"Timing is spot on" - assume you mean cam timing and it was dialled in then? (although using the "dot" would be close enough on a mild cam/standard skim head)
Did you run the cam in properly?
Have you checked and confirmed ignition timing since the rebuild?

Chances are its going to be one of two things:
-Ignition timing (12btdc on Premium (VPower) fuel)
-Cams wiped a lobe because... well Piper cam