07-01-2018, 04:00 PM | #1 |
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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! |
07-01-2018, 05:27 PM | #2 |
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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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07-02-2018, 01:51 AM | #3 |
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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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07-02-2018, 08:12 AM | #4 |
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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. |
07-02-2018, 11:02 AM | #5 |
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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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Bob Bidonde |
07-02-2018, 02:15 PM | #6 |
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Re: Model b carb
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07-02-2018, 09:06 PM | #7 |
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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 |
07-02-2018, 09:44 PM | #8 | |
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Re: Model b carb
Quote:
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07-03-2018, 08:17 AM | #9 | |
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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:
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