Low engine load soft exhaust backfire or popping

[budman100]

Bought a bike that sat for 1.5 years. Replaced all fluids, plugs, timing belt, valve adjustment, cleaned tank and replaced fuel shutoff with Pingel unit. Carbs were rebuilt by an expert plus installed Cobra exhaust. Bike ran good but had loud exhaust popping. Reinstalled stock exhaust (rejetted back to stock). Tried the slow tightening of the exhaust nuts but popping continued. Took bike to a repair shop about a year later that actually works on older bikes and they said the carbs needed rebuilding again. They performed the rebuild, bikes starts and runs great although I still have some slight exhaust popping under low load conditions. Mechanic said that was something he couldn't stop due to the engine desmog, the unburnt gasses/fuel would ignite in the exhaust without the system intact. Any suggestions to eliminate the exhaust popping? Annoying more than anything.

[WintrSol]

You didn't mention synchronizing the carbs; if one (or more) is set a little different, throttle plate wise, you can get these small pops.

[98valk]

"Mechanic said that was something he couldn't stop due to the engine desmog, the unburnt gasses/fuel would ignite in the exhaust without the system intact."

run from that mechanic as fast as u can.

All IC engines backfire on deceleration, when the air/fuel mixture goes lean, the more open the exhaust is the more noticeable it is.

if under acceleration or just steady state throttle, then usually that is a timing and/or ignition system problem.
pulling the carbs out could have caused the spark plug cables to come out of the ignition coils which are moved around to get the carbs out.
each spark plug cable has a resistor in the plug cap. rare reports of one failing.
when timing belts were changed being off one tooth can cause your problem.

[Jims99]

Also check air cut valves on carbs. Not to many people check them when rebuilding. If one or more are leaking, that can also cause the backfiring on deceleration.

[budman100]

carbs were synched after the rebuild.

[budman100]

whats the best way to check the air cut valves?

[98valk]

whats the best way to check the air cut valves?

most likely not the problem.

what jetting, exhaust, air box mods, and air filter are u running?   air/fuel ratio can cause more or less backfiring in the exhaust. there can be rich and lean backfires. Mikuni states any set carb jetting is only good for +/- 25 F. 

when I ride in 30-35F I'll little more lean backfiring actually lean misfire than at 35-40F. if i put a lot of restriction of the amount of air getting into the air box, I can reduce the lean backfiring to like i get at 35-40F when riding in 30-35F.

[Jims99]

whats the best way to check the air cut valves?

https://www.southbayriders.com/forums/threads/101750/page-9
There are others if you search more.

[budman100]

Everything is stock regarding exhaust, airbox and carbs except for the engine desmog