I recently aquired a none running 1981 Honda CB750c.... The work needed to make it run is in the carbs.... So when I got insided the float bowls and learned the floats ARE NOT adjustable..... Hmmmm....The problem is. I can have the bike idling on the center stand. At this time it's running just fine with the gas thats in the carbs. At this time the fuel petcock is NOT turned on. With bike idling I turn fuel petcock on.... The engine will do one of two things. RPM increase or it kills the engine. The float needles will not seal......... OK so I purchased new floats and needle.... Same problem..... Float will not shut off the flow of gas. Next I decided to purchase different floats and needle that are adjustable. I've got that little tab bent to the max...... And same problem persists. I'm wondering if anyone else has done this battle with none adjusting floats.
1981 Honda CB750c
- Thread starter ziller
- Start date
This is almost always a float needle/seat sealing problem or fuel level too high, not the idea that the floats “aren’t adjustable.”
On the Honda CB750C carbs:
New needles can still leak if the seat O-ring is bad or the seat is worn/loose
If fuel instantly kills or floods the engine when petcock opens, the bowls are overfilling immediately
Bent tang won’t help if fuel is bypassing the seat entirely or floats are sticking
Most likely fix: replace/verify needle seats + O-rings and set fuel level (not just float height spec).
On the Honda CB750C carbs:
New needles can still leak if the seat O-ring is bad or the seat is worn/loose
If fuel instantly kills or floods the engine when petcock opens, the bowls are overfilling immediately
Bent tang won’t help if fuel is bypassing the seat entirely or floats are sticking
Most likely fix: replace/verify needle seats + O-rings and set fuel level (not just float height spec).
Raggaren
CB750 Enthusiast
Did you clean fuel valve seat good with carb cleaner and a Q-tip
gunk and corrosion will cause valve not sealing properly…
Go here for detailed carb cleaning/rebuilding instructions.
https://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=9513
From MacGregor carb cleaning manual
gunk and corrosion will cause valve not sealing properly…
Go here for detailed carb cleaning/rebuilding instructions.
https://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=9513
From MacGregor carb cleaning manual
Last edited:
Raggaren
CB750 Enthusiast
Also wanted to add ..
I went through my carbs recently 1980
80-83 floats are non adjustable..
I ordered 1 set of floats but got sent 2.
Good thing they did. Because a couple of the floats were not molded correct
And fuel valve would not fully seat/close…
You said you changed to adjustable floats??
I also had and tried a set of the adjustable 1979 floats and fuel valves with spring clips…
They don’t work/fit….
the fuel valve Pintel is to short. And don’t shut off the fuel flow ….
I went through my carbs recently 1980
80-83 floats are non adjustable..
I ordered 1 set of floats but got sent 2.
Good thing they did. Because a couple of the floats were not molded correct
And fuel valve would not fully seat/close…
You said you changed to adjustable floats??
I also had and tried a set of the adjustable 1979 floats and fuel valves with spring clips…
They don’t work/fit….
the fuel valve Pintel is to short. And don’t shut off the fuel flow ….
Yes.... I did the effort with the qtip in drill with brasso.... That didn't help. I saw another rather interesting trick on youtube. You take a steel bb about 1/8 in diameter.... Put it in the valve seat. Now take punch with flat end and hammer rap the punch a few times. The fellow doing the video showed the valve leaking and fixed solution after BB..... Needless to say that did nothing for me.
Raggaren
CB750 Enthusiast
One more thing.
When you have carbs upside down
The float tab touching the small spring loaded pintle ….
If you touch/apply slight pressure on float
That pintle should compress some ….
When you do that. The floats should not touch the carb castings.. if they do. Then valve is not getting enough pressure against seat…
That’s the reason why I had to switch a couple floats I received..
When you have carbs upside down
The float tab touching the small spring loaded pintle ….
If you touch/apply slight pressure on float
That pintle should compress some ….
When you do that. The floats should not touch the carb castings.. if they do. Then valve is not getting enough pressure against seat…
That’s the reason why I had to switch a couple floats I received..
"When you do that. The floats should not touch the carb castings." And that is my problem! The carb casing has a thing that stops the float. When upside down... The floats bottom there and still don't seal. So .... I took a whole new approach. My question is. Can I get a proper seal in any fashion possible? So I did a bit to crazy. Just to see if it would work. I used rubber bands to hold the floats down.... Now I turn that carb rack over with no float bowls on. I now feed gas into the line.... If they are still leaking they would drip , drip..... With the rubber bands. There is no leaking anywhere. Remove rubber bands , put bowls back on and do same test.... Leaking everywhere. It's almost like the float needle is too short. At this point , I do not have a solution.
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Raggaren
CB750 Enthusiast
you need to go back to the original style non adjustable floats and the longer fuel valves
