that you check the valve timing and clearance every so many hours. Your instructions should tell you how to do this. Basically you turn the prop until it is at TDC on the compression stroke at that time the valves should be closed and there should be a small amount of clearance between the valve arm and the valve stem. The hard part is determining when the engine is in the correct position. It is critically important that the arm not touch the valve stem. Remember that when the engine is hot the clearance may be smaller do to expansion. If the valves are left open even the smallest amount they can be burned, you also lose compression and power.
ENGINE REFUSES TO FIRE
Weak or dead battery; bad connector, wires, or burned out glow plug. Check the circuit, the glow plug should glow bright red/orange. If all the above checks out, is the engine getting any fuel?
ENGINE STARTS, SLOWS DOWN, STOPS
Engine is too rich. Close the needle valve and flip prop until engine fires. It should run for a few seconds then die. Repeat a few more times until it won't fire, then reopen the needle valve to restart the engine.
ENGINE STARTS WITH LOTS OF POWER, RUNS FOR A FEW SECONDS, THEN DIES
Engine is not getting enough fuel; mixture is too lean. Open needle valve slightly and try again. If this problem persists, more may be wrong. Check the carburetor for dirt. If necessary, disassemble and clean it. You could also have pinholes in the fuel line, so check it carefully, and replace if necessary. Finally, make sure the fuel line is not pinched inside the fuel tank compartment.
ENGINE TURNS HARD AND DOES NOT START
It's flooded! Close the needle valve and remove the glow plug. Turn the engine upside down to drain the cylinder, rotate the prop also to help remove excess. Next, replace the plug and try again with the mixture screw leaned out more. An electric starter can help out here, but do not apply the starter and crank, and crank, and crank until the engine starts. Use short "blips" of the starter as this will turn the engine over several times, and help preserve battery life, spinner life, and engine parts. reinstall a new glow plug, reopen the needle valve, and proceed with the manufacturers starting instructions. Make sure you don't prime the engine or choke it too much!