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FS-120SIII Pumped

Old 09-28-2010, 09:39 PM
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Default FS-120SIII Pumped

I have this engine mounted in a Seagull YAK-54. The time that I have on it is last years season and so far this year. It's been absolutely flawless, hardly even needing needle valve adjustments, I have adjusted the intake and exhaust valves two or three times.
I just put the cowling back on it and flew it on Sunday. After starting it, ran it at full throttle for a while but noticed that the RPM would increase approx. 300 rpm (using a digital tach) and would no longer emit as much exhaust smoke as before the idle changed. Then the idle would go back down to where it should be and start smoking what I would call a normal amount. The engine fluctuated up and down this way as long as it was running. I ensured the needle valve was set properly which changing it made no difference. I went ahead a put one short flight on it which I noticed in the air it was acting the same. I could visually see a good exhaust trail then it would go away and just go back and fourth that way.
At home I used a hand vacuum pump and checked the fuel lines, fuel tank, and remote fuel port all as one which did not show any type of leak. I also used the vacuum pump to check individually the three silicone tubes on the engine which all three checked good. Checked all the srews on the engine for anything lose, all were tight. Checked intake and exhuast valve clearence, only had to adjust the exhaust valve. Checked for any cracks anywhere on the engine, non found.

Thanks in advance for any help. Hopefully this isint to long winded.
I only own OS engines. I've had many over the years mostly 2-stroke but the last four years have been only flying 4-stroke. Have'nt ever experienced any 4-stroke problems before so don't know where to go after troubleshooting to the point that I have. Any suggestions welcome.
Old 09-29-2010, 08:59 AM
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Default RE: FS-120SIII Pumped

There could be several things going on. The engine could be set too rich, it may be too lean, it could have cooling problems, the regulator's not working right, the idle's too rich/lean, and so forth.

If the engine will run perfectly without the cowling, and then act up with the cowling, we'd strongly suspect that the engine's not getting proper cooling. Cooling air must go through the fins of the engine, so you must make sure the cowl makes that happen. Many times, the cowl is large-enough that cooling air just goes around the engine and directly to the outlet without passing through the fins. If the air doesn't go through the fins, the engine won't cool.
Old 09-29-2010, 06:04 PM
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Default RE: FS-120SIII Pumped

Thanks for the reply.

It has run equally well with and without the cowling on the airplane. I just checked the fast needle valve adjustment which happened to be 3/4 of a turn open from fully closed, I have to presume that was the setting when it was running but not certain since I have removed the engine from the plane.

I checked the manual, it states that you should "turn the mixture control screw back exactly 2.5 turn" from closed. I'll try that.

The fuel tank was checked for leaks which turned out ok.
If there were a pin hole in the fuel line inside the tank, do you believe that would cause these symtoms?

Thanks again
Old 09-30-2010, 08:55 AM
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Default RE: FS-120SIII Pumped

If air can get into the fuel line, the engine would go lean and tend to detonate and/or overheat.
Old 09-30-2010, 08:14 PM
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Default RE: FS-120SIII Pumped

Today I took the carb off and cleaned it, put it all back together and ran it again. No change, but leaving the cowling off till things are worked out.
Also brought along another fuel tank, tried it with no change.

While I had the carb off of the engine I removed the intake pipe and back cover plate to inspect the gaskets which both looked fine.

Ok, I believe I have done everything I can think of. Can you think of anything else to try? What would be the next logical step?

Thanks

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