Leaking Zenith Carburetor

[TJMack]

Well, I spent most of the afternoon experimenting. I gave a lot of thought to the head pressure theories that Yblockhead proposed, and my opposite version of it and have concluded that neither of them probably holds water. (Pun intended)

By the way, in case anyone is wondering, we’re talking about the pressure generated by a column of liquid; called the “head” here. Not the pressure on my brain from all the frustration.

It shouldn’t make any difference how low the pressure is trying to push fuel into the reservoir, or how high either for that matter as long as it isn’t high enough to overcome the leverage provided by the float on the float valve. Low pressure will just make it take longer to reach the point where the float has enough leverage to close the valve. But it will eventually get there if there isn't a leak.

This is just like a toilet tank. Probably everyone has had a toilet valve that would hiss as the water gets near the point where it shuts off. This is from the valve being almost closed until the leverage of the tank ball floating pushes up hard enough to stop the water flow entirely. If the valve is bad enough the water level will continue to rise to the overflow height and just keep running water into the bowl. Often this isn’t discovered for quite some time because it doesn’t make any noises. A high water bill is sometimes the first clue.

So today I tried a few things trying to find why the valve won’t shut off. I split the carb then took the top half and cleaned it with brake cleaner so that it would dry completely to assist me in finding any leak. I then hooked up the fuel line and put the carb half upside down with the float resting on the float valve. I turned on the fuel valve and watched. No leak.

I then tried it with the float taken out. It still didn’t leak. That verified for me that the float valve that I was using was most likely OK. But when I reassemble the carb with the float level set as prescribed (5/8” to top side of float when inverted which is the same as 1” to the seam) it still leaked.

I went through a number of other fruitless gyrations including lightly grinding the inside surface of the bowl in case there was a bump catching the float. I knew this was silly, but I wanted to remove all doubt.

I took the new float valve apart and cleaned it and retried it. Still leaked.

So, like a leaky toilet, the flow will only stop if the valve gets closed before the overflow height is reached. In the case of the carb this is the height of the jets, as we discussed earlier.

I had tried lowering the height of the float before when I first started messing with this leak but it didn’t work then. But I decided to try I t again since I knew that I had to get the level of gas in the reservoir below the tips of the jets.

I added another washer to the old jet, making a total of three, and gave it a try. So far it appears to be working. Time will tell.

I’m going to make that fuel level gage that someone suggested and see where the level actually is.

Tin Cup suggested trying it with the gas cap off. I failed to mention in all these dissertations that I had already checked that. I can see right through the hole in the cap. I know I should have a gasket in there to reduce the smell of gas when the windshield is open, but I never think of it.

[1930 coupe]

When Ford made new floats back in the day, they all weighed 31 grams.
If you look at original Ford floats you will notice that some of them have round brass weights on top of them to make them weigh 31 grams. Some reproduction floats are one or two grams too heavy or too light. This can change the fuel level 1/4" or more up or down. So when using old or reproduction floats, just measuring the float height does not always work.
The only way to know what your fuel level is, you must use a sight glass. I run the engine and watch the sight glass, and keep watching the sight glass for several hours after the engine is shut off.
Over choking when starting the engine will make gas run out of your carb onto the floor, even carbs that have no leaks.

[daveymc29]

My model A'a revolt at the idea of a fuel filter in the feed line. They all seem to operate better after I put the little pencil filter in the shutoff valve and replaced the screen filter on the sight bowl on the firewall, but out of frustration I got towed home once when I couldn't get gas to the carb in a busy intersection. Once home I completely removed the in line filter, though gas freely flowed through it if I took the bottom connection off of it. Seems strange to me but even when I took the carb connection off not enough gas came through to keep the car running, Brand new filter gave exactly the same result. I just don't use them any more and have very few problems related to gas.

[TJMack]

1930 Coupe, you must be clairvoyant. I was lying in bed this morning before getting up thinking about why adding a washer to the float valve worked this time when I tried it. I was wondering if anyone had ever compared the weight of original floats to replacement floats.


A heavier float would sure explain why the fuel level in the reservoir could be too high allowing it to run out the jets. I was going to take my two floats to the post office to se if they would weigh them for me, but I'm not sure either one of them is an original.


Thank you for posting that info.


Daveymc29, that's very interesting info as well. I have the finger filter in the shut-off valve, and I think I have the screen in the sediment bulb (I can't remember for sure) but I do see some ultra fines in the see-through in-line filter. They're probably small enough to pass through the jets, but the holes in those jets are sure small.


I don't understand why the in-line filter doesn't fill up when the float valve opens allowing fuel to flow. Often when I look it is completely empty with the engine running just fine. I've tried to get it positioned so the fuel has to run up hill to the carb. That does seem to help.

[1930 coupe]

Most original floats will have a Z or a H stamped on the hinge.
Some reproduction floats are lighter than 31 grams because they are made from thinner brass. The picture shows the weight added at the factory to some of the floats. It is unusual for a float to be too heavy, unless it has excess solder on it or has some gas inside it. Make your sight glass then you will not be guessing.
I am not clairvoyant, just 45 years of doing research and working on model A's
A lot of people will tell you that a Zenith carb is no good and you need to get a different kind of carb. This is not true, a Zenith carb runs and idles very good if PROPERLY RESTORED, the problem is very few people have the skills to properly restore a Zenith carb so they get a bad rap.
Also some very old floats can have a dimple wore in it from rubbing on the needle valve, this dimple can cause it to stick sometimes and over fill with gas.

[Tom Endy]

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As in the above post, the weight of the float is an important consideration. The first photo shows an original Holly, and it is right on 31 grams.


the second photo shows a reproduction from Snyder's and it is a gram too heavy. This should not be a problem if you use a glass to view where the float level is set. I have a test stand I use to set float levels on a Zenith. My experience has been that the little sight gauge that screws into the drain boss does not work very well.


The third photo shows the digital scale I use. They are not expensive, I bought this one on-line.


The fourth photo shows a float that will give you grief. Since the crack is at the top the float level can be set, but as you drive the car and fuel splashes around it will slowly fill the float and it will sink and flood the carburetor.


I had this float in a Zenith on a 29 coupe I had. I had been driving the car one afternoon with no problem and parked in in the garage and shut off the fuel valve. The next morning I turned the valve on and tried to start the car and it would not start and I smelled gas and found it was pouring out on the garage floor.


Tom Endy

[Fred K-OR]

Did you check to be sure you had a washer on your main jet and your cap jet? If you do not have them, then the gas will seep out from around them and run out the bottom of the carb. The way to check for leaks on these jets is to take the carb apart and fill the bottom bowl with gas up to the top of the jets and let it stand for a day or so. If it is leaking around the jets, you will see gas in the bottom of the carb. I had a carb where one of the jets threads were stripped and it leaked gas. Another theory!

[TJMack]

Tom Endy, I saw your glass test stand in that excellent article you did on rebuilding the Zenith carb. I'm going to give the easy clear tubing rig a try to see what the fuel level looks like in the actual carb. I'll also give the glass jar test a try.


Thank you for posting that article. I intend to go through the carb fully this summer when the car sits idle while I'm in New York State. With that in hand I feel confident I can do a good job.


Fred K -Or, yes, I'm sure of the gaskets under the jets. One of the first things I did was put in new gaskets and check for leaks like you described. That is great advise for anyone having similar problems.


I sure appreciate all the input I've gotten on this subject! It has been frustrating, but intriguing at the same time.

[Tom Endy]

Originally Posted by TJMack

Tom Endy, I saw your glass test stand in that excellent article you did on rebuilding the Zenith carb. I'm going to give the easy clear tubing rig a try to see what the fuel level looks like in the actual carb. I'll also give the glass jar test a try.


Thank you for posting that article. I intend to go through the carb fully this summer when the car sits idle while I'm in New York State. With that in hand I feel confident I can do a good job.


Fred K -Or, yes, I'm sure of the gaskets under the jets. One of the first things I did was put in new gaskets and check for leaks like you described. That is great advise for anyone having similar problems.


I sure appreciate all the input I've gotten on this subject! It has been frustrating, but intriguing at the same time.

My experience with that little sight gauge tube the vendors sell was that no matter how many gaskets I added or subtracted the fuel level in the tube stayed the same. That was why I built the stand.


If you don't want to go to the trouble to build a stand, an idea is to find a glass jar that can be attached with a large rubber band while on the car in place of the lower casting. I think it would be a more accurate reading.


Tom Endy

[1930 coupe]

I have never used the sight gauge that the vendors sell, the home made one that I show above works very good. If you add gaskets it will show a different fuel level, when you start the engine it will go down a very small amount then quickly go back up. When the engine is reved up the fuel level goes down a small mount, and when it returns to idle it comes back up. When the engine is turned off the fuel level goes up slightly. It seems that the fuel level is about 1/16" lower with the engine running than when the engine is off.
I do not know why the ones sold by the vendors do not work properly, mine might have a larger diameter tube and is not affected by capillary action like a small tube, just guessing.

[Benson]

Originally Posted by 1930 coupe

I have never used the sight gauge that the vendors sell, the home made one that I show above works very good. If you add gaskets it will show a different fuel level, when you start the engine it will go down a very small amount then quickly go back up. When the engine is reved up the fuel level goes down a small mount, and when it returns to idle it comes back up. When the engine is turned off the fuel level goes up slightly. It seems that the fuel level is about 1/16" lower with the engine running than when the engine is off.
I do not know why the ones sold by the vendors do not work properly, mine might have a larger diameter tube and is not affected by capillary action like a small tube, just guessing.

I found the same problem with the capillary action of the smaller diameter tubes in the most common gauges being sold.

My solution was to make one with a tube that is about 3-4 times larger.

The last I heard was that Dave Renner was selling the larger diameter models on his website called Renners Corner.

I will have to dig out that gauge that I made ... if I can find it. I lost track of it along time ago and I have been using one of Dave's gauges for a number of years.