Carb overflow

[DAVE81CB750]

I've got slight drips from carbs 1 and 2 overflow. Happens when fuel on or off. I have replaced the bowl drain screws and still happening so I was thinking it's maybe a hole in float bowl or overflow tube . Any thoughts guys? Has this happened to any of you ?

 

[brettp]

Overflow is a symptom of the float valve not stopping the fuel from entering the bowl. You need to take those carbs apart and clean the valve and fuel inlet where it seats.

 

[DAVE81CB750]

Yes but the fuel that is left in the bowl after turning fuel off eventually drips out so was thinking it got to be a tiny hole in the bowl tube or just at the bottom of the tube ?

 

[brettp]

Nope. After turning off the petcock you still have fuel in the lines that will eventually drain.

 

[DAVE81CB750]

Oh yes thanks brett . I will have to inspect, great ! Bloody carbs !

 

[kirky]

let the fun begin staggy i had the same problem and ended up taking them off for major surgery, but i did them when it was nice warm weather , they are a swine to get off and back on again when the rubbers are cold. just noticed you haven't got the air box? maybe a lot easier then

 

[brettp]

I also found the carbs a hell of a lot harderto remove if the boots are old. And if you're taking the carbs off, might as well change out the fuel rail tube o-rings.

 

[DAVE81CB750]

I have drained all the float bowls and measured the fuel and they are all the same . So I was thinking that this wouldn't be a float valve problem? And maybe the float bowl. That's what is leading me this way before stripping carbs.

 

[brettp]

Look at the overflow tube. Sometimes they crack and fuel drains out of them that way.
Oh, I've seen fuel rails leak because the Orings go bad and the fuel runs down the carb body and drips from the bottom, appearing like it is coming from the bottom.

 

[Brianvance]

Oh yes thanks brett . I will have to inspect, great ! Bloody carbs !

Just buy new needles and seats from cb750 for a reasonable cost. You don’t need to take much apart. Pull the carb bank and just the bottoms off the carbs and leave them connected. I wouldn’t dismantle any more if it runs fine. An hours project so no big deal. Of course look for corrosion but 99% of the time this fixes them.

 

[brettp]

Just buy new needles and seats from cb750 for a reasonable cost. You don’t need to take much apart. Pull the carb bank and just the bottoms off the carbs and leave them connected. I wouldn’t dismantle any more if it runs fine. An hours project so no big deal. Of course look for corrosion but 99% of the time this fixes them.

I would add that bench testing the carbs is a good idea before reinstall. Put a fuel line on the primary inlet, continue to fill the fuel line until the fuel stops draining into the carbs. Wait for an overflow. If nothing, you're good.

 

[Brianvance]

I would add that bench testing the carbs is a good idea before reinstall. Put a fuel line on the primary inlet, continue to fill the fuel line until the fuel stops draining into the carbs. Wait for an overflow. If nothing, you're good.

Bench test never hurts. I just blow in the fuel inlet and if they seal when you push the floats up they are probably good.

 

[Brianvance]

Bench test never hurts. I just blow in the fuel inlet and if they seal when you push the floats up they are probably good.

I should note that needles and seats are available from Vintage CB750 for less than $8.00 each.

 

[brettp]

That is probably true, never tried that. But a bench test with fuel will show you if the fuel rail needs fixing or if any overflow tubes are cracked.

 

[dirtdigger]

The brass overflow tub is a common issue for them to crack or get a pinhole

 

[DAVE81CB750]

I should note that needles and seats are available from Vintage CB750 for less than $8.00 each.

Thanks Brian, will these fit 1981 model

 

[Brianvance]

Thanks Brian, will these fit 1981 model

No. I just grabbed one. Check with Vintage750 for the correct ones. It is amazing what you can buy at reasonable prices for the old 750s.

 

[dirtdigger]

If your bike is a 81 I will move this post to the correct category.