Air Balanced Zenith

John

John

Senior Member
Does anyone know what modifications are made to the Zenith carburetors for air balancing? Several suppliers are now offering this modification to the original Zeniths. The result of these modifications reduces the enrichening of the fuel/air mixture due to installing an air filter on the Model A. I understand the principles, but do not know what actual modifications are made to the venturi and lower half of the body.

Thanks,

John
 
No modifications are needed for the venturi. On the Zenith carburetor there is a vent hole about midway the lower housing on the engine side that vents the bowl part of the carb to the outside atmosphere. A tube can be connected to this vent, a hole needs to be drilled in the choke end of the lower housing , just past the choke butterfly and connect the tube from the vent to the drilled hole. This balances the air in the bowl with the air coming through the choke end like newer type carbs. Have you ever wondered what that little tube that sticks out past the choke on later model carbs was for? If you have, its a balance tube. The balance tube helps to some extent if an aftermarket air filter is used. Balanced or not, any air filter, even a mesh air maze filter will still to some extent restrict air flow and cause the engine to run richer to SOME extent. Filters make some feel better but sure as hell doesn't cause the engine to run ANY better. After driving and fixing model A's for most of my life I know that air filters are not needed on the model A.
 
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Hi John,

An article was written in the Restorer some time ago with photos where the author drilled a small hole in the top/driver's side if his Zenith; and drilled another small hole in the driver's side of the round rear throat of his Zenith adjacent to the moving choke baffle.

He next tapped both holes & threaded both holes, & screwed in two (2) 90 degree brass fittings made for small rubber tube connections.

He then ran a small rubber tube between the outer ends of both fittings in order to maintain the same atmospheric pressure between the Zenith carburetor bowl up front & the rear throat of the carburetor after his small air filter was installed.

Before this tube connection, after his small air filter was installed, he reported that his fuel mixture became too rich & caused carbon deposits on the plugs & inside the combustion chamber which became progressively worse after his filter collected dirt; however, after balancing inside air with this tube, he claimed the carbon problem & running too rich problem was resolved.

Another most interesting area to look for many articles on Model A carburetors is www.Model-A.org , (where the author does acknowledge the long debated controversy surrounding Model A air filters), & reports that this venting is not required when one uses a larger air filter which does not restrict air flow.

Then to acquire even more scientific carburetor lab data on carburetor air restriction, & air flow K&N filters supplies very interesting reading.

Carburetor filter air passage engineering is no different than air passage engineering for filters on a simple residential central air conditioning system. If the air filter is too small, or if the filter is allowed to reduce air flow because of dirt, or with the incorrect type of filter, the system will not perform with efficiency.

Under "Search", under the above web site, & after reading K&N reports, one can find enough discussions & articles to read all day & into the night.

Filtering Model A carburetor air depends on one's individual choices; however, in my opinion, the best part about the "Model A Filtering Air Religion" is that no matter which Religion you choose, you will not be tied to a post & burned at the stake like Joan of Arc.

Hope this helps to decide which way to go.
 
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Bill Goddard said:
Can't the mixture be adjusted with the GAV control? Or maybe change to leaner jets ?
Click to expand...

Only to the point where your GAV is all the way closed. Since a normally set up Zenith runs with the GAV closed when hot, you have no room to lean out any further.

You could change to a leaner main jet, but there is yet another problem. The induced negative pressure from a filter is variable with engine speed and load. You would need to constantly ride the GAV between cruising and accelerating.

Making the atmospheric pressure in the fuel bowl equal to the slightly reduced pressure within the filter addresses the above.
 
I run an original Zenith. Nice tight engine with plenty of compression. I live and drive on a lot of dusty gravel roads. I use an original AirMaize air filter with a fiberglass element wrapped around the AirMaze. I run my GAV at 1/2 when starting, 1/4 for regular driving. 1/8 or nearly closed for highway driving. If I close it completely it is too lean. There are definite differences in different engines and carbs. My plugs stay perfectly colored and have never fouled one yet. I can see absolutely no advantage to running without the filter, can tell absolutely no performance difference except for more intake noise without the filter. I know this is a bit off subject from the original post and I apologize but my point is: What works for one, isn't necessarily the "rule" for all. The equalized carbs talked about sounds like a good idea, but in my case I don't feel its necessary. One other point, I bought my A with the carb as it is now, I have no idea if there have been modifications made or??? It seems there must be something different because it does not seem to be the norm. Someday I'm going to pull it apart to see why.
 
Bill Goddard said:
Can't the mixture be adjusted with the GAV control? Or maybe change to leaner jets ?
Click to expand...

What is affected is the fuel curve and the fuel timing (when the fuel is available at the booster). All those little holes and bleeds etc are sized according to a certain amount of air pressure, change that pressure and both the quantity of fuel and when that fuel is available is affected.
Just like everything automotive, it isn't as simple as it should be.
 
Here's a picture of a B carb modified as HL Chauvin described. The normal bowl vent, described by Purdy, had to be drilled, tapped, and plugged. (Still didn't run well.)

 
I made the same modification on my Zenith 1 carb. I used 3/16 copper tubing instead of fuel line. The engine has never run better. I am using a high-boy air cleaner.
 
Hi Steve

Picture in reply no.8 is what the Restorer article was about -- thanks.

FWIW:

1. I presently have a B carburetor on my Model A engine without the tube balancing device indicated above; & as far as I'm concerned, if the air filter is large enough to prevent air constrictions, this tube is not needed; &,

2. I have installed about a + 7 inch tall or so new K&N air filter (forgot K&N Model No.), &,

3. Said filter is installed with the "Hi-Boy" filter set up offered by some Model A vendors or on E-bay, whereby the filter sits "above" the carburetor & blends nicely into the adjacent firewall as a backdrop & adjacent to the fuel bowl as opposed to sitting out in open air like that of the Model A distributor; &,

4. Consequently no gas can drain upwards into this hi-tech K&N filter & get it wet with fuel thus causing dangerous fuel fumes to be exposed to the hot exhaust system and/or a possible engine backfire.

I see absolutely no difference in starting, running, plug fouling, and/or pick up with or without this large K&N filter on my Model A or B carburetors -- it lets in lots of air & it is also really oversized to compensate for not having to clean it as often.

With or without this large K&N filter, I start up cold with my B carburetor with GAV 3/4 open, & after a few minutes, close GAV all the way & never open it again until the next engine cold start.

After I installed this air filter arrangement I noticed that nobody came by with a bundler of rope, a basket full of kindling & matches to burn me at the stake .............. so I figured since it worked so well I decided to leave it on.

After just buying my wife a new car, both she & I did not want to throw away her air filter either because we both think air filters can work very well to extend engine life on Model A's & expensive new cars -- even inside a clean house, A/C filters get dirty -- one can imagine what would happen if one placed his A/C filter outside the house near the ground or near the street or highway.

Opinions are always interestingly different.
 
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There"s an article in Secrets of Speed mag vol18 # 4 April 2009 by Dave Renner about pressure balancing a B carb with no external pipes / hoses.
 
Thanks 34Fordor, that was what I was looking for. Does anyone have the article and can scan it to send to me?


Thanks,

John
 
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