Topic: #2 carb on '78 cb750k leaking from overflow

[huyzer]

Hi everyone.

New owner here.  After rebuilding the carb bank the #2 carburetor won't stop fuel from pouring out of the over flow pipe.

Details:
1978 cb750
Stock carbs
Stock everything.

New rebuild kit. New float needle.
Adjusted the float to 14.5 mm as specified and fuel is still overflowing. 
Tried increasing the height of the float to about 20mm above the gasket flange to see what happens and it still is overflowing.
At this point I'm thinking of replacing the float valve, but can't find a replacement part. Seems about right as it looks press fit in place.
Has anyone replaced the valve?  If so how??

One other thing I found interesting is that once I installed it back on (after adjusting the float to 20mm, is that it didn't leak right away. I moved the bike back and forth and side to side a bit and then it'd start overflowing. After that, only way to stop the over flow is to turn of the gas at the petcock.
Now I'd rather not run the bike at this crazy float height only to stop the leak and find the gas level incorrect and bike runs poorly.

Anyone know what else I can check/do to stop this thing from overflowing.
It's driving me absolutely nuts!!!! 

Thanks in advance.

[blparker]

Check to ensure you adjusted them with the float barely touching the spring loaded pin on the end of the needle. Second thing I ran into with mine is I could never get after market needles to seal. If there is one thing I have learned. Honda carb parts are the way to go. Same is probably true for other seals and gaskets. My aftermarket float bowl gaskets would always swell. Honda gaskets have yet to do that.

[blparker]

One other thing I did although maybe not advised is I lapped the seats for the needles. I took apart a center punch and used the hardened steel tip in a drill with a bit of polish and touched each seat while spinning it. They never leaked after that.
Just my 2 cents.

[huyzer]

I did set the floats with the float just barley sitting on the spring of the needle.

The needle I bought are crappy aftermarket and after hearing what you said, I'm starting to suspect the needle not sealing properly. Although the the other three were fine. Strange?? Or just lucky I guess.

Don't know what else it could be.

I've also tried lapping the bottom of the valve to ensure a good fit. That did prove to be any better.
Maybe with a genuine part I may have better results.

Anything else I can probably try/ think of that could cause the problem? 

[madmtnmotors]

Aftermarket needles are a crapshoot. I just threw away a brand new set of float needles that began to leak profusely after exposure to ethanol laced fuel for just one week. There are suitable aftermarket needles out there but they must specify "Viton" rubber tips to tolerate ethanol blended fuels.

I highly recommend against "lapping" the seats. There should not be any wear on the seats from rubber tipped needles. If there were, it would be so marginal that new rubber tips should still seal. I would recommend Viton bowl gaskets as well.

[blparker]

Aftermarket needles are a crapshoot. I just threw away a brand new set of float needles that began to leak profusely after exposure to ethanol laced fuel for just one week. There are suitable aftermarket needles out there but they must specify "Viton" rubber tips to tolerate ethanol blended fuels.

I highly recommend against "lapping" the seats. There should not be any wear on the seats from rubber tipped needles. If there were, it would be so marginal that new rubber tips should still seal. I would recommend Viton bowl gaskets as well.

My carbs were in horrible shape. I agree, decent carbs should not show wear. Mine had a lot of tarnish and junk caked everywhere, so my goal was to get an even seating surface. Don't lap the rubber tip lol

[harisuluv]

Other causes for overflow: Cracked overflow drain tube.  The drain screw is also part of the overflow. Just loosening it drains the bowl.  If the screw is badly corroded it will not seal correctly.

[Gene]

Other causes for overflow: Cracked overflow drain tube.  The drain screw is also part of the overflow. Just loosening it drains the bowl.  If the screw is badly corroded it will not seal correctly.

+1 - Seen this a lot on this forum, hairline crack in the tube.  Everything else can be right, but if the tube is cracked or otherwise impaired all fuel flowing into that bowl is gonna leak out.

[madmtnmotors]

Other causes for overflow: Cracked overflow drain tube.  The drain screw is also part of the overflow. Just loosening it drains the bowl.  If the screw is badly corroded it will not seal correctly.

+1

Remove the bowl and "leak test" just the bowl.

[flybox1]

+1 for bad drain screw/seat and/or a cracked standpipe.

[huyzer]

So I've removed the bowl and checked the following:

Looked for cracks in the overflow pipe....none that can be seen with a 3x magnifying glass.
Leak tested the drain screw...works fine
cleaned and checked seating of drain plug screw....works like a charm. 
mocked up a set up to simulate overflow (with water) with just the float bowl and found that it works as it should...ie no leaking if fluid level is below the overflow pipe level.  As soon as fluid level rises above the overflow pipe...it goes straight out of the drain tube.

my main suspect right now is the float needle as it was a crappy aftermarket piece.  I ordered OEM replacements from jetsrus.com.

from all the responses here...its seems as though I've checked off all that I can for causes.  Not unless fuel can come from another source other than the Float/needle valve???  not likely but stranger things have happened. 
     

[madmtnmotors]

from all the responses here...its seems as though I've checked off all that I can for causes.  Not unless fuel can come from another source other than the Float/needle valve???  not likely but stranger things have happened. 

Only other potential culprits that come to mind would be:

>Unusual wear patterns, on either the float tang (that bears against the float needle) that might look like a "divot", or maybe wear or a groove or a lip on the side of the float post that could catch on the float bracket causing it to momentarily "hang". Maybe the float bracket bent out of shape when compared to the others. Maybe the float pivot pin bent out of shape preventing the float from pivoting freely. These would be diagnosed by just a careful visual inspection for anything out of the ordinary.

[flybox1]

Be sure the brass bracket that holds the two floats is not twisted.
this is common if the tang has been adjusted improperly.
I hope harisuluv jumps back on and links his float leveling photos.

you should have viton-rubber tipped float valves. 

when filling the bowls for the first time, have a chunk of wood or rubber mallet to tap on the carb backing plate. 
impacts here can usually free up a stuck(down) float.

here is a good picture of a float tang divot needing to be treated, and a worn float valve tip...

[huyzer]

from all the responses here...its seems as though I've checked off all that I can for causes.  Not unless fuel can come from another source other than the Float/needle valve???  not likely but stranger things have happened. 

Only other potential culprits that come to mind would be:

>Unusual wear patterns, on either the float tang (that bears against the float needle) that might look like a "divot", or maybe wear or a groove or a lip on the side of the float post that could catch on the float bracket causing it to momentarily "hang". Maybe the float bracket bent out of shape when compared to the others. Maybe the float pivot pin bent out of shape preventing the float from pivoting freely. These would be diagnosed by just a careful visual inspection for anything out of the ordinary.

Checked out all these...and yes the pin looks good, the float tang in good shape, no signs of wear on the on the tang either. 
The float does operate freely as it should. 

[flybox1]

its a simple flow system, just like in the back of a toilet.  the float lifts and closes a valve, shutting off flow.
if your flow is not shutting off, either the float is not 'floating' or the valve is not closing.

[DaveBarbier]

While the fuel petcock is off, can you just remove the bowl, gently lift the float with your finger and turn the petcock on. That might tell you if the valve isn't sealing properly. As the fuel is leaking from the overflow tube, rap on the bowl with a screwdriver handle and see if the leaking stops. If so, the float is getting stuck.

I've also seen some people want to actually see what's going on by removing the bowl and taping/rubber banding up a clear plastic bowl and watch it and see what it does when the fuel is on.

Don't smoke or grill near by while doing this.

[harisuluv]

http://forums.sohc4.net/index.php/topic,149845.msg1709647.html#msg1709647

[flybox1]

This^^^^

[huyzer]

While the fuel petcock is off, can you just remove the bowl, gently lift the float with your finger and turn the petcock on. That might tell you if the valve isn't sealing properly. As the fuel is leaking from the overflow tube, rap on the bowl with a screwdriver handle and see if the leaking stops. If so, the float is getting stuck.

I've also seen some people want to actually see what's going on by removing the bowl and taping/rubber banding up a clear plastic bowl and watch it and see what it does when the fuel is on.

Don't smoke or grill near by while doing this.

this is good advice...this is why I ask you guys on this forum!!. 
I will definitely try this to make sure what's happening...hopefully its as simple as a stuck float and worse case a bad float needle.

[harisuluv]

Worst case would be a bad SEAT.  Why?  Cause it's pressed in and non replaceable (practically).

[huyzer]

Worst case would be a bad SEAT.  Why?  Cause it's pressed in and non replaceable (practically).

more because I can replace the float needle and a bad part is causing all this headache.

lesson learned here....buy genuine parts when you can.  Even a simple as a float needle and the poor quality rubber on it can possibly be the source of all this trouble.   

Thanks for all the advice...its been amazingly helpful. 
Action taken so far is that I've purchase new OEM replacement float needles from jetsrus.com...I'll update the group when I get the parts and slap her in.   

Thanks!!!!

[flybox1]

lesson learned here....buy genuine parts when you can. 

Amen, brother!

[huyzer]

Hey folks

Here's an update from my issue with the overflow.

So all my troubles came down to a bad float valve needle.
Basically after rebuilding I used a cheap after martket part and never really sealed the vavle.

Swapped all the new parts with OEM replacements, set the float heights, and it seemed to have solved my problem.

Moral of the story....by OEM replacements when possible!!!

Thanks to the guys that replied to my post...it helped me work through it without going crazy.

Time to get riding!!
Huy

[madmtnmotors]

So all my troubles came down to a bad float valve needle.

Good news indeed.    There are some components that will tolerate replacement with aftermarket knock-offs with substandard components (turn signal lenses for example), while other components are less forgiving to compromise.

Rubber tipped float needles are one of those less tolerant components!   

Cases like this can have you chasing your tail simply as a result of substandard materials. Mine were leak-free for one week, after complete disassembly, sonic cleaning, and reassembly they performed flawlessly for six days. On the seventh day they began to leak profusely. Upon removal of the float bowls, the bowl o-rings were already swollen and distorted to the point of being rendered useless so the root of my problem was painfully obvious. New Viton tipped float needles along with Viton bowl gaskets and I've been good for over 1000 miles and two months now.

All part of the learning process. I've had mine for almost 30 years now and I'm still learning!