Topic: CB750 Runs rough when hot

[Patrick]

OK, maybe I don't ride my old beauty as often as I should. I try to get it out every weekend, but I don't like to ride in rush hour traffic so I sits during the week except for the occasional evening ride. And most of my riding this summer has been riding my old dirt bikes. My son is finishing high school this year and next year he'll be going away to college. He wanted to ride dirt bikes, so I fixed up a vintage Suzuki ('74 TS250)  and a vintage Yamaha ('69 DT1) and we trail ride on those a lot. I offered to let him ride my 750, but he wasn't particularly interested. "I only like the ones that fly," he said. He's 17. In a way, I can appreciate his point. Anyway, that has taken most of my riding time this summer. Those of you with kids will understand, Those of you without kids are probably too busy with your big houses, fancy cars and exotic vacations to care.

But I digress. When I finished resurrecting my '75 CK750 K last winter it ran beautifully. It still does, to a point. Lately when I ride it, after about 20 or 30 miles it starts running sort of rough. Not really missing or anything, but, well, I suspect you know what I mean. I can gun it and there's power and it doesn't die or misfire or pop or anything radical, but it just feels kinda, well, rough.

It always starts very easily and for the first half hour or so it purrs very well. It idles well when I stop and I don't have trouble restrating it when I turn it off. I checked the plugs and I'm running a bit rich. Not black-soot rich, just a medium to dark brown rich. I have good oil pressure and the bike doesn't seem to be running particularly hot. It just sorta feels like it's rumbling. I don't like my baby to run rough. I like it when she purrs.

Any suggections, anyone? Or have I just spent so much time riding screaming two-stroke, extreme high torque machines so that my mind is making me think that the growl of a four stroke is unnatural....

[scondon]

         If your running with stock points/condensers it may just be time to re-set the point gaps and engine timing. My bike would run optimally for 2 months(ridden every day) and then the performance would start to decline so I switched to an electronic ignition(Dyna). Also, you might check the airfilter and replace if necessary. A clogged airfilter and/or a de-tuned bike would certainly cause the symptoms you describe.

[Lumbee]

...u could check u'r compression for kicks...if its low could be time for a top end rebuild...

[Gordon]

A general tune-up and carb synch should clear up most of that.  You also might want to lean out the idle mixture adjustment a little.  One sign that you're running too rich at idle is if you don't need to close the choke to start the bike when the engine is cold. 

[eldar]

I would say to check the valves and timing first just to make sure the points are ok and that the valves are adjusted correctly. I did my valves and got more power. Then do a sync and tune the idle mix. A dwell meter is best for timing and most have electronic tachs built in which is ideal for doing your low speed mix.