still a bit of backfiring on the overrun

I'm still playing with the lean off idle that's causing some backfire with slightly openned throttle on over run. Closing the idle air mixture screws did help, but the idle is obviously not right at that point. Going down a hill I rev'd the engine then let off. When the backfire started I closed the choke and within about a second the backfire stopped. The choke doesn't effect the idle circuit, so I suspect the lean problem is air leaking past the throttle slide (original 932 Amal stainless sleeved by Gaylord 9 years and 17,000 miles ago). Needle jet and needle are new and the needle is set as high as it can go (bottom notch). The valves were re-conditioned by Rabers in San Jose last year and the engine has 185psi compression both sides ('72 750 combat - new rings on +.020 bore less than 1K miles ago). The carb balance pipe was just replaced in case it was leaking. Pipes are tight with new seals. Ignition timing using a RITA is set at 28 degrees maximum advance. Plugs are tan colored, not white a lean condition. The problem has appeared in the last year - no backfiring for the first 8 years after restoration. Any tuning genius out there can help?

Is it a real backfire or just spitting back?

In my experience, leanness in the carbs will cause more of a spitback than an actual backfire.

The only times I've experienced consistent backfiring has been with clearly failing exhaust components, apart that is from the old Boyer connections syndrome but that is usually much less consistent and more like a case of the blind staggers, usually under load.

Is there any significance in the valve overhaul a year ago ? A sticking valve could cause the problem.

Still backfiring

It's a pretty loud "pop-pop"

Pls explain what you mean by failing exhaust components.

I'm running a RITA not a Boyer. The valve overhaul was due to a failed left exhaust guide (came loose). The problem was not cured by the overhaul.

Could be loose slides...but I bet it is still the correct adj on the air bleed screws.
Is the throttle, from full shut, to 1/8 open, absolutely free of "dead" spots, a place where the engine just doesn't run quite right and ran better/faster at the point it was a bit less open? I mean, is the increase in revs a smooth/gradual thing? No places where it wants to sort of die...between a closed throttle and giving it gas? I had to play with mine. Eventually found one carb out of sync with the other (Slides not same height ), and at startup, cold motor, I had to slowly increase the motor speed, and when the dead spot was there, hold that place and fiddle with the air screw till the motor ran right and the dead spot was gone. Had to do it more than a couple of times to get it right. Listen/feel the blowing out of the exhaust pipe, note which side is guilty and fool with that side only. Carried a small screw driver too around for awhile too, and listened which pipe did it downhill, and tweak only that side on the side of the road at the bottom of the hill.
All popping gone now, though, and a nice gradual throttle response, which makes shifting much easier without having to mentally take into account the dead spot and dying motor. Only other popping problem has been the nice new gas taps, which starved the motor so much for gasoline, that the motor popped downhill too...got new taps on it????

Is the popping in the exhaust? could it be air leaks at any point itn the ex. system?

Have you checked the o-rings on the pilot screws?