Oh boy.... I remember a problem that sounds just like what you are experiencing. It was at an airshow 2 years ago. Kolby Kuipers was is name, and the idle wouldn't want to come down all the time. Now if I remember correctly, I didn't personnaly fix it, but I did watch. He had taken off the whole carburator off, and a screw for the butterfly valve somewhere a little loose, it seemed like it was hard to tighten but then he put it back together. The motor ran just fine, at any speeds and the idle was just awsome!

Sorry I wouldn't be able to answer any questions, all I remember is that there was a screw loose somewhere in the carburator.

LeBlanc