There's a trick to tuning my MVVS 40. Unlike any Asian engines I have, you need to tune the engine's high speed needle while the plane is vertical. Doesn't make any sense but works perfectly. Makes the engine sputter and hesitate when horizonal on the ground (static), but for some reason the minute it's airborne, the engine unloads quite a bit and runs normally! In any case, I've just run 1 gallon of FAI fuel through my 40 and started using 15% nitro... Without any problems. The little 40 has unbelievable thrust with an 11x4 prop. Enough to hang my Morris the Knife ARF and pull out vertically with authority. And ah, the sound of that mini-tuned pipe is music in my ears.