You've stated you've already checked it but I'd double check the cam timing. It's hard to do because you really should take the throttle body off to line up the dot on the crank web to the right spot so the dot on the cam is in the right spot too.

Tappet clearance is another thing I'd double check, too little you should be able to tell straight away without pulling the engine apart. You mentioned pulling the throttle barrel, I found the seal arrangement meant I could only put mine back in from the opposite side it comes out from otherwise it'd take a chunk out of the seal..

It sound's like it's getting plenty of fuel but not burning it right.

You did put the ring back on the piston after the first teardown ??

PS. Double check the disk valve in the backplate is located to the crankpin properly and not offset to either side of it. I'm not sure from memory if it's possible to have the crankpin locate to one side of it's correct slot but it's worth a look..