Hi guys,
I have a further request for help with an OS Gemini 120.
BACKGROUND - I purchased this old engine via e-bay, it has "OS one one side of the crankcase and "FT-120" on theother, the carb has an idle mixture screw in the centre of the carb barrel, the main jet is three and three quarter turns open, the idle needle is about one turn open from being fully engaged when the throttle is closed.

THE PROBLEM - The engine will start and run OK but there is a fair amount of unburnt fuel coming out of the exhausts (Vario helicopter silencers) but there is a tendancy for one cylinder to stop firing, this is more likely at low carb openings but can happen at full throtle. Applying power to the glow plug (OS F) will usually "restart" the non- firing cylinder. The problem can happen with either cylinder - no pattern that can discern. If the cylinder stops firing at mid throttle and the throttle is opened in an attempt to restart a really horrible knocking ensued until lower carb opening is restored. Valve clearance is OK I think (Checked three times).

CONTRIBUTORY FCTORS - When I purchased the engine, all the internal bearings were very heavily corroded (It felt rough when turning by hand so I stripped it down before attempting to run the engine. In this process the camshaft came out of the engine with the backplate and so the valve timing was lost. I now have the valve timing set as follows, The "center pop" mark on the crankshaft, the center of the crankshaft, the center of the camshaft, and the hole in one side of the camshaft geat are all in line with the "center pop" and the hole outermost. The Cylinder heads, valves, valve seats, cylinders and piston rings appear to be in good condition but I have little experience of 4 stroke engines.

The engine is destined for a Vario Bell 47G helicopter but with the present problem of one cylinder "going out" I cannot even think if installing it.

Any help or advice would be very much appreciated.

Paul