I have cleaned up numerous carb with great success. I have come across a stubborn one and would like some advise. I usually just give the carb a good gasoline bath and scrub it real good with a toothbrush. I clean out the little hole on the bottom, in the brass thing. I blow thru the plastic pickup tube to make sure it is not blocked. I replace the diapragm and call it good. I noticed that there is a screw on the side, looks like an adjustment screw but it is screwed all the way in, what is it? I have played with the old pull choke style carbs that have a high speed idle in about the same location, but i think this is something different.

http://i96.photobucket.com/albums/l193/purplewindworm/Briggs/100_1642.jpg

http://i96.photobucket.com/albums/l193/purplewindworm/Briggs/100_1640.jpg

Sorry about the pics, I'll have to get some better ones.

This engine runs great at half to 3/4 throttle, but when i crank it to full throttle it sounds as if it is struggling. It is model number 91212 , think it is actually a 3 or 4hp.

you will prolly need to disassemble the carb (removing welch plug also) jet adjustment screew and soak it in carb cleaner for a few hours. use the wire from a bread tie and run it through all passages. If your reusing the jet adjustment make sure there isn't a ridge on it.