Motorcycle Repair: wont run smooth, yamaha virago 750, 1981 yamaha virago 750

Question
My 1981 Yamaha Virago 750 v twin has new coils, rebuilt carbs,fresh gas,new plugs and new air filter. I rode it to a motorcycle shop because it was backfiring and would run on one cylinder until I got up to about 20 mph. It kept backfiring at all speeds. First,they  rebuilt my carbs, but it ran no better.Then they ordered a set of rebuilt carbs for it, but could not keep it running long enough to synchronize the carbs. I finally had to tow the bike home because they can't fix it. I checked all the wiring, fuses and removed the spark plugs. I poured a little rubbing alcohol in each cylinder, replaced the plugs and it started right up but soon quit and would not restart on gasoline. What could the problem be?  

Answer
Hi Chris,

I would like to see the carburetors. I suspect they were not thoroughly or properly cleaned.

Did the shop perform a cylinder compression check? If so, do you know the results? When was last time valve clearances were checked and adjusted on the Virago?

Reply to my email address: mshively1@woh.rr.com

Respectfully,
Mark Shively