A Carburetor Story

Penthode

Penthode

Member
I had been frustrated for sometime with stalling of my '29 Tudor when pulling up to a stop. I have performed all the standard tests and remedies including checking ignition , fuel flow and removed and examined the carburetor to no avail.

I finally bit the bullet and decided to carefully and methodically go over the Zenith carburetor. First step was examining all the passages. I used the paperclip trick forced down the passages after disassembly. The upper casting idle path was clogged. I poked and rotated the paperclip through each passage. The main, cap and compensator jets were too large and so I soldered them up and used the micrometer to select the appropriate size drill to match the original Ford dimensions.

The long and short of it is I went for a drive tonight and the performance has been transformed. I can pull up to a sharp stop when cold or warm with out a stutter of a problem. Lots of power and she idles smoothly at very low revs.

Conclusion? Patience and perseverance. And don't give up on the Zenith!
 
Working through the same problem and finding the stall-at-stop issue is very common. Would welcome any additional details you can provide on the assessment and measurement of jet size. Good post !!
 
Mulletwagon said:
Working through the same problem and finding the stall-at-stop issue is very common. Would welcome any additional details you can provide on the assessment and measurement of jet size. Good post !!
Click to expand...

I think the main problem causing the stop stall is the idle circuit in the Zenith.A good understanding of the operation of the Zenith is important to diagnosis and this site helped me:

http://modelabasics.com/carb basics 2.htm

A clue that the idle circuit is not working is when the idle screw has little effect on the idle. I found it should optimize the idle when 1/4 to about 3/4 turn open. After cleaning out the top casting passages it now works as it should.

To address the stalling, I would closely examine the little passageway from just above the throttle flap which is drilled diagonally to the idle screw as it was fully clogged. Disassemble the carburetor and remove the idle adjust screw. Then poke a small paper clip wire down the diagonal hole until you reach the end brass plug. No need to remove the plug. Try forcing air through the passage. To ensure it was clear, I put my finger over the idle jet hole (with the idle jet removed) to block it and covered the large intake port at the venturi end with my hand. Place your mouth over the manifold flange end and blow! Air should flow out the idle air input depicted.Then try adjusting to the seat the idle screw and blow and virtually no air should pass so long as the throttle flap bushings are not worn!

Don't forget to check the idle jet is clear and the hole is of exactly the right dimensions.

This little job transformed my car from a staller on just regular stops to now being able to jam on the brakes without touching the throttle lever! She'll pull up purring like a kitten!
 
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I wouldn't recommend restoring the idling jet by just using a drill that is of the correct dimensions. All jets should be flow tested per Steve Pargeter's Zenith Carb Restoration Guidelines. The idling jet is the smallest and the hardest to get to the recommended flow rate of 44-48 ml/min. When I have to change the flow rate of a jet, I use the micro sized drills like a round file. I insert the drill in the orifice and then draw it back out. Just one cycle is enough to change the flow rate 2-4 ml/min. If you simply just drill out the jet you have no idea what the flow rate will end up being. The flow rate is greatly influenced by the kind of corner edge you leave inside the tip of the jet after you have used the drill.


The idling adjustment screw is an air adjustment into the idling circuit. From experience I like to have the idling jet passing 44 ml/min because at this low end of the range, the adjusting screw is only opened about 1 -2 turns at most. If nothing else, you don't risk having the screw fall out and there is plenty of spring tension on the screw. When you are establishing the flow rate of the idling jet, you are also setting the optimum adjustment setting of the idling screw. Good luck, Ed
 
Penthode said:
I think the main problem causing the stop stall is the idle circuit in the Zenith.A good understanding of the operation of the Zenith is important to diagnosis and this site helped me:

http://modelabasics.com/carb basics 2.htm

A clue that the idle circuit is not working is when the idle screw has little effect on the idle. I found it should optimize the idle when 1/4 to about 3/4 turn open. After cleaning out the top casting passages it now works as it should.

To address the stalling, I would closely examine the little passageway from just above the throttle flap which is drilled diagonally to the idle screw as it was fully clogged. Disassemble the carburetor and remove the idle adjust screw. Then poke a small paper clip wire down the diagonal hole until you reach the end brass plug. No need to remove the plug. Try forcing air through the passage. To ensure it was clear, I put my finger over the idle jet hole (with the idle jet removed) to block it and covered the large intake port at the venturi end with my hand. Place your mouth over the manifold flange end and blow! Air should flow out the idle air input depicted.Then try adjusting to the seat the idle screw and blow and virtually no air should pass so long as the throttle flap bushings are not worn!

Don't forget to check the idle jet is clear and the hole is of exactly the right dimensions.

This little job transformed my car from a staller on just regular stops to now being able to jam on the brakes without touching the throttle lever! She'll pull up purring like a kitten!
Click to expand...

THanks for your info but I couldn't open that link. I got Error 404 (Page not Found)!
 
Thanks for the tips. I have a Zenith that will not idle well and often stalls out from a stop. I have checked over the parts but have not check all of the passages.

By the way, a Tittotson carb on my car works very well (after taking care of the warp).

Marc
 
I want to remind you from my earlier post, I did not redrill the idle jet. But the key was making sure the passages in the upper casting were clear. Upon earlier overhauling my carburetor, I failed to clear the blockage for the idle jet feed.

Yes I know a proper flow test is recommended. But the stalling problem many of us have faced is due to an overall blockage of the idle circuit. Fine tuning the jet may be the desirable next step once it flows! The performance of my car was transformed once the stalling stopped after clearing this vital passage in the casting.

Here is the link again. The URL has spaces in it and suggest copying and pasting the entire link up to and ending with the .htm in your browser.

http://modelabasics.com/carb basics 2.htm
 
I fixed a lot of “die out on stop” by just flattening out both flanges and a new gasket. That idle mixture can’t take any extra air. That should be your first move and it is easy to do. All you need is a big file and a mirror.
 
But as I said, I had checked the idle jet but not the passage way in the upper casting. Not until I forced the wire from a paper clip through the passage and made sure it was fully clear did I get a good idle.

A thought I had was unlike the other secret passageways which always have fuel in them, this one is carrying only the fuel air mix and is therefore subject to more corrosion hence blockage. I therefore suggest that if you have any stall upon pulling up to a stop problem, you should check this passage way.
 
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