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OS FS-120S-E Won't Run

Old 04-04-2004, 01:38 PM
Lucky Dog
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Default OS FS-120S-E Won't Run

Hello Bill,
I've got an older FS-120S-E that won't run for more than 15-20 seconds at a time. (Another FS-120S-E works just fine on the same aircraft). I checked the gap on the valves, they were fine. I have partially dis-assembled the engine and carb. Upon disassembly of the engine, I don't think I see much out of the ordinary. The only thing that seemed odd was after removing the head, when I turned the crank over there was an audible hiss and a small amount of fuel seems to come out from a point on one side between the piston and cylinder wall. If you were looking at the top of the engine, with the prop shaft being at 12 O'clock, it was coming out at 9 O'clock. Bad ring maybe? Cylinder walls seem to look smooth. There was a fair amount of thick fuel residue (looked like green paint) on the top of the cylinder. One of the O-Rings for the Push Rod Covers needs to be replaced (torn).

Valves springs seem tight (I guess). Have to press pretty hard to move them. One valve (the smaller of the two) maybe, just maybe, doesn't go as flush as the other when closed. There did not seem to be a lot of carbon build up. I have not removed the valves.

Carb looked pretty clean. However, there looks to be a couple of small cracks in the O-ring of the needle valve, not torn thru though.

It was also suggested to me that the symptoms might be consitent with the timing being off. How does one go about checking this?

I look forward to hearing from you.
Old 04-06-2004, 01:24 PM
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Default RE: OS FS-120S-E Won't Run

You'll need to check the proper setting of the rocker arms to the valves. If they're too tight, then your engine won't run well, and the valves may not be fully closing. If the rocker arms are set correctly, and you still have a lot of valve leakage, the engine may never have been broken-in correctly, and you have a valve that's not sealing properly.

Model four-stroke engines will all have some compression leakage when turned over by hand. The compression helps seal the valves shut, but a slow turnover doesn't allow the compression to build high enough or give a hard enough 'push' to seal the valves.

Running in very short bursts usually means that the engine's not getting fuel. If you have an identical engine that runs well, try swapping carbs. If the engine then runs, you know that it's not getting fuel. It may be that the carb had been disassembled and the incorrectly assembled. Something to check.

Also, as stated in the duplicate post you made in the general engines forum, you'll need to check the engine for proper assembly, especially the timing.

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