83 750 twin backfiring.

My bike has taken up the habit of backfiring when you let off the throttle and allow it to use engine breaking. I put new plugs and wires and that didn't help.I blocked offthe crossover and it is as I suspected it is all coming from the left side. When I changed the plugs (again) the one from that side was badly coated in carbon and/or soot. Also it spits and sputters under hard acceleration in the 6K RPM range.
Where do I need to start? This sounds like a carb or possibly a timing issue to me but this is the first multi cylinder anything I've ever worked on.

Weird. Backfiring on deceleration is a sign of lean-ness, or perhaps an air leak. Carbon sooty plugs is generally the opposite.

Blocking off the exhaust x-over? That's funny. I'm doing everything I can to return the crossovers to their original state on both my 750s, after I learned what a performance benefit it provides to the one bike.

Well, there's a few pieces of advice I give to every twin owner who asks:

1.) set/validate your float levels,
2.) replace the coil/wires, and
3.) do the well-known coil-power mod.
...Oh, and inspect your starter clutch, but that's a different topic. :blink:

In your case, from the sounds of it, you should also check for exhaust header leaks, cracking carb boots, and that kind of stuff....

Just had a similar problem with my 1979 KZ750 B4 Twin. I read through the service manual and reset the pilot screws. I turned them in all the way until they were lightly seated. Then turned them out 1 and 3/4 turns. Works great for my bike.

Hope this helps. If thats not it, try spraying starter fluid around the carburetors with the bike running. If the engine speed increases you have an air leak.