If you're talking about MVVS engines from the Czech Republic, I've had GREAT luck with 'em. Own 4 in total (the only other manufacturer from whom I own more engines is Thunder Tiger), one 40, two 49s, and one 91. The 40 with an 11x4 prop gives a 5lb profile plane unlimited vertical. The 49s is more powerful than the Asian 46s out there, and the 91 is literally a stump puller. Most of the issues that people post are in regards to the carb settings. But I've found once broken in (takes about 1/4-1/2 gallon using FAI, you can run 10-15% after running in) and tuned, they are as reliable as anything else I own... My current arsenal includes TT, OS, YS, Rossi, Webra, and MVVS glow engines.