QuestionHi Bill,
I have a 1973 CB750 with about 32000 miles that I’ve put a lot of effort into, but I am still having troubles with. I have cleaned the carbs (not rebuilt), set the valve clearances, put in new plugs, replaced the plug caps with new NGKs, and replaced the points. I was having trouble getting good spark before I changed the plug caps (timing light was sporadic at higher rpms on 2-3 cylinders). I have set the timing and rebuilt the petcock. I have also installed aftermarket pipes, which are pretty well straight pipes.
The bike idles great, and runs perfectly at low rpms (under 3000), but starts to hesitate at higher revs. When I crack her open at idle, she revs right up with no hesitation. Town riding is great, and it will pull nicely in first, second, or third gear (under 3500 rpm). BUT..it is difficult to get her to highway speed. There is absolutely no chance of joining the one ton club! There seems to be a dead spot between 4-5000 rpm, and I have never had her at higher than 6000 rpm. It’s like she is not getting enough gas, but I have the throttle at ¾ at least just to stay at highway speed.
Here’s hoping you have an answer or, at least, something for me to try.
Thanks for your time.
Brent.
AnswerBrent, if you are still using stock jets with straight pipes and/or air filter changes, you are running WAY TOO LEAN. Not sure what series carbs you have, but I imagine that you will need some 120-130 main jets to get it into the ball park. Don't run it lean, otherwise you can seize the pistons.
Verify spark timing at idle and full advance with a dynamic timing light. Also check your coils to see if they have around 4.5 ohms resistance for the primary windings. Both coils should check out nearly the same.
Be sure that the gas cap vent system is clean and clear. They will rust up and the tank creates a vacuum inside, halting fuel flow.
I am hoping that you have four cylinders with around 160-175 psi in each one.
Bill Silver
