Erratic high speed OS 52 surpass

by Mike
(Greenlawn NY)

The high speed mixture is constantly changing. Rpms will raise and lower erratically from 1/4-full throttle. Idle seems to be perfect. I’ve tried: Multiple new os type f glow plugs throughout my testing/tuning. replacing the fuel line and filter. Replacing the o-rings in the carburetor (all of them).

Replacing the fuel tank. Inspecting the exhaust pipe for cracks loose fittings etc. and adjusting the intake and exhaust valves. Nothing has helped.

Compression is good when cold and hot. I’m using the same fuel I use in my os 91fs and os ft-160. Neither of them have any tuning issues so I doubt it’s the fuel. I’ve tried everything I could think of.
Any suggestions?

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