I've built this great little aerobatic plane, my own design weighing in at just 2lb and flys just great on an OS15 LA (.41 hp). I wanted a bit more power so I bought a cheaper SC15 ABC at .67hp. The engine runs fine for about 2 minutes then starts losing and gaining engine revs constantly, almost to the point of stopping then it starts going faster and so on. Does anyone have any clues to what might cause this effect, considering the OS15 LA works really well.

Just to note the SC15 really dies when I go into a vertical climb.

Fuel used on both engines: Fully synthetic, 10% nitro.

6oz square tank very close to engine.

Running to lean . Any air bubbles in the fuel line? Make sure tank is well isolated from fuse. Could also be an airleak.

The tank is squeezed into the fuse due to it's small size. I'll get a smaller tank and wrap it in foam.

There are quite a few tiny bubbles but I didn't think they would make much difference. Perhaps it's generating more later on in the sky.

I'll take the tank out and check for leaks too, many thanks.

As the engine unloads in the air more bubbles are formed causing the engine to slow down. Less bubbles when it slows down making it go richer causing it to rev up again, and so on. Bubbles are displacing fuel the engine cant get. Taboo. If it's only touching formers you might be able to grind out a bit. Goop also works good for an insulator.