Hey Gang, 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.
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. One of the O-Rings for the Push Rod Covers needs to be replaced (torn).
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 consistent with the timing being off. How does one go about checking this?
Any thoughts would be greatly appreciated.
Many thanks! Mike "Lucky Dog"