Carb Leaking -- New to Vintage Bikes

[lnuhring]

New member here!

I recently acquired a 1979 cb750k on the local classifieds--not running, but otherwise in great condition with 12k miles on the odometer. I have no experience with older motorcycles, particularily carbs.

The throttle was "stuck" and I discovered (after pulling the carbs out) that the outermost clutch side carb's butterfly valve was seized up (this was why the throttle was stuck).

I was unable to get it unseized and so I bought a stripped outermost clutch side carb on ebay. The "new" stripped ebay carb is off of 1980-1983 cb750k, and did not incude the float bowl.

I assemebled the carb bank with the new ebay carb and installed all new parts.

The good news: I got the bike running for the first time in a few decades.

The bad news: the carb that I replaced is now leaking a steady stream of fuel from the overflow tube.

I have tried a number of combinations of differnet valve needles. I currently have the 1980-1983 valve needle installed in the car (the kind with no metal wire on it). I currently have 1979 float installed and I have played with adjusting the float height as well. All with no success.... the carb continues to leak.

Can anyone point me in the right direction?

I suspect that maybe the issue is the combination of the 1980-1983 carb and a 1979 float bowl and/or float?

Any help is appreciated!


Thank you!

 

[fg1972]

Hi there and welcome.
If it’s leaking out the drain tube then it sounds like the bowl drain screw is not doing it’s job. There is an oring on it.

 

[lnuhring]

Hi there and welcome.
If it’s leaking out the drain tube then it sounds like the bowl drain screw is not doing it’s job. There is an oring on it.

I removed, cleaned, and installed new O rings on each of the drain plug screws when I rebuilt the carbs--so I dont think that its the drain plug screw.

Additionally, when I turn the petcock on, it takes about 1.5-2 minutes for the gas to start leaking (which would indicate that the bowl is filling up during that time).

I played around with the float level yesterday, as the 79 cb750 float is adjustable. We shall see what happens when I install the carbs again in a few days.


Thanks!

 

[amc49]

The bowl drain screw does not control the overflow vent, it's a double-sided system. The vent tube would be there for nothing if it did.

If the needle seat is damaged in the carb body you can change needles till earth ends and not fix it. Likely why the couple minutes before leak thing, a small trickle going on there. Common where ethanol is used in the local fuel. Water traps right at the needle when float cuts off.

 

[George Mooyman]

I have the same problem and have stripped and cleaned the carburetors and fitted new parts. If you Google MacGregor carb cleaning Volume 1 Issue 6 you can print out 42 pages of very helpful instructions for free.
The needle seat in mine is fixed and not designed to be replaced and the seat was badly damaged you need a good torch and a magnifying glass to inspect it. I polished mine with a cotton bud in a battery drill with metal polish anything will do but Brasso is maybe best.
Set up the carb on the bench with the tank connected and test it before you put it back in the bike. You will also need to replace the O rings on the little tubes that give in between the carburetors as they can leak also.
I used the metal polish on lots of parts to get the crud off even inside the pucks, make sure they move up and down freely.
The worst part of the job is getting the airbox connected up it would have been good if Honda made them a little shorter.

 

[scoleshi]

New member here!

I recently acquired a 1979 cb750k on the local classifieds--not running, but otherwise in great condition with 12k miles on the odometer. I have no experience with older motorcycles, particularily carbs.

The throttle was "stuck" and I discovered (after pulling the carbs out) that the outermost clutch side carb's butterfly valve was seized up (this was why the throttle was stuck).

I was unable to get it unseized and so I bought a stripped outermost clutch side carb on ebay. The "new" stripped ebay carb is off of 1980-1983 cb750k, and did not incude the float bowl.

I assemebled the carb bank with the new ebay carb and installed all new parts.

The good news: I got the bike running for the first time in a few decades.

The bad news: the carb that I replaced is now leaking a steady stream of fuel from the overflow tube.

I have tried a number of combinations of differnet valve needles. I currently have the 1980-1983 valve needle installed in the car (the kind with no metal wire on it). I currently have 1979 float installed and I have played with adjusting the float height as well. All with no success.... the carb continues to leak.

Can anyone point me in the right direction?

I suspect that maybe the issue is the combination of the 1980-1983 carb and a 1979 float bowl and/or float?

Any help is appreciated!


Thank you!

Before turning on the petcock to "fill" the carburetors, make sure the bike is on the main stand and level. Use the original parts is also the best idea especially with needle valves, seats, floats etc. Use the other carburetors on the bike as a guide. They should all (4) be the same. I have the same bike.