Isn't it possible that he's not getting enough tank pressure with the pitts muffler? I've heard that can happen, and other than his needle position I don't see a good indicator of the engine set rich or lean.

I would try the stock muffler just to see if anything changes. If the muffler is the cause, try plugging one of the exhaust ports on the pitts muffler to increase exhaust pressure to the fuel tank.

Just my thoughts...