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Old 07-01-2018, 04:00 PM   #1
DMontague
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Default Model b carb

Gentlemen,
I just rebuilt the engine in my 28 AA. It's a diamond block, bored .020 over, touring grind cam, crank grind and all new bearings. I also installed a 6:1 snyders head. When I got the truck it had a model b carb on it. The only way I can get it to run decent is with the gav at least a full turn open. It's my understanding that you shouldn't have to run a model b open?
Thank you!
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Old 07-01-2018, 05:27 PM   #2
40 Deluxe
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Default Re: Model b carb

Keep in mind that your carb was originally calibrated to run on real, undiluted gasoline with the GAV closed, or nearly so. It could also run with a mix of ethanol and gasoline by opening the GAV to compensate for the leaning effect of ethanol. Most gasoline today comes already diluted with ethanol. Thus the need to richen the fuel mixture for smooth running.
Ethanol releases oxygen when it burns which has the effect of leaning out the mixture, so additional fuel must be added to get the right mixture. Modern cars do this automatically, which is why using pure gasoline gives noticeably better fuel mileage over using the various ethanol blends.
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Old 07-02-2018, 01:51 AM   #3
Chuck Sea/Tac
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Default Re: Model b carb

You need to flow test the jets. Remember, the flow test info is for a stock engine. Mods usually mean not enough gas. You may have to enrich some jets to get it to run correct. But you need to know where you are to start.
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Old 07-02-2018, 08:12 AM   #4
Terry, NJ
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Default Re: Model b carb

It sounds as though you're thinking about going back to the A carb. My recommendation is don't do it! The model B carb is superior in every way, First, it breathes better having larger throat and throttle diameters, Second, the float is positioned to move sideways to the motion of the car. A carb floats move with the direction of the car and can be affected by sudden braking. Thirdly, a B carb has an accelerator jet which the A carb does not have. Is your intake manifold bored out to B size? (1.250-1.300) This is in the primary runner that feeds the other two runners. If not, you may not be getting the full advantage of the B carb.
Terry

Last edited by Terry, NJ; 07-02-2018 at 08:25 PM.
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Old 07-02-2018, 11:02 AM   #5
Bob Bidonde
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Default Re: Model b carb

I second Terry's comment.


It is a good thing that you have a diamond block motor because these are aftermarket engines with all of the service bulletin changes. The 1928 and early '29 Model A engines had some significant design flaws you can read about in the factory service bulletins.
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Old 07-02-2018, 02:15 PM   #6
Ed in Maine
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Default Re: Model b carb

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I own a Model B and a Model A. I like the operation of the GAV on the Model A much better than on my Model B. I like to adjust the carb for a smooth operating engine on the Model A. The GAV on the model B is simply a bypass around main jet to get a much richer mixture during startup. Then the GAV should be fully closed. The Ford Service Bulletins spell out this starting procedure. It was a problem for Ford dealers in 1932 that customers were leaving the GAV open and then complaining their cars where using too much gas! I might add that my Model B Carb does have flow tested jets and it does run great but there is no further adjustment after the car has been warmed up. Another thing I don't like about my Model B is that after a few days of not running, it starts hard. I suspect the gas has evaporated some out of the carb and the fuel pump has to fill up the carb bowl before it starts. Ed
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Old 07-02-2018, 09:06 PM   #7
DMontague
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Default Re: Model b carb

When I went through my carb I just got the rebuild kit from snyders, are these not flow tested? If not, where is the best place to get good jets? I didnt know that the intake needs to be bored out. I just measured it at 1.189" at the carb flange. How far do I need to bore it, just to the "y"?
David
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Old 07-02-2018, 09:44 PM   #8
Chuck Sea/Tac
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Default Re: Model b carb

Quote:
Originally Posted by Terry, NJ View Post
It sounds as though you're thinking about going back to the A carb. My recommendation is don't do it! The model B carb is superior in every way, First, it breathes better having larger throat and throttle diameters, Second, the float is positioned to move sideways to the motion of the car. A carb floats move with the direction of the car and can be affected by sudden braking. Thirdly, a B carb has an accelerator jet which the A carb does not have. Is your intake manifold bored out to B size? (1.250-1.300) This is in the primary runner that feeds the other two runners. If not, you may not be getting the full advantage of the B carb.
Terry
I understand what you’re saying regarding the B accelerator jet, and it’s better than an A, but the secondary well on an A carb serves the same purpose, that is giving an extra shot of gas on opening the throttle.
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Old 07-03-2018, 08:17 AM   #9
Terry, NJ
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Default Re: Model b carb

You will note it is a cast finish. I drill the complete length of the runner with a 1.250 + drill that usually winds up with a around 1.281 (1 9/32) bore. I locate with a large pin, from the top of the bore to the bottom. I then polish it out with a 1" drum sander with a extended shank and polish it from top to bottom.
Since there's no practical way to drill out the full length of the cylinder runners, I only go into them as far as the extended shank will allow and then clean them up to prevent any turbulence in the flow. This, according to Pirianos, will increase the HP with both the A and B carbs, although more so with the B.
Google PirianosMod A dyno tests results
Terry






Quote:
Originally Posted by DMontague View Post
When I went through my carb I just got the rebuild kit from snyders, are these not flow tested? If not, where is the best place to get good jets? I didnt know that the intake needs to be bored out. I just measured it at 1.189" at the carb flange. How far do I need to bore it, just to the "y"?
David
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