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 .

My MVVS 40 also required a longer break in period than I was used to (hey most of my engines are TTs and OSs). It took nearly a quart of FAI before I was comfortable in the air with it. Probably another half gallon before it started to transition well. But now with over 1/2 gallon of FAI, 1/4 gallon of 5%, and 1/4 gallon of 15% run through it, it's become a wildly fun engine to run, especially with the growl of the tuned pipe!

BTW, I took out the plug that came with the engine shortly after receiving it because it wouldn't idle worth a damn with it and put on an OS #8. I recent put the Rossi R5 back on the engine and started using 15% nitro without issue (and now it idles fine with this plug also). From what I understand, I have a newer 40 (bought it in October from Morris) which may be more tolerant to nitro than older models.