Britbike forum Forums Thread Like Summary - Carbs or AAU?

My 68 T120R starts first kick, ticks over like a clock, runs like s%#t (low power & backfiring) until 3.5K then...BAM....lights off and runs like a new bike the rest of the way on up. When I back off and the Rs drop below 3.5K she turns back to s#%t- (low power and back firing) Opening the throttle up does not help. Maybe make it worse. Down shift to get the Rs back above 3.5 and...BAM...she's new again. Last year I installed new Amal premiums and set them up so...??? I've double checked the sync and air screw setting (1.5 seams about right). I've checked for air leaks and can't find any. Plugs are sooty around the rim with medium brown electros. I bought a new Pazon Surefire IE thinking it may be a bad AAU. But before I rip out the existing ignition system on an Easter egg hunt I'd like to know what you guys & gals think might be going on.
Thanks in advance for your help!
Randy

I seem to recall similar symptom(s) on my bike a few years ago. It was a loose jet holder in one of the carbs. After I got off the slide/needle jet circuit, the bike would run just fine, but once I was on the pilot/needle taper/slide circuits, it ran like sh!t....

Cheers,

Steve