About a year ago I bought a Magnum 30 four stroke engine after reading a glowing review in an RC mag. (lesson #1: R/C mag reviews are about as honest as B.C.) The thing had almost as much power as a fox 15. 
I wrapped it up in an oily rag and tossed it in the engine box. A failed experiment. Actually it would have been just about the right power for a Flight Streak but it is waaay too heavy... Last week I dug it out to give it another try: I mounted it on my Primary Force and fueled it up with Power Master Helicopter fuel: 30% nitro and lots of castor oil to carry way the heat...
The combo actually worked! I don't recommend that anyone go out and buy one of these. But, if you've got one - try some high nitro fuel...
I flew it Sunday AM until the wind got too high for me. I've got to try some more trimming: messing with the tank and props etc.