Z mod for air filter

[James G.]

I've seen the venturi and lower casting modified so to use an air filter. I'd appreciate info on how this is done/works.

[Synchro909]

I think you are asking about pressure balancing the carby. I'm sure there will already be a thread on here about it. I've done it on a Zenith 3 and a 1. It's not hard to work out what to do once you realise that what you are doing is closing the float bowl off from the atmosphere and venting it instead to the venturi after the throttle butterfly. The result is the float bowl is kept at the smae pressure as the venturi, not the atmosphere. The venturi pressure is lower than the atmosphere so if the bowl is at atmospheric pressure, the higher pressure will force more fuel through the jets, making it run rich. Once the pressures are balanced, that pressure imbalance does not exist so the mixture remains unaltered by the restriction of the air cleaner.
Someone here will have pictures, I'm sure.

[BILL WILLIAMSON]

I thought they ran a tube from the float chamber vent to the carb, adjacent to the choke butterfly. I saw a pic of one that way.
Bill W.

[Synchro909]

Originally Posted by BILL WILLIAMSON

I thought they ran a tube from the float chamber vent to the carb, adjacent to the choke butterfly. I saw a pic of one that way.
Bill W.

Pressure equalisation can be done the way I described entirely within the carbyand is invisible from outside. Originality is not compromised unless you take the carby apart. For a driver car, there is no reason why an external tube wouldn't work just fine.

[BILL WILLIAMSON]

If you use a quite LARGE air filter, BALANCING is NOT NECESSARY!
Bill Balanced?

[Synchro909]

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Originally Posted by BILL WILLIAMSON

If you use a quite LARGE air filter, BALANCING is NOT NECESSARY!
Bill Balanced?

You're right on that - not enough restriction to make balancing necessary.

[Tom Endy]

Attached photos show a modified Zenith that I believe is the subject of this post. Note the slot milled into the casting above the secondary well. There is also a hole drilled into the forward part of the casting through to the inlet throat of the carburetor. The venture has had a channel machined around it to let the air from the hole pass around the venture. Also the original two vent holes have been plugged off with a threaded brass plug.


This new path of air does not enter into the inside of the venture. The only thing I can see that this new adventure provides is the reservoir air pressure would be the same as the air pressure behind any kind of filter that would be installed.


This is the first I've seen of this modification and I do not understand the theory.




Tom Endy

[mhsprecher]

Renner's Corner will make the mod. There was a presentation on this at the MARC meet in Gettysburg. I have been thinking about having a carb done to see how it works? A good test for Model A Times.

[Bikerider58]

Since I run with a filter, I had it done, Renner’s Corner.

Worked great on fixing the rich mix. I then had the spare carb done.

[Synchro909]

Originally Posted by Tom Endy

Attached photos show a modified Zenith that I believe is the subject of this post. Note the slot milled into the casting above the secondary well. There is also a hole drilled into the forward part of the casting through to the inlet throat of the carburetor. The venture has had a channel machined around it to let the air from the hole pass around the venture. Also the original two vent holes have been plugged off with a threaded brass plug.


This new path of air does not enter into the inside of the venture. The only thing I can see that this new adventure provides is the reservoir air pressure would be the same as the air pressure behind any kind of filter that would be installed.


This is the first I've seen of this modification and I do not understand the theory.




Tom Endy

Tom, That is the mod I was referring to. As I said, I have done it on a Zenith 1 and 3. It moves the source of air to vent the float bowl from the atmosphere to the throat of the carby which is at lower pressure because of the air cleaner. That means that the air pressure in the bowl is not higher than where the fuel is delivered and so fuel is not pushed out through the jets at too higher rate. As Tom obcerved, the larger and less restricted the air cleaner, the less important this mod becomes.

[Tom Endy]

What happens if you run a carburetor that has been modified like this without an air cleaner?


Tom Endy

[Flathead]

Nothing

[Synchro909]

What Flathead said. That rhymes!

[Y-Blockhead]

[QUOTE=Synchro909;1666330]I think you are asking about pressure balancing the carby. I'm sure there will already be a thread on here about it. I've done it on a Zenith 3 and a 1. It's not hard to work out what to do once you realise that what you are doing is closing the float bowl off from the atmosphere and venting it instead to the venturi after the throttle butterfly.

Originally Posted by Synchro909

Tom, That is the mod I was referring to. As I said, I have done it on a Zenith 1 and 3. It moves the source of air to vent the float bowl from the atmosphere to the throat of the carby which is at lower pressure because of the air cleaner. That means that the air pressure in the bowl is not higher than where the fuel is delivered and so fuel is not pushed out through the jets at too higher rate. As Tom obcerved, the larger and less restricted the air cleaner, the less important this mod becomes.

I'm confused. Seems your saying two different things. In Tom's photos it shows the float bowl is vented to the carb throat before the venturi and throttle butterfly, not after the throttle butterfly.

[Y-Blockhead]

Originally Posted by Tom Endy

What happens if you run a carburetor that has been modified like this without an air cleaner?

Tom Endy

Newer carbs that are designed to have an air cleaner usually have the float bowl vents positioned so the vent is inside the air cleaner so there is not a pressure drop. The Zenith is vented to the outside.

I see Snyder's sells a NEW Zenith that is Air Balanced.

[Synchro909]

[QUOTE=Y-Blockhead;1666806]

Originally Posted by Synchro909

I think you are asking about pressure balancing the carby. I'm sure there will already be a thread on here about it. I've done it on a Zenith 3 and a 1. It's not hard to work out what to do once you realise that what you are doing is closing the float bowl off from the atmosphere and venting it instead to the venturi after the throttle butterfly.



I'm confused. Seems your saying two different things. In Tom's photos it shows the float bowl is vented to the carb throat before the venturi and throttle butterfly, not after the throttle butterfly.

Y,
I did say two different things. Memory is a strange and changing thing. In ythe first one, I should have said "after the choke butterfly". Sorry for any confusion.

[Y-Blockhead]

Gothcha Synchro, Thanks.

[Benson]

If you do this make sure that brass plug in photo #2 in post 5 also plugs the outside end of 1/8 inch hole below brass plug.

Only later carbs have this hole.

[James G.]

I'm a bit confused. The venting is to be from the bowl to the chamber between the choke and throttle butterflys . I see the machined grove in the lower casting and the grove around the venturi. How does this path continue to reach the pre throttle area?
Jim G.

[Tom Endy]

I believe the idea is to balance the atmosphere in the fuel reservoir with the air entering the carburetor input that has the air cleaner attached. The slot milled in the casting, the groove in the venturi, and the hole drilled in the area where the venturi sits is a direct path from the reservoir to the area behind the air cleaner. It has nothing to do with the throttle butterfly or the choke.


Tom Endy