Motorcycle Repair: Carberator adjust on 1979 GL 1000, air passages, bill silver

Question
The cycle backfires and #3 spark plug is fouled.
When running the #3 sparkplug can be disconnected and the backfire ceases until plugged back in.
I think the #3 carberator needs adjustment but I am not conversant with how to accomplish.
The spark plug fires but I think it is running too rich.
Please advise!
Thanks,
Vance

Answer
http://www.classicwingclub.org/  may be a better source of info on this issue.

Running rich can mean too much fuel, due to high float level, loose jet, sinking float or not enough air due to blocked air passages.

Could also have a bad spark plug cap, defective plug or there could be some issues with air cut valve diaphragm or accelerator pump diaphragms if equipped on your bike.

Unless, for some reason the idle mixture screws have been tampered with, there probably isn't much you can do externally to "adjust" the problem, other than carb synchronization.

I alway suggest doing compression test, verifying the ignition timing and spark advance and electrical system integrity, then working on carbs last.

Bill Silver