Topic: Cb750 carb overflow wont stop leaking.

[la ca fe]

I rebuilt these carburetors using new Honda parts and I can not stop #3 from dripping from the overflow valve.
I replaced the float, valve, jets, gasket... I even opened the gap of the float past the 15mm requirement to 16mm to shut the valve but still a drip, drip, drip... I made sure the float isn't rubbing up against the walls of the bowl and getting stuck.
What am I overlooking?

[martin99]

Might have a split in the brass tube inside the float bowl?

Might be a bit lean on that carb at 16mm float height btw, probably a good idea to clear tube them once you've got your leak sorted.

[evinrude7]

la ca fe, could be a split brass tube as stated above.  could be the float valve is upside down.  i've had both happen to me.  if it's a split you can use a jb weld for fuel use or silver solder.  i used jb weld and it worked.  i'm waiting on a float bowl from a fellow member because i don't trust to leave the jb in there for long.  i've also had an issue with float height but you state that's not the problem. 

[flybox1]

Check that you dont have a dried out or 'ringed' rubber float valve tip.  That'll cause it to not seal well on those PD carbs.
Dont forget the 'debris' in your tank which might be causing a float valve to not fully close.
(easy to clear this - just get your overflow tube into a jar, turn your pingel ON, and then crack open the bowl drain screw.  The 'wash' of fuel exiting the overflow tube will usually carry out any debris.)

[la ca fe]

I've swapped float bowls with a donor carb set, so I think the problem is something else??

[flybox1]

I've swapped float bowls with a donor carb set...

...and the problem continues?
Are you sure the gas is not running down from somewhere else, and just collecting off the overflow pipe?
Floats should be the same height as the pilot jets on PD carbs.  14.5mm, but you installed aftermarket valves, so
Perform clear tube test and set the floats based on the desired 3mm below the bowl gasket seam fuel level.
The 15 or 16mm you set is only a starting point.

[la ca fe]

Alright, I am taking the carbs off the bike again and determined to figure this drip problem out.

I'm going to check the valve again.
I'll check if any cracking is visible with the brass overflow tube.
I'll reset the float height to stock level.
The carbs are really clean, but I'll check for any debris causing the valve to not seal properly...

[flybox1]

Alright, I am taking the carbs off the bike again and determined to figure this drip problem out.

I'm going to check the valve again.
I'll check if any cracking is visible with the brass overflow tube.
I'll reset the float height to stock level.
The carbs are really clean, but I'll check for any debris causing the valve to not seal properly...

Dont reset the floats to stock level.
Clear tube test your carb bowls and adjust them based on the goal of a fuel leve 3mm below the bowl gasket seam.



Except you will attach the tube to your overflow pipe, and then turn open the bowl drain screw for gas to fill.
Do this BEFORE removing the carbs from the bike, and then again, on your work bench, after you make any float height adjustments.
Continue to test and make adjustments off the bike until the fuel level is correct. 
It wont overflow if you get them on 3mm.

[la ca fe]

Ok, will do that.
I'll let you know what I find.

[flybox1]

Info added above.

[jonda500]

While you're there, make sure the float tang that pushes the needle valve is not dimpled or twisted.
John

[tommychains]

What was the cause and fix in the end?