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Old 09-13-2013, 07:39 AM   #1
my2nd40
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Default Downdraft Carb suggestions

I want to put a downdraft carburator on my stock model a engine. Which one should I look for? 81,97 or 94, and what is the best way to set it up?
thanks
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Old 09-13-2013, 07:56 AM   #2
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Default Re: Downdraft Carb suggestions

a weber ! best by far .......
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Old 09-13-2013, 08:01 AM   #3
BRENT in 10-uh-C
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Default Re: Downdraft Carb suggestions

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I want to put a downdraft carburator on my stock model a engine. Which one should I look for? 81,97 or 94, and what is the best way to set it up?
thanks
While the 81 will be closest to cubic inch size of the Model-A engine, most folks often find they are still too large for a stock engine. More cam and larger valves allow for the 81 to be more useful, and more compression allows the additional fuel to become more useful. My personal experience for a stock engine, all of those downdrafts are a tad too much. If you are adamant you want a downdraft, there were some Bendix manifolds made to adapt the Rochester single BBL carb found on engines such as the 216 ci Stovebolt engine from the 30's.

Just out of curiosity, have you considered installing a Model-B carburetor & manifold for a little more airflow? With those, there is not a need for plumbing in a fuel pump when used on a Model-A.

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Old 09-13-2013, 08:37 AM   #4
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Default Re: Downdraft Carb suggestions

Is there a quick way to ID the B manifold? Headed to a swap meet today.
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Old 09-13-2013, 08:48 AM   #5
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Default Re: Downdraft Carb suggestions

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Is there a quick way to ID the B manifold? Headed to a swap meet today.
its flat on top compaired to a model a which is rounded
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Old 09-13-2013, 08:53 AM   #6
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Default Re: Downdraft Carb suggestions

I use a 97 just because its the best carb I had around. It does run a little rich but its not too bad. Still gets about 20 miles per gallon.
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Old 09-13-2013, 09:35 AM   #7
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Default Re: Downdraft Carb suggestions

I have found the opposite of what Brent says. The 81 is adequate, and better than the stock carb, but the 97 is better, and cheaper. They usually need to be totally stock but sometimes may need a slight main jet change. I've tried them all.
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Old 09-13-2013, 10:12 AM   #8
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Default Re: Downdraft Carb suggestions

I have a Weber on mine, and it works great. Can't compare it to any other DD, just haven't used them.
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Old 09-13-2013, 10:15 AM   #9
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Default Re: Downdraft Carb suggestions

Stromberg 48,81,97 all run good on an A block. Of course you have to jet it correctly, but that's not hard to do. On a flat head you want the mixture to be a little rich, rather than lean. Lean will burn up a motor. People often refer to the 94 as a stromberg, but it's actually a Holley. It's he cheapest one out of all of them, but they tend to run leaner than a stromberg. I have a 48 and I love it, I had an 81 once and traded it for a weber and boy was that a mistake. The webers are fine, but the strombergs give you faster acceleration in my opinion and run better down the highway. The webers do get better gas mileage though. But not by much. I've tried most of the popular downdraft conversions, and the stromberg has been my favorite so far. You can usually pick up a 48 or a 97 in decent condition at a swap meet for about 75-100 dollars that needs rebuilding. For an 81 you can expect to give 250-300 for one that needs rebuilding, or closer to 400-500 for a rebuilt, if you can even find an 81. They reproduce the intake manifolds, and I think they're right around $200 for the manifold.
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Old 09-13-2013, 10:16 AM   #10
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Default Re: Downdraft Carb suggestions

I run dual updraft with B carbs. Its easy, bolt them on and thats it. I use original numbered jets. Gravity flow works fine, no pump or pressure regulator needed. This setup will run good, even on a completely stock engine. For best results with any mods on the model A, a higher compression head should first be added to really feel the improvement.
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Old 09-13-2013, 11:05 AM   #11
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Default Re: Downdraft Carb suggestions

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I run dual updraft with B carbs. Its easy, bolt them on and thats it. I use original numbered jets. Gravity flow works fine, no pump or pressure regulator needed. This setup will run good, even on a completely stock engine. For best results with any mods on the model A, a higher compression head should first be added to really feel the improvement.
Purdy can you post some pics of that setup? It sound WAY cool.
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Old 09-13-2013, 11:30 AM   #12
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Default Re: Downdraft Carb suggestions

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I have found the opposite of what Brent says. The 81 is adequate, and better than the stock carb, but the 97 is better, and cheaper. They usually need to be totally stock but sometimes may need a slight main jet change. I've tried them all.

Ohh come on now Jim, when have you EVER had a stock engine??
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Old 09-13-2013, 11:31 AM   #13
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Default Re: Downdraft Carb suggestions

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Is there a quick way to ID the B manifold? Headed to a swap meet today.
In addition to being flat on the top there was a boss and threaded hole for the clamp that held the Model B choke cable. There were two Model B intake manifolds. The early one had a large threaded hole that was originally intended for a vacuum line to the vacuum assisted clutch that was never adopted. The later version had a vacuum line for the windshield wiper that went to the rear and would probably look better on a Model A.

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Old 09-13-2013, 11:58 AM   #14
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Default Re: Downdraft Carb suggestions

The B manifold is also "clocked" a few degrees toward the motor. How do you overcome this in an A and hook up the GAV?
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Old 09-13-2013, 12:26 PM   #15
Purdy Swoft
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Default Re: Downdraft Carb suggestions

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The B manifold is also "clocked" a few degrees toward the motor. How do you overcome this in an A and hook up the GAV?
Snyders has a kit to solve this problem. Look on page A-136. Part number A-9510-K Lists for $31.00 . Its kinda pricey for what it actually is. I cut the mounting flange off the manifold, rotated it as needed and brazed it back on. Be careful if you try this. The brazing job has to be good or you will end up with vacume leaks.
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Old 09-13-2013, 12:55 PM   #16
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Default Re: Downdraft Carb suggestions

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Purdy can you post some pics of that setup? It sound WAY cool.
Here is a view of the setup on the speedster. The manifold is a cast iron uniflow that was made in the thirties. This manifold has larger runners and performs real good. I got this manifold from Berts, several years ago.


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Old 09-13-2013, 01:10 PM   #17
Purdy Swoft
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Default Re: Downdraft Carb suggestions

Here is a picture of the cast iron Trojan that I bought on eBay. This was taken when I was building the roadster. I've got better pics on photo bucket. Photo bucket seems to know which pictures that I want to post and most of the time won't let me. I had it working good late last night but not today. These pics are on Blue Melon. If I can get Photo bucket working again I'll post more.
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Old 09-13-2013, 01:28 PM   #18
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Default Re: Downdraft Carb suggestions

Thats a really neat setup. Who cares if you cant make photobucket work, if you can make two B carbs work!

For Photobucket I always use the IMG link by the way, never had a problem
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Old 09-13-2013, 03:16 PM   #19
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Default Re: Downdraft Carb suggestions

A club member recently told the membership about his misfortune with a downdraft carburetor. His Model A is equipped with a downdraft manifold, downdraft carburetor, and an electric fuel pump. His thinking was that the fuel pump would block any fuel flow from the tank to the carburetor when the engine was shut off, therefore there was no need to shut off the tank valve. Wrong assumption.

He had not driven the car for about three weeks, when he went to do so he found that about four gallons of fuel had flowed into the intake manifold and into the engine, including the oil pan.

His advice is to always shut the tank valve off. One would also hope that the tank valve has the ability to shut off 100%.

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Old 09-13-2013, 04:31 PM   #20
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Default Re: Downdraft Carb suggestions

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Originally Posted by Tom Endy View Post
A club member recently told the membership about his misfortune with a downdraft carburetor. His Model A is equipped with a downdraft manifold, downdraft carburetor, and an electric fuel pump. His thinking was that the fuel pump would block any fuel flow from the tank to the carburetor when the engine was shut off, therefore there was no need to shut off the tank valve. Wrong assumption.

He had not driven the car for about three weeks, when he went to do so he found that about four gallons of fuel had flowed into the intake manifold and into the engine, including the oil pan.

His advice is to always shut the tank valve off. One would also hope that the tank valve has the ability to shut off 100%.

Tom Endy
Looks like a really good argument for an electric shut off valve!
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