Dear AnnMarie,I take on board what you said about exhaust systems causing overheating problems ,as I said in my original thread, I use the standard exhaust on my OS 91 fx. I know now of two people with the same problem with this engine and,obviously a few more judging by the web site.Please be honest,is there a problem with this particular engine?,nothing is worse than irritating engine performance but, if one is on a hiding to nothing then, that makes it all the more annoying.Does OS have a problem with this engine?????