Motorcycle Repair: rough idling and backfiring, honda cb750f, inductive type

Question
I have a 1981 Honda CB750F. This bike had been sitting for 4 years in a garage when i acquired it. I rebuilt most of the bike and had it running great when the exhaust cam chain broke. I have a parts bike and replaced the cam chain with the chain from it. I also replaced the valves since some of them had bent when the chain broke. There did not appear to be any other damage to the engine. Now that I have it back together it runs rough idling and begins to backfire at higher RPM's. It will idle at 1,000 RPM's, however, when given throttle after let off it will remain at 2,000-3,000 RPM's for 30 seconds to a full minute. The throttle is not sticking. I removed the carbs and gave them a thorough cleaning, but the problem still remains. I also put in new spark plugs the day before the chain broke. Any advice on what to do next would be greatly appreciated.
Thanks,    Travis
P.S. I have an old portable tachometer that was given to me. It is for 4 cyl and 8 cyl engines and also checks dwell timing and points I think. Will this work on this bike or is a special one needed for motorcycle? If it will work how is it hooked up? According to the instructions it is hooked to ground and the distributer, but not sure where to hook it up on the bike. If this one will not work can you recommend an affordable portable tach? Thanks again.  

Answer
Hi Travis,

Reference the timing light, I can't help you with the antique. I recommend finding an inductive type timing light such as those seen here:

www.jcwhitney.com/webapp/wcs/stores/servlet/ProductDisplay?productId=6913&catalogId=10101&storeId=10101&langId=-1&TID=1544

Check Ebay for better prices. I see them at local yard/garage sales now and then.

The irratic high idle could be an air leak, poor carburetor synchronization, or the carbs need more thorougly cleaned.

Respectfully,
Mark Shively