I had an Evolution 100 that just would not run right upside down. I played with it for two or three months, got very frustrated with it. I changed over to an OS 1.20 AX and mounted that upside down and it ran like a champ, no problems at all. Same installation, same plane, but different engine and it was great.

I don't know much about the ST 51 you are planning on using but do know that the Evo 100 was no good in that application. It was very fussy about needle settings, idle set then HS set it would run fine at full throttle but not idle. Set the idle and it would not run good at HS. Fly it, and it would die after a few orbits. Even the club pro's could not get it to run right, and we have some real pro's in our club. As someone said above, you will definitely learn to tune an engine that way. I learned a lot with the EVO 100 that I applied to the OS 1.20 AX, and it was worth it. The plane, an Excelleron 90 pattern plane, has pretty much unlimited vertical and runs forever inverted. With the EVO 100, I was happy to get a couple orbits out of it before it started to act up. Pain in the butt.

As I said, all I did was change to the OS 1.20 AX and nothing else.. and it ran like a champ. All depends on if you know enough about the engine and can get it to run right at all settings and speeds. No one runs at full throttle all the time, so mid range is important... so the transition zone between idle and WOT is a critical area that some engines just don't tolerate well, and the EVO 100 is one of them. All I can say is try it, and get it set up right, and see if it runs ok. If not.. well, alternative engines may be an option.

DS.