Carb Passage Question - What is it?

[dmaxweb]

There are 3 holes in the lower carb body between the bowl and throttle body. The center one is for the bolt that holds the upper and lower body together. One 'outboard' hole is for the cap jet. What is the 3rd hole for? It seems to be open to the air and it has gas dripping from it.

 

[Dave in Boise]

I think that is a float bowl vent or air balance passage to the float chamber.. I assume the carb gurus will gove you a better answer

 

[Tom Wesenberg]

I think that is a float bowl vent or air balance passage to the float chamber.. I assume the carb gurus will gove you a better answer

Dave is right and if you have gas dripping from it the float is leaking or the needle and seat are leaking and allowing the gas to raise too high.

One more though, is the gas boiling due to heat and comming out the hole, or does it do this even in cooler temps or while setting still?

 

[dmaxweb]

Dave is right and if you have gas dripping from it the float is leaking or the needle and seat are leaking and allowing the gas to raise too high.

One more though, is the gas boiling due to heat and comming out the hole, or does it do this even in cooler temps or while setting still?

It does it when cool and sitting still. I will check it today. I suspect it is the float vavle but if the float itself has a leak, can it be soldered?

 

[Tom Wesenberg]

It does it when cool and sitting still. I will check it today. I suspect it is the float vavle but if the float itself has a leak, can it be soldered?

Yes, I've soldered several. I use a hair drier to heat the float and turn the float all around to find the leak, then hold the leak down so the hot air drives the gas out.

When you get all the gas out clean the leak area and solder the clean brass as quickly as possible to avoid getting the float too hot. If it gets too hot the hot air will want to keep escaping the float and you'll wind up with a soldered repair that still has a leak.

You'll often see a small solder spot on floats. This is a small factory vent hole that lets the air escape while the seam is soldered. Then with the float at room temp the small hole is quickly soldered shut.

On a couple of floats with cracks I've had to unsolder this vent hole, then solder the crack, let the float cool, then go back and solder the vent hole.

 

[dmaxweb]

Yes, I've soldered several. I use a hair drier to heat the float and turn the float all around to find the leak, then hold the leak down so the hot air drives the gas out.

When you get all the gas out clean the leak area and solder the clean brass as quickly as possible to avoid getting the float too hot. If it gets too hot the hot air will want to keep escaping the float and you'll wind up with a soldered repair that still has a leak.

You'll often see a small solder spot on floats. This is a small factory vent hole that lets the air escape while the seam is soldered. Then with the float at room temp the small hole is quickly soldered shut.


On a couple of floats with cracks I've had to unsolder this vent hole, then solder the crack, let the float cool, then go back and solder the vent hole.

I removed the float and submersed it in water and it has no leaks. When I removed the glass sediment bowl on the firewall I found a big crude deteriorating leather gasket that only had a small hole in the middle and no screen. Found grit in carb screen and cleaned it. Still getting fuel overflow. Next step is to check the float level and the float valve to see if grit has damaged the float tip or seat preventing it from sealing.

 

[Purdy Swoft]

If you are talking about the small hole (less than 1/8 inch) that is behind the lower drain plug that covers the bottom of the main jet, it is not a bowl vent. I call this a choke drain hole. The leaks could be caused by too many gaskets under the main jet. This will lower the tip of the main jet below the level of gas in the bowl and it will continue to leak. It could also be caused by a cap jet that is TOO short, this has the same effect as too many gaskets under the main jet. T he bad repro cap jets that I have found have a bullet shaped end. Other causes could be improper float level, bad or loose needle valve seat, trash in the needle valve, hole in float or in some cases a cracked lower case

 

[dmaxweb]

If you are talking about the small hole (less than 1/8 inch) that is behind the lower drain plug that covers the bottom of the main jet, it is not a bowl vent. I call this a choke drain hole. The leaks could be caused by too many gaskets under the main jet. This will lower the tip of the main jet below the level of gas in the bowl and it will continue to leak. It could also be caused by a cap jet that is TOO short, this has the same effect as too many gaskets under the main jet. T he bad repro cap jets that I have found have a bullet shaped end. Other causes could be improper float level, bad or loose needle valve seat, trash in the needle valve, hole in float or in some cases a cracked lower case

With the lower body removed, the holes I am talking about are in the top center of the lower body. There are three 3/8" holes. One is for the bolt that comes up to hold the lower body to the upper body The one furthest from the engine is where the cap jet screws in and the other hole closest to the engine is empty and goes straight through to upper body and then to the ouside. It does not appear to have any internal passage to anything. Could this be an overflow for the bowl?

 

[Purdy Swoft]

There is a hole about mid way on the engine side of the lower half of the Zenith carb. this hole is the bowl vent. it could be several things that would cause a leak at this location. Fuel rises in the bowl, if the needle valve can't close, the gas has to go somewhere. Usually when they leak at this spot the float has sprung a leak and sunk.

 

[dmaxweb]

There is a hole about mid way on the engine side of the lower half of the Zenith carb. this hole is the bowl vent. it could be several things that would cause a leak at this location. Fuel rises in the bowl, if the needle valve can't close, the gas has to go somewhere. Usually when they leak at this spot the float has sprung a leak and sunk.

Thanks, that was my guess about the purpose of the hole.

I took off the float and submerged it in water. It floats and there are no bubbles. I then removed the float valve and when I blow through it, it does not completely seat (leaks air). I am going to replace the float valve tonight and make sure nothing is binding on the float arm.

 

[30Tudor]

On the leak front here are some ideas.

The following is an excerpt from an article I wrote on Carburetor issues. This article was written is to discuss some of the things I learned that weren’t mentioned in the reference materials.

Leaks

After assembling the carburetor, the first thing I had to deal with was a chronic intermittent carburetor leak. I fooled around for weeks with my float level setting and also tried every type of float valve available (an expensive experiment). Despite trying both a modern needle float valve and the ball valve (the one that I eventually landed on), I would still occasionally open the fuel valve to find fuel dripping from the carburetor down the splash pans unto the garage floor or my engine running very rich. I have also heard with the needle valves that you need to seat them in by lightly tapping on the valve or using toothpaste to lap them in.

One leak-fix tip that was given to me related to a small indent getting worn into the top of the float hinge where it contacts the float valve. If this indent is too large it will apparently catch on the float valve and inhibit the float movement. Although the indent on my float was minor, I fixed it by soldering a piece of brass shim stock over the indent to make the float valve contact area smooth – but still no success. Finally, it was suggested that the raised casting at the end of the float bowl may be interfering with the float travel. I had checked my float movement a number of times using the “rotate/shake carb and listen method”, however, at this point I was desperate to try anything. Out came my Dremel and I ground the raised area down smooth. Finally the leak problem was fixed!

Another leak source is the Gas Adjusting Valve (GAV) needle housing where it screws into the lower casting. This leak source can be deceptive because it is slow and the gas ends up at the drain plug as with other leaks. I used gasoline resistant thread sealer to fix this problem. Tightening the needle valve housing down resulted in the needle valve being difficult to turn. To allow the GAV needle to turn more freely, remove or open up the retainer wire at the end of the housing.

 

[Ron in Quincy]

If the bowl fills high enough for fuel to exit the vent hole, you would have gas running over the top of the main and cap jets .
There are various differences in carburetor lower bodies; mid ones don't have the vent hole drilled directly into the bowl; the air enters thru the large hole in the upper body into the cavity where the idle jet is located; in this cavity is a small hole drilled into the bowl so the bowl breathes; this cavity also provides air to the idle air adjustment needle.
On occasions I find a problem with the gas line connection leaking, this is easily detected as the bowl gasket will be wet, the gas will travel to the back of the carburetor and drip near the vent hole.

Etc, Etc, Etc.

Ron