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O.S. FS-120S-E Won't Run

Old 04-04-2004, 01:43 PM
Lucky Dog
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Default O.S. FS-120S-E Won't Run

Oh Great and Wise Ones [sm=wink.gif],
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 consitent with the timing being off. How does one go about checking this?

Any thoughts would be greatly appreciated.
Old 04-06-2004, 12:25 AM
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Default RE: O.S. FS-120S-E Won't Run

With the piston at top dead center, the dot on the cam should be in line with the push rods. Not straight up but slightly to the rear.

Take off valve cover
Remove rocker arm stand
Remove cam cover
Push exhaust cam follower up out of the way. So you can see the dot.
There should be a make on your prop drive hub that will align with the casting seam on the case when the piston is a top dead center.

Reassemble and adjust the valves.


Old 04-06-2004, 04:57 PM
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Default RE: O.S. FS-120S-E Won't Run

Lucky Dog,

You did not mention how the compression was on this engine. What’s it like in relation to the other engine?

Not too long ago I got another Ebay engine, OS 91 with pump and it arrived very clean but with little to no compression. This engine had way less compression than a Satio .65 that I’ve ran for about 8 to 10 years. Upon inspection of the cylinder and piston, they looked nearly brand new.

Took head apart and did homemade valve job (little toothpaste under the valves and rotate them a few times on their seat). Make sure to thoroughly clean valves after doing this and use a good quality toothpaste….beware of denture cream :-)

Upon reassembly the engine had tons of compression and started as soon as the pump sent fuel to the carb. Engine ticks over like a clock and had plenty of power in the air.


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