Motorcycle Repair: honda cm400t, honda cm400t, rubber diaphragm

Question
Mark,

I recently purchased a 1980 Honda cm400t. It is in pretty good shape. This is my first foray into motorcycle repair - but I bought a repair manual and have been having fun with it. It used to bog down when accelerating but I took off the carb and cleaned it thoroughly and actually got it back together with no problem. It runs much better now with smooth acceleration, but with a new problem: whenever I start to accelerate or decelerate it spits and backfires repeatedly. It runs smoothly when cruising along at a constant higher speed (above 35 or so) but as soon as I touch the gas up or down, or cruising at a slower speed it starts to backfire and spit out the tailpipes. I'm guessing I have something adjusted wrong in the carb but don't know where to begin. Any help would be greatly appreciated!

Chris

Answer
Hi Chris,

Visit the following link and see part number 1, this is the air cut-off valve. If this rubber diaphragm is aged, damaged, varnished, or improperly installed, it will cause backfiring. The backfiring usually occurs on deacelleration, but may apply otherwise. Also, consider synchronization of the two carbs. If the carbs are not balanced (synch'd), they may cause the problem.


http://houseofmotorcycles.bikebandit.com/partsbandit/oem_schematic_view.asp?sche...

Respectfully,
Mark Shively