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Old 08-05-2019, 02:39 PM   #1
Papa4Nine
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Default 31 Carburetor Adjustment

I have a late 1931 Model A Pickup with indented firewall. Rebuilt original engine. Having difficulty getting carburetor adjusted to where it will idle and not stall. First issue is that to get engine to start and run I have to pull choke rod out about 3/4 of the way and hold it there. Engine stalls out if choke is allowed to return. I have checked for intake air leaks and cannot find any (new intake and exhaust manifolds and gaskets). Installed a rebuilt carburetor with side bowl. Carburetor was purchased as rebuilt so I have not checked the jet sizes. New gas valve and have good gas flow. Also re-timed engine with light per Les Andrews. Air mixture screw set at 1 1/2 turns. Spark advance Lever works correctly as engine speeds up when Lever is pulled down however you must hold choke rod out..... Where do I go from here?
- Brad
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Old 08-05-2019, 03:05 PM   #2
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Default Re: 31 Carburetor Adjustment

If you have a spare carb (doesn't have to be sidebowl), put it on and see what it does
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Old 08-05-2019, 05:58 PM   #3
1931 flamingo
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Default Re: 31 Carburetor Adjustment

Pulling the choke out tells me you have a vac leak...........What gaskets(s) did you use and how tight did you tighten the nuts??
Paul in CT
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Old 08-05-2019, 06:28 PM   #4
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Default Re: 31 Carburetor Adjustment

Did you check the faces of the manifolds where they bolt to the block with a straightedge, or have the assembly milled by a machine shop, to make sure that the 5 surfaces all at the same level?

When you installed the intake/exhaust manifold assembly, did you use gland rings? If so, did you ensure you have enough depth for the gland rings to NOT bottom out before the manifold is clamped tightly to the block? This is critical. The rings can hold the intake manifold from sealing tightly to the gasket and you have massive vacuum leaks.
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Old 08-05-2019, 07:15 PM   #5
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Default Re: 31 Carburetor Adjustment

We do this with our old Harleys checking for an intake leak. Spray a little WD40 near the manifold inlets while its running and see if you notice any change in rpm? Otherwise I think I would be pulling the carb and checking my jets. Sounds like you purchased the carb rebuilt so Lord only knows what you might find. Let us know what you learn.
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Old 08-05-2019, 07:21 PM   #6
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I concur with the others here. Had the exact same issue with mine. Three, manifolds and a carb later to realize my manifolds were ALL slightly off! Super frustrating!

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Old 08-05-2019, 07:31 PM   #7
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Default Re: 31 Carburetor Adjustment

Block and mating surfaces of manifolds were machined. Used copper gaskets. Used gland rings. Sprayed WD40 around intake and no change in rpm. FYI.. also plugged the vacuum line at inlet manifold.
I have the original carb that came with the truck when I bought it. I will install it to see if there is a difference.
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Old 08-05-2019, 07:36 PM   #8
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Default Re: 31 Carburetor Adjustment

biggieou, thanks for the reply. Helps to know this has happened to someone else. What did you do to fix once you figured it out?
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Old 08-05-2019, 07:40 PM   #9
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Default Re: 31 Carburetor Adjustment

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Originally Posted by Papa4Nine View Post
biggieou, thanks for the reply. Helps to know this has happened to someone else. What did you do to fix once you figured it out?
I got the manifolds as close as I could get them when bolting them together and then lightly filed down the surface till I could run a straight edge over them. I also used the two piece copper gaskets as I've personally had better luck with them.

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Old 08-05-2019, 08:30 PM   #10
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Default Re: 31 Carburetor Adjustment

I checkeck the machined faces and all was good. What I didn’t check was the thickness of the common bolt flange. I will pull the manifolds when I pull the carb and check it. May need to match thickness of inlet and exhaust manifold flange at common bolt.
Thanks
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Old 08-06-2019, 03:49 AM   #11
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Default Re: 31 Carburetor Adjustment

back to first priciples.. ( assuming the fuel quality is OK) . it's running weak yeah? [and gas gap venting ok?] thus it's either ....
A) too much air .... eg manifold/to block or manifold to carb airleak check as suggested - I use Easystart aerosol ( an ether compound) to check for leaks - warning - don't mess too much with it it's explosive!
If not A) then it must be .....
B) restricted fuel . check fuel supply to carb ( disconnect supply pipe at carb should be about 1 pint per min ). If fuel supply to carb ok then problem is within carb..... blocked passage/ jet/ filter/ float valve / crud/ present , too small a jet(s) , fuel level far too low etc etc etc..

Last edited by johnbuckley; 08-06-2019 at 03:56 AM.
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Old 09-15-2019, 11:06 AM   #12
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Default Re: 31 Carburetor Adjustment

Thanks to all. Problem resolved. Hole in manifold set at rear stud was not aligned right and manifold would not seat perfectly against block. Reamed out hole to allow manifold to slide over stud and mate with block. Problem compounded with jets in carburetor not being correct. Idle jet and cap jets were too small and had to drill out to spec. Starts on first cylinder fire and idles perfect now.
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