Geoff,
I would still try dropping the jet needle one notch. If that doesn't work, That leaves timing and exhaust. If you have access to a dial indicator and can get to the alternator without a lot of trouble, it is easy enough to check. The exhaust is one area a lot of people overlook. It is a tuned system and has a lot to do with scavenging the cylinders. A system that has been modified from the original, or is leaking or clogged can make a big difference in performance.
Bill.