I read a post somewhere that a guy had similar issues and it was plugged up exhaust system. Is that worth looking into? And as added info, back when this happened FIRST (over a year ago before doing any carb rebuilding, etc.), the motor ran great and then it didn't. We were out running it wide open and all of a sudden, this started. I messed with it just a bit back then to try to resolve it, but never did. Then, this year, started the process of getting it back in order. Thought maybe that part of the story (when and how it happened) might help ID the culprit.