Topic: Help with 81 CB750C - won't rev over 7K????

[bikebitzofvt]

I kindly offered to overhaul the carbs on a 81 CB750C for a fella at work.  Pretty straightforward job, nothing out of sorts, just some rust from the tank had built up in the float bowls.  POR15'd the tank (first time using it and wow is it awesome!!) cleaned the carbs, new float valves and put it all back together.  I took it for a short ride and found it to need a clutch.  However, it won't rev over 7K.  I haven't worked on one of these in well over 15 years or so.  If I remember correctly that is a sign the igniter boxes are going bad??

Thanks in advance for any help!

[w1sa]

If the igniter/spark units alone are going bad it should (I think) cause some soot fouling on the plugs.......Check the plugs (plug chop) after some sustained running in the range where it doesn't want to increase speed under load.

Could also be float/fuel levels...........check using the clear tube method

Could also be malfunction in main needle/piston vacuum function, preventing adequate main needle lift.

Has it had a complete engine/ignition tune-up lately...?

[cb650]

Try running with the airfilter cover off.   Also might be the plugs.  Both easy to change check.  My 650 once would hit 6k and it was like a rev limiter.  Screwed with the carbs with no improvment and finnally put in new plugs and good.  The 81/82 650's also had problems with the airflow to the carbs.    Same carbs as the 81 750 mostly.

[bikebitzofvt]

Thanks for the input!
I'll be checking some things tomorrow afternoon and let you know how I make out!

Tom

[bikebitzofvt]

If anyone's interested in an update I finally figured out the solution to this problem.  I ended up adding a relay to the "hot" side of the ignition coils.  The voltage drop to the coils through the stock harness, ignition switch, and kill switch, was 2.1 volts!!!!  Now with the 40 amp relay in place and 14 gauge wire supplying the power to the coils it pulls like a freight train right up to redline, and only looses .2V!