There are issues with leaking needle valves and Evo "factory settings" that aren't even close. In some cases the slow speed idle was completely closed off!! I bought an Evo 61 to try. The engine would change setting in the air and I finally got it to run decent by replacing the carb with one from a TH 75 engine. Add that to the price of the original engine and the price of the fantastic running OS FX/AX engine is a bargain!! The Thunder Tiger are also very good. Either engine in this size are very good. In the next larger size engine look at the ASP 52 Super series. Scary power output and smooth running.