Motorcycle Repair: 1980 honda 400, honda cm400t, valve diaphragms

Question
I'm having an idle problem and a miss at constant speed with exhaust popping on decel.  could this be a coil or other ignition problem or is it the carb idle jets?  Do you know what the wings on the bottom of the carbs do?  They turn about 300 degrees but don't seem to affect anything.

Answer
Clem, the "wings" on the idle mixture screws are limiter cap stops, to prevent tampering of the idle mixture screws (DOT/EPA emission control regulations). If you put a hot soldering iron in them for 20-30 seconds, the glue will melt and you can pull them off. Then you can manually adjust the idle mixtures. Or you can use some end nippers and cut the wings off, carefully. The idle mixture screws are brass and very easily damaged. That only works if the carbs are clean and all jets are functioning properly and there are no air leaks in the intake system.

Your "exhaust popping" on decel may indicate one or both of the air cut valve diaphragms are leaking. These are also known as anti-backfire valves. There is one one each carb and the inside of the right side is very difficult to access without carb removal and partial disassembly.

http://www.cmsnl.com/honda-cm400t-1980-usa_model7286/partslist/E++2001.html   #14

When the diaphragms fail, you get a constant vacuum leak into that side intake tract, which leans out the mixture at idle and most other engine speeds.

It doesn't hurt to check the spark plug caps for 5k ohm resistance values, but ignition problems are somewhat rare on the later generations of the 400-450Ts.

Bill Silver