Whatsup with carb setting?

[Ray64]

My car runs pretty good. When I run the air setting screw all the way in it makes little difference. Its a ZENITH 1 So can someone explain whats the problem and a fix. Looked in the archive but no specific answer there. Thanks RAY

[John Butts in CT]

Is the idle adjustment screw original or reproduction? How about the spring?

There are different length repro idle adjustment screws and springs. Combine a too short screw and a too long spring and the spring will never touch the seat.

First try the screw without the spring.. If that works, great... shorten the spring to match. (The spring merely discourages the screw from turning on its own.)

If that doesn't work, look for a longer screw and repeat the process.

[Duffy1]

If post #2 doesn`t solve the problem ,then look for a vacuum leak.

[Purdy Swoft]

All true but..The air adjust screw only effects operation when the Zenith carburetor is runing on the idle circuit. Turning the screw clockwise as in closing richens the mix. As Duffy1 says a vacume leak would cause this condition. If its running lean, closing the air adjust screw would richen and try to compensate for the lean condition.

[Mike V. Florida]

Originally Posted by Duffy1

If post #2 does't solve the problem ,then look for a vacuum leak.

The adjustment regulates the air into the venturi. Too much air and the car will not run correctly too little (all the way in) and it will not run right.

If you close off the air and it still runs than the adjustment on the throttle plate may not be correct and/or there is a air leak. Check the clearance on the throttle shaft.

[johnbuckley]

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and see the thread last week on 6.24.14 started by "fredski".
The Ford Barn > General Discussion > Model A (1928-31)
idle air mixture adjustment please advise

[JOES31]

I have the same condition as Ray. I have confirmed it is not a vacuum leak. Used different needles and made no difference. If I turn it all the way down it will slow the idle but not stall the engine. When I open screw it makes little difference. When I use the adjustment screw on the tilitson it makes a huge difference. I think it depends on the carburetor. There isn't a brass seat for the needle to close against, (can't understand why that was made that way), so it cannot be renewed. The cast iron seat might be pitted and not getting a good enough seal. That's the only reason I can find for the way mine run. Anyways I just turned it out 1-1/2 turns and left it.

[Tom Endy]

In order for the air\idle screw to be affective you have to turn the idle speed control down to where the engine is just ticking over. This closes the throttle butterfly down to where the small hole in the throat takes over the idle.

Tom Endy

[Mike V. Florida]

Originally Posted by JOES31

I have the same condition as Ray. I have confirmed it is not a vacuum leak. Used different needles and made no difference. If I turn it all the way down it will slow the idle but not stall the engine. When I open screw it makes little difference. When I use the adjustment screw on the tilitson it makes a huge difference. I think it depends on the carburetor. There isn't a brass seat for the needle to close against, (can't understand why that was made that way), so it cannot be renewed. The cast iron seat might be pitted and not getting a good enough seal. That's the only reason I can find for the way mine run. Anyways I just turned it out 1-1/2 turns and left it.

How big is the hole in the carb throat? How well does the butterfly valve seal in the throat?

[KMeredith87]

Not sure if this makes any difference...had a similar problem after a carb rebuild with all new repro parts. Could not get the idle to adjust for the life of me and no vacuum leaks could be found. In the end I found that that the repro secondary well was complete garbage, put an original back and after a thorough cleaning and had all the adjustment in the world...just a thought.

[Tom Endy]

Originally Posted by KMeredith87

Not sure if this makes any difference...had a similar problem after a carb rebuild with all new repro parts. Could not get the idle to adjust for the life of me and no vacuum leaks could be found. In the end I found that that the repro secondary well was complete garbage, put an original back and after a thorough cleaning and had all the adjustment in the world...just a thought.

Original brass secondary wells came in several different machined shapes depending on the casting they were to be used with. Reproductions, as far as I have seen are all of one configuration.

Zenith castings have a hole drilled from inside the tunnel where the GAV assembly installs. The hole is a passage to the secondary circuit. Depending on the casting, the hole can go to the secondary well, or direct to the cap jet. Both are part of the secondary circuit. You can determine how the hole is drilled by running a paper clip bent at an appropriate angle into the hole in the GAV tunnel and see where it exits.

If the hole is drilled to the secondary well it can arrive there at different angles. The machine grooves in the brass secondary well should have reliefs by the hole to allow the fuel in. The same applies to the comp jet, there should be a machined groove next to the comp jet port to allow the fuel in.

It is possible to re-machine the brass secondary well.

Tom Endy

[JOES31]

The hole in the throat is very small. The butterfly closes fine. If I turn the idle speed screw out it will stall. I did adjust it with the engine just ticking over. There is a change when I adjust the idle mixture but not much to speak of. If it can only be adjusted when the engine is just ticking over then it is basically a mute point to even adjust it. If the engine has to be at that low an rpm then why bother because you're not going to run it at that low an idle when you're driving it.

So what the heck is that adjustment good for?

[Ray64]

Update on the carb. Thanks to all for info. Spring under the needle was to long and would not let the needle seat far enough. All is well with old shorter spring. I love this site. RAY

[Mike V. Florida]

Originally Posted by ray64

update on the carb. Thanks to all for info. Spring under the needle was to long and would not let the needle seat far enough. All is well with old shorter spring. I love this site. Ray

great news!!!

[Sparky]

Glad you fixed it. For the benefit of others I'd like to relate a similar problem I just fixed. I was troubleshooting a carb which had identical symptoms of bad idle with the air screw having no apparent affect. It turned out that the lower end of the screw threads in the casting were rusted and not letting the needle seat tightly even though the screw seemed to be snug, so the mixture was always too lean. After running a jet tap (available from parts vendors) through the threads the screw now goes all the way and the needle valve works correctly.

[swede]

Tom, I read your report concerning cleaning out the passageways. The #2 is blocked and I have worked for hours using every kind of wire to try to get to the cap jet. I can only get about 1/16th of an inch into it. I cannot see the passageway hole in the cap jet well. I have tried to use high air pressure as well. Is there any other way to open the passageway?
Thanks for you helpful tips.
Swede

[Mitch//pa]

Originally Posted by swede

Tom, I read your report concerning cleaning out the passageways. The #2 is blocked and I have worked for hours using every kind of wire to try to get to the cap jet. I can only get about 1/16th of an inch into it. I cannot see the passageway hole in the cap jet well. I have tried to use high air pressure as well. Is there any other way to open the passageway?
Thanks for you helpful tips.
Swede

here is a little tutorial on the passageways and an excellent all around carb web site from berts..
http://www.modela.org/lower_passages.html

[jmeckel]

I hand clean the passageways with a gas welding tip cleaner wire of the right size. I clean every passageway I can get a wire through. I then glass bead blast and follow this with a soak in chemical carb "boiling" tank, then blow it all out and run the tip cleaning wires through all of the passageways. I have rebuilt a lot of carbs and never had one yet that I could not get all passageways open.