Author Topic: Never ending gas leak from carb #2 on CB350F (Read 3428 times)

Gibbit · « **on:** December 28, 2018, 02:00:41 PM »

I recently rebuilt the carbs on the CB350F and the bike runs better than ever. But I have a hellish leak from the overflow tube on carb #2.

Rebuild includes all new brass everything including float needle and seat. Rebuild kit was the deluxe kit from 4into1. I did not replace the floats. After taking off the bowls twice, checking everything was seated properly and putting back together the leak remains. The other bowls leaked a little at first but a couple taps on the bowls and they all stopped aside from #2.

Steps done so far:

New float needles and seats, needles drop in seat freely don’t appear to bind
Cleaned inside of bowl where the needle seats press in
Float heights all set to 24mm, checked every time I had the bowls off
Float retaining pins cleaned thoroughly and floats move freely
Checked for cracks in the overflow tube, no cracks and didn’t leak before rebuild
Blew out the 2 vent tubes, when shooting a little compressed air in each vent tube you could see a decent amount of gas being pushed out 1-2 and 3-4 carb overflow tubes

What am I missing?

Couple follow up questions...
The floats are original foam/plastic type floats. Are these prone to leaking? Are they foam throughout or still hollow in the middle?
Are the reproduction floats from 4into1 any good?
The float needles are machine tipped, I couldn’t find and Viton tipped needles for the 350f. Does anyone make Viton tipped needles for this model?

Next time I have the carbs off I’m going to swap the needle and seat between the #2 carb and another carb to see if that might narrow it down to the needle/seat. Any other ideas?

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Duke McDukiedook · « **Reply #1 on:** December 28, 2018, 02:25:51 PM »

I have been battling a leak in #1 and #4 on my carbs and they were aftermarket float needles.
For whatever reason people have been reporting issues with aftermarket needles so I bought an OEM set but have not installed yet.
Try that out and see if that helps.

innovativems · « **Reply #2 on:** December 28, 2018, 06:24:39 PM »

I had the same problem on my Cb400f. I replaced the float needles with a set of oem ones and never had a problem since

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Don R · « **Reply #3 on:** December 28, 2018, 08:42:35 PM »

I had a leak on an overflow tube I couldn't see, a float that moved to the side and hung up, at least a couple float valves that wouldn't hold. One float that didn't, that's all I have.

Robbo · « **Reply #4 on:** December 29, 2018, 08:56:01 AM »

Could also try swapping the floats between 1 & 2 if the leak is still there after swapping needles.

Swap one component at a time.

If/when the problem moves to carb 1, it becomes much easier from that point to replace the suspect part or adjust the float height on carb 1 since you have better access to the outer-most carb.

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Tim2005 · « **Reply #5 on:** December 29, 2018, 09:37:22 AM »

Shouldn't they be set to 21mm not 24mm? I doubt that's much to do with the leak though, but you never know...

flybox1 · « **Reply #6 on:** December 29, 2018, 05:30:47 PM »

24 mm, and then clear tube test to confirm. 21mm sets the fuel level too high....and is incorrect.
anytime you go to aftermarket float needles, it is prudent to clear tube test the fuel level, and then adjust accordingly.

Gibbit · « **Reply #7 on:** December 29, 2018, 08:35:39 PM »

Thanks all. Going to start swapping parts to see if/when the problem moves to another carb. If narrowed down to the needle I’ll probably go ahead and replace all needles with genuine needles.

Might be a couple days till I can try and report back, got family in town for the new year.

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awrawr · « **Reply #8 on:** January 01, 2019, 12:35:44 PM »

The needle valve seat is fitted with an o ring. (4mm ID 1.5 or 1.0mm ? CS)

Is that seated properly?

Gibbit · « **Reply #9 on:** January 02, 2019, 11:12:13 AM »

Took the carbs back off. Swapped the needle and the seat one at a time to carb #1. When turning the gas back on, with the carbs back together on the bench, #2 still leaks and all other carbs leak on and off. I also turned the gas on with the bowls off and one at a time lightly pressed the floats up and gas stopped on all 4... so the crevice the needle seats are in and o-rings are probably fine.

At this point, I think the aftermarket needles and seats are garbage and I'm going to order genuine needles, seats and o-rings.

With the carbs apart I also dunk tested the floats and none of them showed signs of leaking. No air bubbles and they all floated in gasoline for the better part of 30 minutes with no issues. Also filled the carb bowls with gas up to the edge and the overflow tubes did not leak at all (i.e. no cracks on the tube brass).

Hopefully the genuine needles and seats take care of the leaking. And if not, at least I can have peace of mind that they're likely not the problem and genuine again.

Report back in a week or two when the parts arrive.

Deltarider · « **Reply #10 on:** January 02, 2019, 11:35:55 AM »

What happened to the OEM jets and needles that were in there? They hardly wear if at all.

flybox1 · « **Reply #11 on:** January 02, 2019, 12:30:12 PM »

Why test the carb floats....they're solid?!?

How do you know the aftermarket needle and seat (and OEM needle and seat) are gabage?

Did you clear tube test the bowl to see if the float height is so off to make the bowl always leak?

Gibbit · « **Reply #12 on:** January 02, 2019, 12:41:55 PM »

Quote from: Deltarider on January 02, 2019, 11:35:55 AM

What happened to the OEM jets and needles that were in there? They hardly wear if at all.

Hey Delta, I’m referring to the float needle and seat, not the jets.

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Gibbit · « **Reply #13 on:** January 02, 2019, 12:45:57 PM »

Quote from: flybox1 on January 02, 2019, 12:30:12 PM

Why test the carb floats....they're solid?!?

How do you know the aftermarket needle and seat (and OEM needle and seat) are gabage?

Did you clear tube test the bowl to see if the float height is so off to make the bowl always leak?

Hey Fly, I couldn’t find an answer to the question in my first post about if the floats are solid or hollow... so I tested to be sure.

As far as the aftermarket needle and seat, I’m basing that simply on the comments above about using genuine float needles versus the aftermarket.

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flybox1 · « **Reply #14 on:** January 02, 2019, 12:59:50 PM »

The only aftermarket parts to NOT use, are jets, and slide needles. Everything else is fine to use.
The only time you will have a float valve and seat fail, is if their is a wear line on the conical tip of the float needle...rare, but it happens.
Aftermarket float needle sets typically have different overall lengths than OEM, so, to be sure bowl fuel levels stay the same, floats need to be adjusted. How much (or little) to adjust can only be shown by performing a clear tube test.

Really....clear tube test the fuel level in that bowl....

Gibbit · « **Reply #15 on:** January 08, 2019, 07:41:56 AM »

Finally replaced all the float seats and needles with genuine Honda parts. Set the floats to 24mm and did a clear tube test. No leaks at all and the clear tube test shows all bowls withing 3-4mm below the bowl lip.

I probably could have gotten away with the aftermarket needles after some float adjustments, but now I have some piece of mind using new genuine parts. The aftermarket needle and seat were both longer / taller than the genuine parts and the spring in the needles did not have as much travel as the genuine needles.

Attached some pics of my clear tube setup. It's not pretty, but it works.

Duke McDukiedook · « **Reply #16 on:** January 08, 2019, 07:51:52 AM »

I have had nothing but leaks with the some of the aftermarket needles I have used lately.
Seems that others have had these problems as of late.

flybox1 · « **Reply #17 on:** January 08, 2019, 08:18:59 AM »

tough to tell readings as there is too much tubing.
if you can route the tubing closer to the bowl, and vertical, you'll get a nice concave meniscus of water to read against the bowl seam.

Aftermarket float valves can be troublesome. the stock float value goes out the door, and setting float heights based on clear tube test readings is the only solution.

Gibbit · « **Reply #18 on:** January 08, 2019, 10:09:10 AM »

Quote from: flybox1 on January 08, 2019, 08:18:59 AM

tough to tell readings as there is too much tubing.
if you can route the tubing closer to the bowl, and vertical, you'll get a nice concave meniscus of water to read against the bowl seam.

Aftermarket float valves can be troublesome. the stock float value goes out the door, and setting float heights based on clear tube test readings is the only solution.

Reading on the tubes are good, they’re just that way for the sake of the picture and so the gas doesn’t leak out when I’m not holding them. I moved the tubes around and closer to bowls to get good reading.

Carbs are back on the bike, no leaks, runs great. Plug chops up next but that’ll be a little while when the weather warms up.

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Robbo · « **Reply #19 on:** January 08, 2019, 01:59:41 PM »

Good to hear

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Author Topic: Never ending gas leak from carb #2 on CB350F (Read 3428 times)