Engine Troubleshooting, Part IV

[Editor’s Note: this information was contributed by Rick Scott and was obtained through an Internet search. The source is unknown. Additional segments may appear in future issues of Glitch Busters]

Fuel Tanks

Most fuel systems work by having pressure from the muffler pressurize the tank in the airplane, pushing the fuel out of the tank into the engine. Some engines have pumps or even use gravity feed.

If you look inside the air intake of the engine you will see a small brass tube that runs across it. This is the port that the fuel is sprayed from into the carburetor. The center of your fuel tank should be at the same level as this port no matter how the engine is mounted. If the tank is too high it will have what is called a positive head and too much fuel will flow into the engine. Sometimes the fuel will even siphon into the engine and leak out of the carburetor when the engine is off. If the center of the tank is too low the engine will run lean and have trouble getting enough fuel when the fuel is low in the tank.

If you can not get the tank high enough you might look into buying a small fuel pump, they mount onto the back of the engine and vibration pumps the fuel, they do not need electric power..

Most problems with fuel tanks are caused by leaks in the tank or hose. Even a pinhole will keep an engine from running. Leaks in the bottom of the tank or output hose are obvious as the fuel will leak out, although some small leaks may not leak fuel until the tank is under pressure. Unless the leak is huge it will not effect the running of the engine, however some holes leak air into the fuel line, not fuel out, this is what will cause your engine to run badly. Watch your fuel line with the engine running. If you see bubbles in the line you may have a leak or the fuel is foaming. Leaks in the pressure side are harder to find, but effect the pressurization of the tank. Pressure problems can also come from the muffler. If the pressure port is clogged or the muffler is loose you can loose pressure.

The best way to check out a tank is to disconnect the hose from the engine and blow through the tank. You should feel very little backpressure. If you do, check for pinched hoses or the pickup hose in the bottom of the tank kinked. Now pinch closed the pressure hose on the muffler and pressurize the tank like you were blowing up a balloon. Now hold the pressure. You should not feel any pressure loss. If you do, you have a leak. IMPORTANT: let go of the pressure hose FIRST, before you release pressure with your mouth or the pressure in the tank will pump a mouth full of the worst tasting cocktail you have ever had into your mouth!!! (Of course if you have ever done this before you already know that.)

Another problem is fuel foaming. If you have too much vibration the fuel will mix with air in the tank and foam. If this foam gets into the fuel line the fuel mixture will be screwed up. This often happens during a high speed run when the fuel is getting low. It is almost impossible to see this problem on the ground, but if you see bubbles in the fuel line and there are no leaks this may be the problem. A soft engine mount or padding the fuel tank and balancing the prop may help a lot. An engine that is set up correctly and suddenly stops in the air especially right after increasing altitude probably has this problem.

On your first flight of the year pump some fuel into the tank then shake the airplane. Then pump the fuel out again and discard. You do this to clean the tank up, the fuel and oil left in the tank all winter can turn to goo.

One other problem I had using a small 2oz tank: the fuel hose inside the tank between the clunk and the fuel fitting was so stiff that the clunk would not reach the bottom of the tank. It just stuck straight out. I had to add weight to the clunk to get it to the bottom.

 

 

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