CM400 Carb Leaking Overflow

jpweaver

jpweaver

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Joined
Sep 13, 2025
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Raleigh
Hi guys, wondering if anyone can help my diagnose my leaking overflow on my right side carburetor. Some details below...

Only right side carburetor is leaking, substantial leak upon start up (rebuilding bike and have not gotten to properly idle yet)​
When I removed it, bowl was full of gas - making me think it's the float/plunger that is malfunctioning and not the overflow pipe​
I switched the plunger and float between the two carbs - and the issue stayed with the right side carb. Now making me think it's a problem with the sealing surface and the plunger.​
I checked the hole that the plunger seats in and everything looked fine​

Any ideas on this one? Thanks for your help!

SOLVED: Thanks for your help guys! It ended up being that I needed to thoroughly clean the sealing surface of the float needle valve - not the valve itself, but the carb body. Per a recommendations below, I cut a cotton swab in half, put it in my drill, and used carb cleaner and the swab to clean the sealing surface.
 
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Fill bowl with mineral spirits in the bench, rubber stopper on top of over flow tube, pump about 5 to 10 psi with a vacuum pump/mityvac from the drain and look for bubbles. Thats where the Crack is in the standpipe. Solder appropriately.
 
It seems like I’m getting flow around the outside of the plunger even when the float has closed it, is there supposed to be an o-ring at the bottom of the gas feed hole?
 
jpweaver said:
It seems like I’m getting flow around the outside of the plunger even when the float has closed it, is there supposed to be an o-ring at the bottom of the gas feed hole?
Click to expand...
The "plunger" is the float needle (or float valve) and some have rubber tips, some do not. I know of no float valve seat that has an o-ring seal. Most aftermarket float needles supplied with kits are incorrect length or have an imperfect tip, often ground at an angle, so they don't seal well for many possible reasons. Here is some interesting reading on it.

 
It sounds like the float needle isn't seating. You can use a small roll of fine sandpaper, @600 grit, with one end protruding a little and work to polish the bore and seat of the float needle housing. Some people use a Q-tip. Toothpaste is a mild abrasive that can be used with the Q.
 
His intro thread says a 1980 so assuming the carbs are original then it would be the non-adjustable floats which means you definitely need the OEM ones.
 
OT: but yeah, if I had a choice 78 or 79 carbs. Accel pump is overrated in my opinion. Getting the gap right is annoying on replacement. I never noticed any real performance improvement on the models that had it.
 
Thanks for your help guys! I’ve got some OEM float needles on order. When I get home today I’ll polish the sealing surface. I’ll post an update when I’ve got something
 
If you had aftermarket float needles then OEM is probably all that needs to happen. Just take a q-tip with some carb cleaner and run it in there and put the new float needles and see what you get.
 
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