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Motorcycle Repair/1981 honda cb750 hesitation issue


QUESTION: to follow up on earlier conversation.i tried running the bike with the airbox cover off and the situation was worsened indicating a lean mixture present at high rpm.i the blew out the jets from the top and now i am not sure what to do. the main jet is a 68.the secondary main jet is a 112.the bike has a kn air filter and 4-2 pipes on it from honda.the idle to midrange is good but if i quickly open the throttles from 4500 up the motor bogs.if i open the throttles partially then the bike will run up to 8000 no problem in it's time.i have replaced the coil for the middle 2 cylinders with a performance one.


It sounds like you may be losing voltage at high rpm,
have you tried a new battery or chacked the charging system?
Some new plug wires can help and also using Iridium spark plugs.

The fuel these days is different and bikes tend to run leaner.
Some #75 main jets may help but I am tending more toward
a lack of coil voltage at high rpms The performance coils
may even require more power than stock coils.

Your bike revs okay until you put it under load where the spark
has to work harder. Try iridium plugs or if you can get ahold
of a strong battery for testing might help.

The charging systems are known to be a problem spot for these bikes.
The electronic ignitions need strong power to work properly.

You can try the larger main jets but if that doesn't work then
voltage is not firing at high revs imo.


---------- FOLLOW-UP ----------

QUESTION: I have new wires  and plug caps installed already with iridium plugs.i have a voltage meter installed on the bike and the voltage  never dropped below 14.5 volts.which jets are normally changed ,the main jet(68)? Or the secondary main jet(112)  the one that  the main needle goes into?most jet kits start at 100. I just took  the bike out in 40deg.f weather and  it seems like under minimal throttle as it gets more vacuum it runs better.there are no vacuum leaks around the carbs(tested with propane)


If you can get some #75 to replace the primary main jets that would
be the ones to try. the secondary jets sound large enough.

an alternative might be to try a stock honda air filter as
these bikes cv carbs are sensitive to air flow.

You can buy tiny hobby drill sets which can allow a person to
enlarge the main jets slightly by hand.

This of course makes the jets unusable if it doesn't work
and you would have to buy new standard jets again or
whatever seems to work best in your trials.

I have even used epoxy to fill a jet and  redrill it with
some reasonable results but new is better for reliability.

It sounds like your electrics should be strong enough so
maybe some jet experiments might help.


---------- FOLLOW-UP ----------

QUESTION: I tried the stock filter back but I think the cold weather right now compounds the problem.i found a o ring leaking for the accelerator pump so I disconnected the spring from the pump  until I  replace it. I will pick up a set of no.75 jets anyhow but now I think if the pump was working properly then my fuel problem may be solved. I am not a motorcycle mechanic but a refrigeration  service engineer who is trying to make a 31 year old  bike run like a  true sport bike.i appreciate all your advice as this is my first bike and I have tried almost everything I can think of with my limited bike experience .thank you very much.

Hondas in general are rather notorious for not liking cold weather.
Most of their engines need a long warm up to run properly.
Sometimes a small thing can cause a problem and
other times it can be a combination of things which
makes troubleshooting difficult.

I hope you can narrow it down with some of the info
I gave you. Good luck with it.


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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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