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.