I read most treads to this pointon this topic some good some not good. Been using my MVVS 40 all summer and fall in my Sig Something Extra At fist I was about ready to toss it out and install my YS 45 . I had the same problem as some of you run in to, no power wide opened up when flying most was just during break in period about it took it about one gal to come around running 10% Powermaster Apc 11-4 prop. After many plugs witch blows often. Especially using cheep ones. Found using short plugs work great. OS 4 the best. the motor would come alive with a new one. also check all heads bolts they come loose due to vibes motor puts out. So don't give up with it I have learned to love it and hate it at the same time. It pulls my Something extra vertical with no problem .