Heres the deal. I had the same symptoms as your having. I changed the spark pulgs because I figured after 800 hours it might need it anyway, made no difference. So I took the air cleaner housing off and saw the casting plug had had came out,,, I dont know if the plug comes in a curaburator kit, but the curaburator is 180$ and Im cheap so I thought about how to fix it. The main carburator jet goes through up to where the plug goes and it looked like there were passeges so some sort of plug was neccesary. I wound up <DONT LAUGH> taking a small split lead fishing weight and breaking it in half filed on it till it could be tapped into the carburator, then took some 2 part epoxy and put it on top so the plug wont fall out. It has worke for about 6 weeks so Im sure its a permant repair.