Troubleshooting gas engines

Model Airplane News, Jan 2001 by Yarrish, Gerry

Gasoline engines are usually very easy to operate. Once they have been properly adjusted, the carb settings can normally be left alone for a long time. Sometimes, however, things can go haywire, and frustration replaces the joy of burning gas. This time, I thought I'd share with you some of the more common problems and solutions associated with running gasoline engines.

TROUBLESHOOTING 101

When you're trying to figure out why an engine doesn't work properly, you need to take a look at the three basic elements that make up the "fire triangle": air, fuel and heat. All three are concentrated around the carb and ignition system.

Let's start with the carb. Fuel and air enter it and mix together. If you can't get your engine started, check the following.

* Needle valves. These should be about one turn out from fully closed for the high-end needle and about 1 1/8 turns out for the low-end needle. Unscrew them completely to make sure nothing is blocking the fuel flow. Then screw them in fully and back them out accordingly.

* Fuel fitting. Make sure the fuel is flowing into the carb. A blockage here is most often the culprit. Also check to make sure that the fitting is not cracked or leaking. If it is, replace it.

Fuel pump. Place your finger over the carb opening (or close the choke), and flip the prop several times. Is the carb working and drawing fuel? If you have recently taken off the carb, make sure that you have replaced it correctly and that the carb heat/spacer block is in the correct position. A small pressure-- feed hole in the block allows crankcase pressure pulses to enter the rear of the diaphragm pump. If the hole in the carb and the hole in the block don't line up, the carb will not work. Also make sure that the holes are not blocked by debris.

* Fuel screen. The small fuel-filter screen in the carb can become dirty after several seasons of flying, and this can affect the engine's performance. Carefully remove the screen, flush it clean and replace it. For more details, see Rick Eyrich's article, "How To Maintain your Gas Carburetor," in the October 2000 issue of Model Airplane News.

* Fuel lines. The rich and lean conditions of the fuel mixture depend on how much air is mixed with the fuel. If the engine begins to sag and overheat, then the mixture is getting too lean. A hole or cut in the fuel line can cause the mixture to lean out, so check the condition of your fuel system's plumbing. Start at the clunk, and work toward the fuel fitting. If you have an in-line fuel filter (a good idea), make sure it is tightly sealed. Also, gasoline is relatively cheap compared with glow fuel, so treat your engine to fresh fuel once in a while. If your gas can has been sitting in the garage all winter, dispose of the gas properly and mix up some new fuel.

Another possible-though uncommon-- reason for the carb to stop pumping fuel is an air leak in the crankcase or cylinder. If your engine starts, runs briefly, then stops after you have squirted raw fuel or starter fluid into the carb, check whether any of the bolts that hold the cylinder to the case are loose. If you have installed a pressure tap in your case to run a smoke pump, make sure that it is properly sealed.

IGNITION PROBLEMS

The most likely problem here is a dirty, fouled spark plug. Check the plug's condition often, and make sure that the insulator isn't cracked. Make sure the grounding wire and kill switch operate properly and, if you have an electronic-ignition system, make sure its battery is properly connected and fully charged.

Make sure that the magneto and fly-- wheel gap is correct. For a quick check of the gap distance, I use three layers of 100grit sandpaper for a makeshift feeler gauge. This may seem crude, but it has served me well for years. Spark-plug gaps seem less critical, and they work well from anywhere between 0.020 to 0.035 inch. Check the coil and the spark-plug wire to see whether they have frayed or cracked. Look inside the spark-plug boot, and make sure that the coil that fits over the top of the plug fits properly and "snaps" into place. If the boot fits loosely, arcing and erratic engine operation will result.

CUSTOM LINKAGE

An ongoing challenge for gasoline-engine users is making and installing throttle linkage. Most Walbro carbs have the industrial web-shaped throttle arm, and this requires you to devise some sort of throttle-- linkage attachment. To make this job easier, Bruce Hanson of B.H. Hanson* has made little, molded-- plastic throttle-arm fittings that fit over the carb's butterfly pivot shaft. Inside the cap is a locking collar that fits over the Zenoah* G-23 pivot shaft and is locked into place with a setscrew. As you can see from the photos, a simple ball link is all that's needed to connect it to the rest of the linkage.

In my inverted-engine setup, I used a simple, flat aluminum mounting bracket to support the throttle bellcrank; I attached the bracket to the engine with the carb-mounting bolts. The rest of the linkage is made from Rocket City* ball-- link clevises, a 4-40 threaded rod and some plastic Sullivan* Nyrod. The hole in the cap's locking collar can easily be drilled out to fit other gasoline engines.