ZDZ40 what rpms are you guys getting ?
I think Fred has it right, it's the muffler. My buddy HT's would turn a Mejzlik 20x10 at 7600 rpm. He was using the B&B (muffler?). Too much backpressure in the muffler delays the mixture flowing through the transfer ports. Get some earplugs, and test it without a muffler....I think the power will return.