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Author Topic: Leaky carb  (Read 978 times)

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Offline steve

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Leaky carb
« on: August 28, 2020, 08:52:49 AM »
I just rebuilt a set of 1978 CB750 carbs, and no matter what I do the #1 leaks out the overflow.
I have probably rebuilt 50 sets of 550 and 750 carbs over the years, but I am stumped.

Any ideas?
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Offline bryanj

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Re: Leaky carb
« Reply #1 on: August 28, 2020, 08:57:01 AM »
Cracked brass standpipe in floatbowl?
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Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline steve

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Re: Leaky carb
« Reply #2 on: August 28, 2020, 11:03:30 AM »
I thought that also.  Tested it and even put another bowl on.  That is not the issue.
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Offline 750K5

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Re: Leaky carb
« Reply #3 on: August 29, 2020, 03:03:40 PM »
I'm currently battling a similar issue w carb #2... intermittent leakage! Pulled today checked float height and cleaned jets blew out everything.  Installed #2 still leaking.  I'm about to pull the rack again.  Let me know what you come up with. The bike is a 75' 750k..
« Last Edit: August 29, 2020, 03:05:17 PM by 750K5 »
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Offline bryanj

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Re: Leaky carb
« Reply #4 on: August 29, 2020, 03:06:58 PM »
Did you use aftermarket parts? They seem to be notorious for not working
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Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline 750K5

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Re: Leaky carb
« Reply #5 on: August 29, 2020, 03:47:00 PM »
Quote from: bryanj on August 29, 2020, 03:06:58 PM
Did you use aftermarket parts? They seem to be notorious for not working

I used aftermarket rebuild kit w/ stock jets and float needle & seat. The leak is coming through overflow tube. I may need to look closer at the needle & seat on #2. I have another set of carbs to rob parts. I'm just bummed to re do s#%t!
« Last Edit: August 30, 2020, 06:37:44 AM by 750K5 »
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Offline steve

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Re: Leaky carb
« Reply #6 on: August 30, 2020, 07:44:40 AM »
So, I take the float bowl off #1 - put fuel to the rack - hold the float up/shut no fuel comes out which is correct.  I open the float - fuel comes out and then when I shut it fuel keeps leaking.
I have tried multiple new float needles.  I am baffled.
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Offline 750K5

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Re: Leaky carb
« Reply #7 on: August 30, 2020, 10:24:00 AM »
Quote from: steve on August 30, 2020, 07:44:40 AM
So, I take the float bowl off #1 - put fuel to the rack - hold the float up/shut no fuel comes out which is correct.  I open the float - fuel comes out and then when I shut it fuel keeps leaking.
I have tried multiple new float needles.  I am baffled.
Is it possible for these floats to have a leak and not float due to filling w fuel?
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Offline steve

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Re: Leaky carb
« Reply #8 on: August 30, 2020, 11:35:03 AM »
Nope.  Checked it.

It is odd.  The needle seals at the beginning if I hold it shut, but once I open it and gas flows I cannot seal it up again.
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Offline bryanj

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Re: Leaky carb
« Reply #9 on: August 30, 2020, 01:22:49 PM »
Has to be a bad needle or needle seat then
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Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline steve

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Re: Leaky carb
« Reply #10 on: August 30, 2020, 01:29:19 PM »
If it is the needle seat there is no fix for that - correct?
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Offline bryanj

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Re: Leaky carb
« Reply #11 on: August 30, 2020, 01:32:30 PM »
Nope just exchange, but buy the Honda part
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Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline Maraakate

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Re: Leaky carb
« Reply #12 on: August 30, 2020, 04:46:33 PM »
In some rare cases you may be able to stop the leak by taking some q-tips and toothpaste and clean the seat.  Keep doing this until the q-tip comes up clean, you might need 2 or 3 to do it.
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Offline ktm520

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Re: Leaky carb
« Reply #13 on: August 30, 2020, 09:07:14 PM »
Quote from: steve on August 30, 2020, 01:29:19 PM
If it is the needle seat there is no fix for that - correct?

FWIW, the seat could be the issue (OD interface and ID interface with needle), but I have also found in some cases it is actually the dynamics of the float tab on needle applies force at an angle and leads to a hang up of closure on seat. Changing the tab bend usually takes care of it.

All of this can be tested out easily, but you will need to take carbs off again.

Good luck!

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