Motorcycle Repair: carbs/electrics, air mixture, aftermarket exhaust

Question
Hi, Mark I have a kz750B twin and its driving me nuts.Only 2 carbs (sigh) anyway the symptom was and is a backfiring on on deceleration. I cleaned the carbs,checked for manifold leaks, replaced the coil to standard oem and I noticed that as I turned in the air mixture screw and kept the idle at 1000 rpm the backfiring diminished to the occasional spit and bark. the color of the sparkplugs are grey, and it starts on the button and accelerates to 70mph with no drama in 5th.Oh I had the valves checked and I better mention that there is a 2 into one aftermarket exhaust and electronic ignition bolted on.
Im worried that its running lean and seems to lack power at the lower end but if I richen it up with the air /fuel screws then the popping will return agggggggggh. Of course it might be the electrics butit does start and apart from the odd main engine fuse blowing theres not much I can I see there. Any help would be appreciated.

Answer
Hi John,

Sorry for the long delay in responding. My PC has been down for several days.

Send me your email address as requested. I'll return several files on carb tuning, cleaning, synchronizing, and a coulpe electrical files. Send to: mshively1@woh.rr.com

Respectfully,
Mark Shively