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Old 08-06-2019, 09:26 PM   #1
DannL
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Default Carburetor leak . . .

After returning from the trip to get gas I examined the carburetor and noticed something interesting. On the back side of the carburetor, in line with the Idle Adjust, it was "boiling and spitting" gas out onto the carburetor body . . . and occasionally it spit gas as far as the engine block. That doesn't sound normal. The carburetor was to hot to touch any length of time. Once the carburetor cooled down it stopped spitting gas. Ideas? I think it's time for a rebuild kit.


Any idea why there is a hole on the back of the carburetor for gas to come out?
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Old 08-06-2019, 09:38 PM   #2
F.M.
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Default Re: Carburetor leak . . .

Well for one it looks like part of the intake gasket is missing ??.. I would say just put a rebuilt Zenith back on it as original..
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Old 08-06-2019, 10:33 PM   #3
1955cj5
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Default Re: Carburetor leak . . .

There is a small hole back there....that screw is an air-bleed adjustment and the hole is the air source.
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Old 08-07-2019, 10:29 AM   #4
Purdy Swoft
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Default Re: Carburetor leak . . .

It sounds like it is either running way too lean or the timing is running too retarded. Either of these problems would cause the engine and carburetor to run hotter to the touch . Heat causes expansion . Retarded timing or excessive use of the spark lever in the retarded direction causes loss of power and overheating . I've never had that problem with a Tillotson carburetor but do prefer the Zenith carb .
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Old 08-07-2019, 10:00 PM   #5
Bob from Northport
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Default Re: Carburetor leak . . .

Those carburetors are made of an alloy that warp easily when hot. Get a good Zenith
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Old 08-14-2019, 07:47 AM   #6
DannL
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Default Re: Carburetor leak . . .

Okay. I've taken the advice of some here and ordered a rebuilt zenith. Should be here today or tomorrow. Will be relieved to put the smell of gas behind me. Once thats installed I will hit the timing and maybe I'll still be able to make it to a local show Saturday.
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Old 08-14-2019, 09:24 AM   #7
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Default Re: Carburetor leak . . .

Quote:
Originally Posted by DannL View Post
Okay. I've taken the advice of some here and ordered a rebuilt zenith. Should be here today or tomorrow. Will be relieved to put the smell of gas behind me. Once thats installed I will hit the timing and maybe I'll still be able to make it to a local show Saturday.
And please get rid of that paper air filter. Why Vendors keep offering these fire hazards is beyond comprehension.
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Old 08-16-2019, 07:33 AM   #8
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Default Re: Carburetor leak . . .

I surmise this is a common thing. Swapped the Tillitson for a rebuilt Zenith and the Zenith leaks fuel via the air intake once the engine is shut off also. Heres what appears to be happening . . . the caburetor get hotter when the engine fan is off and as the heat from the manifold migrates. The fuel expands and must go somewhere. This only lasts for a few minutes or so until the carburetor has cooled down.
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Old 08-16-2019, 07:59 AM   #9
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Default Re: Carburetor leak . . .

Actually, the fuel bowl of ALL carburetors have a vent - otherwise they might have difficulty in filling with gas from the fuel tank.

One of the modifications that is frequently done for those using an air filter is to "plug" the vent hole, and provide another vent hole into the carburetor intake. A so called "balanced" carb which is sold by some of the carburetor rebuilders.

The use of an air filter can cause a low pressure at the inlet to the carb, and this changed vent connection "balances" the air pressure between the inlet area and the fuel bowl - and prevents flooding caused by the vacuum of the engine when starting.

The "boiling over" you have seen may be incipient "vapor lock" which is when the gasoline in the fuel bowl vaporizes, and can't flow through the jets to the venturi. It goes where it could - out the vent?

The classical solution to vapor lock was to wrap strips of sheet metal alongside the fuel line feeding the carb - and holding these with wooden snap-type clothes pins. The motion of air alongside would "cool" hot gasoline coming down the pipe from the fuel tank and before it hits the carb.

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Old 08-16-2019, 08:12 AM   #10
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Default Re: Carburetor leak . . .

I noticed that I could easily touch the carburetor and intake manifold when the engine was running. Only after the engine was off did both rise in tempurature then followed by a slow drip from where the filter housing meets the intake of the carb. The Tillotson did that plus spat fuel from the vent opposite the idle adjust.
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Old 08-16-2019, 09:39 AM   #11
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Default Re: Carburetor leak . . .

Quote:
And please get rid of that paper air filter. Why Vendors keep offering these fire hazards is beyond comprehension.
There was one of those on my CCPU when I bought it, it didn't last too long, fell off somewhere. So I purchased and installed a "high boy" air filter. It didn't last too long either, lost it some where after 2 or 3 short trips. If I ever decide to get another air filter I'll put a tether on it in case it tries to go AWOL.
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Old 08-16-2019, 09:54 AM   #12
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Default Re: Carburetor leak . . .

Maybe its trying ton tell you that it doesn't want a filter .
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Old 08-16-2019, 09:57 AM   #13
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Default Re: Carburetor leak . . .

Quote:
Originally Posted by Purdy Swoft View Post
Maybe its trying ton tell you that it doesn't want a filter .
Could be.
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Old 08-16-2019, 11:47 AM   #14
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Default Re: Carburetor leak . . .

If I added a plate with cooling fins between the intake manifold and carburator, it might prevent the heat from migrating down. Hmmm . . . thats a thought. This intake manifold being tied directly to the exhaust manifold may be the problem. Good for northern climates but not here where we're over 100 degree in the summer.
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Old 08-16-2019, 11:53 AM   #15
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Default Re: Carburetor leak . . .

Gasoline is formulated "seasonally" now. Different gas in hot weather. You don't say how old the gas is or whether your issue is being caused with "old gas" from the cooler months. Perhaps try running it down or replacing it with current stock?

Also check your float valve for non-leaking. Lap in place with abrasive toothpaste? Or replace with a viton tipped version if necessary? Some like the Grose-Jet version (I don't care for this as multiple balls upon which it relies give multiple places to hang up.)

Some of us habitually close the gas tank source valve when shutting down.

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Old 08-16-2019, 12:03 PM   #16
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Default Re: Carburetor leak . . .

Check your private messages on this website.
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Old 08-16-2019, 12:15 PM   #17
Purdy Swoft
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Default Re: Carburetor leak . . .

In the past , a lot of the reproduction cut off valves under the tank never cut off completely . I tried replacing the packing in an original cut off valve and solved the problem . Other causes are usually trash in the valve , missing or leaking gasket under the needle valve assembly or too short reproduction main or cap jet . If the main or cap jet is too short it will be below the level of the gas in the bowl of the carburetor and will continue to leak , especially if the cut off valve is also defective . A stand up filter placed in the cut off valve will solve the problem except in worse cases the tank may need to be removed for cleaning .
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Old 08-17-2019, 09:44 AM   #18
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Default Re: Carburetor leak . . .

A little experimenting came into play. Before shutting the engine down last night, I turned the shut off valve to off but still allowed the engine to run until it emptied the carburator bowl some . . . just before the engine started to sputter. Then I shut the engine off. Observation: No leaks, No drips. Yipeee!
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