Engine Troubleshooting, Part III

[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]

Needle valve settings.

Setting needle valves is somewhat of a "Black art" most engines have two needle valves: a "High Speed" and a "Low Speed". The names are descriptive, i.e., the low speed is adjusted when the engine is running at low speed. Setting one valve will affect the other, but it is minimal in most cases.

Always leave the low speed valve alone until you get the high-speed valve set. If your engine has run correctly before, don't touch the low speed valve until you try everything else first; it is probably correct.

Before you start, unscrew the high speed valve all the way out and take a good look at the end of the needle it should be smooth and slightly rounded, in addition the entire thing should be straight with no bend. I have seen needles break off just the last 1/32 of an inch. This makes it almost impossible to adjust the valve if you have an engine that just will not get lean enough this may be the problem especially if it runs with the valve all the way closed; or you may have damaged the valve seat..

1. Open the high speed valve about 3 turns from full closed. Note: Do not over tighten the needle valve when finding full closed; you can and will damage the valve seat.

2. Start the engine. If it dies immediately after starting, try opening the valve about a half turn. You can't hurt anything opening the valve; you can by closing it.

3. Once started, open the valve until the engine starts to sputter, cough and smoke.

4. Close the valve until the engine is running fast and there is no smoke. Immediately start opening the valve again until the smoke just comes back. No smoke is a indication that the engine is too lean; this is very bad. What you want is the engine running the way you think it should and still putting out light visible smoke.

5. Fine-tune the engine for the best sound with it still putting out smoke.

6. Now with the engine running fast, pinch the fuel hose so the fuel is cut off for a second.

a. If the engine speeds up then slows down after you pinch the hose, close the valve some.

b. If the engine speeds up and continues at the same speed after the pinch you are close; the valve should be closed just a little more.

c. If the engine does not speed up and continues running at the same speed after the pinch you have the correct setting.

The low speed valve is very touchy. Make only tiny adjustments and write them down if you can. Trying to remember if you turned the valve clockwise and how much after a ten-minute flight ending in a dead stick is hard to do. Make sure your engine is broken in and warm before you try to adjust the low speed needle valve. If your engine is still new and tight it will not likely idle correctly at any adjustment. Make sure you have adjusted the high-speed valve before you start on the low speed valve.

Never try to adjust the low speed valve with the engine throttle open by more then a third. Ideally it should be at the idle setting..

If the valve has not been adjusted since it left the factory work from there; skip the rough adjustment. If you are sure it is wrong use one and one half turns from closed as a starting point. When you are finished you want the engine to idle smoothly with just a little smoke coming out of the muffler.

First you have to rough-adjust the valve. Open the valve first until you start getting raw fuel coming out of the muffler and the engine dies as soon as it goes to idle. Now start closing it down until you have little or no smoke. Now fine adjust it no more then 1/8 of a turn at a time until you see some smoke and the engine will run at the speed you want it to run at. New fliers need someone to help them determine what this magic speed is. Do not rush this. Remember, the engine is using very little fuel so adjustments are slow to take affect, count to 30 before you make another adjustment. Try the idle with different amounts of fuel in the tank. It will be most important when the fuel in the tank is low because that will be when you are trying to land and need a good idle. Once you have the low speed adjusted, try not to mess with it again. It should not change unless you change fuels, altitude, or air temperature.

If you cannot get a good idle, try a smaller or less pitch propeller. If the engine is under too high a load it will not idle at a slow speed. Another thing that will keep an engine from idling properly is if the engine has bad bearings or has a bind. Take the glow plug out and turn the engine over slowly it should be smooth and not bind at any point. ABC engines will have tight points, but they should not bind. Now check for play from side to side at the point that the crankshaft comes out of the engine. There should be a small amount, more if it is a bushing engine then with ball bearing engines. Any excessive slop will allow air to leak into the engine around the shaft and indicates that the bearings are worn.