Motorcycle Repair: 1980 cx500d lacks power, hick ups, rubber diaphragm

Question
Jim, I have a 1980 Honda cx500D.  I just got it from a friend. It hick-ups around 2500rpm. the other problem is that it has is it sputters really bad at 5000rpm and will not rev above 6000rpm. its absolute top speed is 60mph then it falls on its face. We have replaces coils, stator, plugs, my friend went thru the carb, said he saw nothing wrong.  he is not a Honda tech.  I rode the bike 150 miles with sea-foam on the tank.  Its feels better at lower rpms. It seems to miss really bad on the left cylinder. It dosent care to much when the left plug wire is removed. It however does have spark. less spark at 5500rpm. The right side does the same when i checked it.  When i pull the right plug it wants to die or really sputters bad. Am i fighting 2 separate problems.  I see these are notorious for loosing power at 5500rpm. I'm kind of at a loss. Bike runs good cold with the choke pulled out also. doesn't like being worm. Idles great. all of the time.  Smells rich and backfires at 5500-6000rpm. sends out black smoke. Jim please save me. Thanks.


Answer
I would first take another look at the fuel/intake system, especially on the left cylinder. It sounds like the carb might still be plugged up. Look carefully at the operation of the float valve and make sure the float height is set correctly (per the service manual). Remove all the jets and clean out the tiny holes in them with a strand of copper wire. Take a look at the tank for rust which can cause future carb issues. The rubber parts on the carbs should also be replaced. Make sure the slider on the carbs is operating smoothly with no hangups. The slider controls fuel supply in the power range when you accelerate past 5k RPM. Make sure there are no tears or leaks in the rubber diaphragm which operates the slider.

Put a voltmeter on the battery and check the state of the battery and regulator as the revs increase. You want it to stay between 12.5 and 14.5 volts with a newer battery.

I would also check valve clearances and do a carb sync. These are needed to get a good idle.