I have an FS .70 Four stroke that idles perfectly and runs very good all the way up through 2/3 trottle. At times full throttle is steady but it eventually stumbles. I put in a fresh plug and this improved the engine somewhat. The other night I just spent a few hours adjusting the valves and got it to run as good as I can but it still stumbles a bit at full throttle.

The engine is on a Great Plans Piper Cub which I bought already built. The plane happens to have a DuBro remote fueling valve on it and this thread has me thinking that maybe the valve is leaking air when the fuel flow is at its highest point. I will have to try connecting another fuel tank and running the engine again. Could the fuel valve be part of my problem? I bought the plane from the builders son who sold off all his father's equipment after his father passed away so I don't know how long the plane sat before I bought it. Maybe the rubber seal in the valve dried out too much in the mean time.

Anyone have a similar experience?