Early Model A Holley carb info

[Tony Hillyard]

Good afternoon everyone,

I have an early 1928 Model A standard roadster with a holley made carb. It runs like a pig. I changed the carb for a Zenith carb from my 1928 Model A Tudor Sedan and she runs beautifully. I took the holley carb apart but can find no jets numbers stamped anywhere. I suspect that originally the holley carb may have been a twin venturi model that has been modified (fumbled with).

Has anyone any drawings or technical information on my holley carb please. Even better, where I might buy a set of new correct jets and perhaps the original twin venturi set-up.

As always, many thanks in advance.

P.S. Is it going to be cheaper to buy a second hand standard Zenith carb and rebuild that?

[Tom Wesenberg]

Can you post pictures of your carb? Good shots of the throat area should let us know if it's a double venturi carb.

[Tony Hillyard]

Hello Tom,

Thanks for your quick reply. Please find attached (if I can get it to work) three pictures: the base of the carb, the venturi fitted with badly made slot on the side and the underside of the top.

Thanks

Tony

[Benson]

The idle jet and cap jet are swapped around. I see this a lot ... do not feel too bad!

The lower casting is in fact a double venturi as there are no threads to install the brass secondary well.

The main jet has the "domed" end which will dribble gas when the engine is not running but this is a minor problem.

If you swap the cap jet and idle jet to their correct positions it will work better provided that all of the ports in the castings are clear and not plugged. Note avoid drilling out plugs at all costs ... too many castings are ruined while drilling.

Although the venturi is botched up ... the good thing is that they did not grind off the pin in the lower casting as such all you have to do is replace the venturi with the correct one!

[Tony Hillyard]

On the face of it i would agree. That is how the jets were fitted. I am more than a little baffled as when I compare my Zenith carb I get the impression that the Holley carb has all the wrong jets fitted - perhaps Zenith jets. Hence my request for any technical information, jet sizes or if I'm lucky, photographs of the inside of a Holley.

No wonder she ran like a pig!

[Tony Hillyard]

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One other thought, reading Snyder's catalogue it mentions that the early twin venturi carbs had no secondary well fitted. Mine has no secondary well or thread for one.

But... my carb has been fitted with a single venturi.

[Tom Wesenberg]

A&L sells the double venturi, and I bought one for my carb. I haven't assembled mine yet to try it, but the double venturi is said to work very well.

[Tom Endy]

The carburetor is definitely an early one. It does not have a screw-in secondary well and has the notch for a double venture. In the photo the idle jet and the cap jet have been swapped. The carburetor also has a brass GAV seat in the lower casting in the GAV tunnel. Quite often the comp jet (the little button jet in the reservoir) is swapped with the GAV seat because they look similar. The main jet is a poor reproduction with the end bullet shaped instead of concave.

Bratton's sell the double venture. They also sell the correct jets.

Tom Endy

[Tony Hillyard]

Thank you all for all your advice. I am starting to get the picture.

As I understand it, there were several manufacturers of the same carb and the jets etc should all be interchangeable. With the exception of the early twin venturi carb's venturi. I have a stock of various Zenith type jets which I will try and as suggested I will buy a twin venturi part and fit as original.

All your help has been greatly appreciated, thank you.

[Wagga A'er]

Thanks guys. Your answers have been a great help to me also.

[Richard Lorenz]

The Holley double venturi main jet had a rounded end. It had what I call a torpedo shape to it. The rounded end had a taper leading up to it, not like the repro mains that were rounded only at the ends. These original main jets are very rare, partly because the repros got so much bad publicity that the real torpedo-shaped mains were thrown away.

I do not post pictures on Ford Barn, but I can email a picture that shows the difference to anyone who wants to post it on Ford Barn. My email is [email protected] .

[johnbuckley]

Since we've touched the topic of double venturi... Has any one found running the correct double vs an incorrect single makes any difference? I ran my '28 carb which should have had a double venturi with a single venturi for years. When double venturis became available again I put in a double venturi in - it made no difference.

[Richard Lorenz]

I made some air flow resistance measurements a few years ago. The Holley double venturi carburetor had less flow resistance than a Zenith 1 or 2. The single venturi sticks down further in the carburetor throat, and I think that is what creates more air flow resistance. You would notice the difference only at near-full-throttle operating conditions. The difference might be only a couple of miles per hour.

[Tony Hillyard]

Good Afternoon Everyone,

An up-date for anyone that has not had a look inside a double venturi Holley carb. As mentioned above, I ordered a replacement double venturi for my '28 Roadster Holley carb from Snyder's. It arrived to-day. I thought it might be of interest to to upload a couple off photographs for reference.

Thanks again for all you help and advice.

Tony

[PEASE]

What are the correct jet sizes for a twin Venturi 28 carb?

[Richard Lorenz]

The main jet shown in Toni's pictures has the rounded tip. This is correct for the double-venturi Holleys. It is not one of the repro jets that has a similar rounded tip. The Holley double-venturi carburetor original main jets have more of a "torpedo" shape to them. Sorry that I am not yet able to post pictures.

[Ron in Quincy]

Because there is no removable secondary well the comp. jet should be a #18 (.o36). The original idle jet was a #10 and slightly longer than the later #11, however the #11 (.021) will work just fine. The original cap jet had the orface at the bottom, a later cap jet, with orface at top, will work just fine; I recommend the cap jet opening works better if the opening is .038 or .039; just my opinion.

If I can be of any help just send me a Private message.

Good luck,

Ron

[Tony Hillyard]

I'm sorry PEASE, I double checked and there are no numbers on the jets on my double venturi carb. Thankfully Ron and Richard were able to give us all the right information.

Good luck PEASE with your jets.