Backfire thru muffler

[BaronBigDog]

Coming home from a very short ride yesterday my 75 GL1000 started backfiring through the left muffler. I have a split system, no crossover. It pops pretty loud. (Sounds like an old Harley with the timing too far up) If I hold #2 carb closed and open the throttle on the remainder and it does not backfire. As soon as I let off the linkage and it opens up it starts backfiring. My carbs are relatively fresh from a professional rebuild--Old School Carbs--and until yesterday were perfect. Everything is virtually new on the bike in the realm of spark and fuel supply. Any suggestions before I start tearing into it?

I looked at the troubleshooting guide in the service manual and it suggest timing, but I think that if the timing was off it would affect both 2 and 4. It appears to be 2 only, but this may be a misdiagnosis.

Just to be certain that you know the following are new.

Plugs
Wires
Coils
Plug caps
Electronic ignition
Rebuilt carbs
Electric fuel pump
Running no ethanol regular gas
A total of 226 miles on the above.
A whole lot of other new stuff that would have no affect on this problem.

[Aussie81Interstate]

make sure #2 carb is receiving fuel - undo the drain screw to confirm there is fuel in there (motor off - on the centrestand - fuel tap off) - catch the contents of the bowl and compare the level to the other carb bowls - should be the same. also make sure there are no vacuum leaks around the carb inlets

[RBGERSON]

Gl's are a wasted spark system sounds like extra fuel in the cylinder being ignited at the wrong time..IE when exhaust valve is open.