runs poorly until 4500 rom

Why would my Commando have low power with lots of popping and backfiring until it hits about 4500 rpm, at which point it takes off like it grew the proverbial third cylinder (as hobot would have put it). I just cleaned out the carburators, which are fairly new, the spark plugs are dry, I have the needles set at the top groove. The battery has 12.2 volts, and it stars on the first kick. Boyer Bransden ignition that I installed back in 1995. I've been running avgas for quite a while now, but these symptoms are new. It's interesting that about the time it started happening, my disc brake wasn't resleasing very well on a ride. I wonder if I stressed the engine while nursing it home under load? (Brakes are now rebuilt.)

That sounds very much like your Boyer is holding full advance. I would suggest checking and cleaning all the connections, might even be worth running wires past the ignition switch straight to the unit.

Regards,
David

What is the voltage across the Boyer box + and -, with the engine running?

carbs......

First, why are your needles in the top notch?? if it's a 750 with amals, it should be in the middle groove... and you should put it there so you have a known point of reference to work from. IF,....... later you have a rich mixture in your midrange and want to try the top groove then that would make sense, but start out with the recommended settings so you havea frame of reference to start with.

Next, the fact that it pops and backfires at low RPM, but then comes on strong when you open the throttle sounds like you have a clogged idle ciruit. If you've cleaned it out recently, iike maybe with a toothpick, then you know it's clogged now... I like to use a sewing needle because it won't leave behind any shards of material when you poke the idle jet. A toothpick is the worst thing you could use. It will clog the idle jet if you poke it with toothpick.

Needless to say, you need to make sure your timing is correct, and since you can get your bike up to 5000 rpms, you should put a timing light on it and make sure your timing is good. Sometimes the magnetic rotor that fires the timing pick ups for the boyer ignition slips on the camshaft so it messes up the timing... A timing light check will tell you if that has happened.

AV gas... I used to be part of a dirt race car team and would mix half 108 leaded race car gas with half pump gas and my bike loved it probably because of the cooling effect of the extra octane and cushioning effect of the lead on the valve train. What is AV gas's octane rating? is it also leaded?

If your battery only has 12.2 volts that would be an easy and inexpensive place to start the troubleshooting.

Yes, 12.2 volts is a very low battery voltage.
Sounds like until the alternator starts to chime in, its pretty marginal and starving for volts.