I have an OS 70 series four stroke engine in my Cub which has begun to run a bit erratic lately. When I fly it tends to surge ever so slightly even with a full tank. About two years ago a friend of mine at our field adjusted the carburator fuel mixture control valve on the carb and it was running fine. I fly off the lake at my cottage and last summer I crashed in the water and destroyed the plane. I have since built a new plane and the engine has been running well, accept for the slight surging. I suspect the mixture control valve. I don't suspect any damage caused by the crash, the engine quite before impact. Also, I had to remove the high speed needle valve to install the new cowel. Upon replacing the hs needle valve, and setting it properly, it would idle and accelerate fine. The problem is the slight surging. It did this before and I don't know how to adjust it. My friend is no longer with the club. I downloaded the owners manual but the way the adjustment is to be made is unclear. This appears to be an obsolete engine so I am at a loss at what to do. Any advise, vidios maybe would be appreciated.

Do you see any air bubbles in the fuel supply line ? If so, look for leaks in your fuel system. If you find any, repair them, and the surging should go away.

OS engines often have a small O ring on the adjusting needle. It is worth while replacing it and see if things improve.

Needle settings are generally straightforward. Surging can be the result of a very rich setting, but if a simple tweak doesn't fix it, the problem is elsewhere. There could be an air leak in a fuel line, a bad oring, or a crack somewhere. Bad fuel is a common cause of your oroblem too.

make sure your tank venting is good......no pinched line. also check for a rich low speed needle setting.