Motorcycle Repair/1980 honda cb750 k runs bad
stock 1980 Honda cb750k idles good & runs well at rpm's below 5000, but bogs down under a load @5000rpm's & above. Sometimes when you ease off the throttle the bike seems to catch & take off. I've had the carbs off & apart several times. I removed ALL the jets, needles, cleaned all pasageways with carb cleaner & air. It ran well for a few days the 1st time I cleaned the carbs, but started to act up again. A repeat clean & inspection of the carbs did not repair this problem. I've switched out ign pick-ups, coils, & ign box's one at a time with no change(these parts off a donor bike#. This problem occurs after the bike has warmed up #10-15 min). Before I cleaned the carbs I installed new plugs & the bike ran well for about 50 miles. I ohmed the plug wires w/reisitors, all below 5K. Ck all elec conn & grounds, all look good. I am an ase certified master auto tech, so bikes are new to me. Any information you can give to point me in right direction would be greatly appreciated. This bike is 100% stock, 9,500 miles, vin RC012100148
Lee, it sounds like you have done all your work well, but perhaps didn't go far enough into the fuel system and/or the electrical system.
If you drain the fuel tank and remove the petcock, you will find a nylon screen pushed up inside the tank. http://www.cmsnl.com/honda-cb750k-750-four-k-1980-usa_model7245/partslist/F++13.
These can become fouled with old fuels/water/debris and cause a fuel flow delivery slowdown to the carburetors, especially under load. Likewise, the gas cap venting system may be causing a partial vacuum condition as fuel demand increases. Basically, the carbs are running out of fuel at higher fuel demand loads.
The DOHC series of engines also has a recognized problem with the electrical charging system. Generally the rotors fail and the resistance goes up, especially as the engine heats up and causes higher resistance on the slip rings.
Because of the high-demand headlights on these models, if the charging system isn't keeping up with demand, then the ignition system starts to misfire as voltage diminishes. The system doesn't always fail completely, just weakens enough to cause low voltage problems with the transistorized ignition components and coils.
Those are my two best guesses, at this point.