Gas Leak from Carb Drain in CB750F

[Rockwork]

How can I stop a slow gas drip from the drain even with the drain plug fully tightened? I swapped drain plugs with another bowl, and get the same drip so it doesn't seem a problem with the plug.

[shultz_1978]

Check your float level and over flow tube on that carb.  Some times the over flow tubes can get a small crack in them.

[shinyribs]

Is it leaking from the drain screw or the overflow nipple?

Shultz is on the money on the cracked overflow tubes. I scratched my head for many nights over one of those.

If it's the drain screw itself,there are two different kinds: one that seals via o-ring,and one that seal via an aluminum crush washer. Chances are it the o-ring or sealing washer that's failed.

[Rockwork]

Leaking from the overflow nipple. It's not leaking from the drain screw. Like I said, I swapped drain screws with another bowl and the drip didn't change. And it's not the the float valve, because when I took off the bowl and it was only half full of gas it still leaked.

Therefore, is there a way to replace the overflow tube?

[icerigger]

Just patch it with some JB Weld. Works okay.

[seestheday]

I believe that most people fix cracked tubes carefully with jbweld.

Shiney, is that what you did?

[Matt Story]

A badly functioning float needle valve is more likely to cause fuel to come out of the overflow tube than the chances of tube being split.

Your should:
Inspect, repair and/or replace your float needle and/or float seat.
Check that your float is equally buoyant compared to your non-leaking carbs.
Check that the float level is correct (and for tuning, even with all the other floats).
Your could check the needle valve function with the float bowl off by actuating the float by hand and observing if the gas gets turned off.

On my 750F I recently had to take drastic measures to get my float needle/seat to seal up by polishing the seats and replacing the needles.  (search my thread re: 750 needle seat needs aggressive polishing)  I wouldn't recommend doing what I did unless you have limited options and you're confident in your ability.  Others will recommend lightly polishing your seat with toothpaste and a Q-tip, and that is normally good enough.

Hoping to be helpful.

[seestheday]

I don't like toothpaste. Leftover toothpaste can cause then to stick. I think brasso or other polish is better.

My valves stuck really bad with leftover toothpaste, but I let my carbs sit for months before reinstalling them.

Sent from my BlackBerry device

From: Matt Story [via Honda CB750'S]

Sent: Thursday, June 6, 2013 7:49 PM

To: seestheday

Subject: Re: Gas Leak from Carb Drain in CB750F

A badly functioning float needle valve is more likely to cause fuel to come out of the overflow tube than the chances of tube being split.

Your should:
Inspect, repair and/or replace your float needle and/or float seat.
Check that your float is equally buoyant compared to your non-leaking carbs.
Check that the float level is correct (and for tuning, even with all the other floats).
Your could check the needle valve function with the float bowl off by actuating the float by hand and observing if the gas gets turned off.

On my 750F I recently had to take drastic measures to get my float needle/seat to seal up by polishing the seats and replacing the needles.  (search my thread re: 750 needle seat needs aggressive polishing)  I wouldn't recommend doing what I did unless you have limited options and you're confident in your ability.  Others will recommend lightly polishing your seat with toothpaste and a Q-tip, and that is normally good enough.

Hoping to be helpful.

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[jamman18]

Can someone show a picture of where the drain screw is located on the carbs?

[TOOLS1]

It's the screw on the very bottom of the float bowel.
TOOLS

[Re-run]

Look at the bottom of the bowl, right where the drain tube connects to the nipple. Of your carbs are assembled right, you will see a lone horizontal screw on each bowl. Carb 1 and 2 face outside left. 3 and 4 face right. Simple flat head screwdriver fits them.

[SnowriderX]

I realize this message was stated back in 2013 but I felt it worth

[SnowriderX]

I realize this post started back in 2013. It's worth mentioning that JB Weld often falls off after sometime & is not a permanent repair. After spending many hours replacing the float valves & adjusting the floats to repair a leak from the overflow Drain. I finally realize that the bowl had a crack in the overflow tube, as the earlier post mentioned. The best repair for this was to braze the tube with solder using an electric soldering gun as torch is too hot. Start by cleaning the tube with small wire brush & prep with flux as you would any plumbing job. Set the bowl slightly upside down & work from underneath heating the tube with the soldering gun. Once the tube is hot apply solder and allow it to run down the entire length of the tube and make sure it's completely covered. Tried to load a picture but it doesn't support my pic format. Disclaimer: Moulton Solder is hot don't get hurt!