Motorcycle Repair: Popping Noise, mixture adjustments, exhaust leaks

Question
Hi Bill,
Thanks for taking my question. I recently restored an old '75 honda that was in storage for several years. I bought new mufflers for it, and now that it's done, i've been riding it around the neighborhood to get a feel for it. I've been hearing a "popping" sound (a series of successive pops) coming from the left muffler. It doesn't happen when i give it gas, but only when I let the throttle go but continue to cruise in gear, and most often after I downshift. If I take it out of gear, the popping becomes quieter and a bit more infrequent. Any ideas on what this could be? possibly synching the carbs? Thanks Again.
~Rob Drew  

Answer
Rob, it would be a little more helpful to know which model you have, however there are general guidelines for this kind of problem.

First I would be sure that the compression is good on both sides. A tight, burnt or leaking valve will cause this type of problem, but that is not all that common. Check the valve clearances if compression is low on that side.

The next most likely cause is an air leak in the exhaust system. If you didn't replace the exhaust pipe gasket or the joint is not air-tight then you will get this kind of popping on decelleration. Check all the joints where the exhaust connects to be sure that there are no exhaust leaks.

Incorrect ignition timing will cause backfiring, so be sure that it is set per specs.

The last one concerns the carburetors and manifolds. Any kinds of air leaks in the intake manifolds or carburetor O-rings. Blocked air jets, incorrect idle mixture adjustments and carbs out of synch can cause irratic running and misfiring, too.

That should keep you busy for awhile...

Bill Silver