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Old 02-25-2019, 11:15 AM   #1
30AAFord
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Default Idle Screw - Running Issue on a '33 BB

Will start this out by saying I've been a Model T and Model A guy for 40 years - am very new to V8s. I have a '33 BB that sat in an equipment shed for 20 years. I got it running, but am having a problem with how she is running. The engine is a 21-stud, but not original to the truck. Generator has been rebuilt, distributor rebuilt (by Bubba's Ignition), running a rebuilt 59A carb (Mac's) and a new fuel pump. Here is the issue:

I can only get the truck to stay running with the idle adjust screw fully screwed in (maxed out). Any adjustment short of that and the engine will die when accelerated. I have played with the mixture adjustments endlessly and it has made no difference. I looked at the Service Manual and now keep them set at the 3/4s off full seat the manual mentions.

I should also note that even at full adjustment of the idle screw, the truck will eventually act like it is starving for fuel and die (usually after running for a while).

Any ideas on what to check would be greatly appreciated - I've been messing with this on/off for a couple of months now and am out of troubleshooting ideas.
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Old 02-25-2019, 01:44 PM   #2
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Default Re: Idle Screw - Running Issue on a '33 BB

Will also add that the plugs are new as well.
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Old 02-25-2019, 02:20 PM   #3
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Default Re: Idle Screw - Running Issue on a '33 BB

Not quite understanding. Screwing the idle screws in, decreases fuel.
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Old 02-25-2019, 02:36 PM   #4
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Default Re: Idle Screw - Running Issue on a '33 BB

I think you are referring to the mixture screws - on the 59A carb, there are two (one for each barrel/bank of cylinders). There is only one idle adjustment screw and that is part of the throttle butterfly. When you screw it in, you are increasing how much the throttle stays open while you are idling. In order to keep my truck running, that screw has to be fully threaded in (until it stops), substantially increasing the idle speed of the engine. Anything less and the engine stalls out. I know this isn't right.
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Old 02-25-2019, 04:22 PM   #5
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Default Re: Idle Screw - Running Issue on a '33 BB

I did misunderstand your reference. I start with the mixture screws 1 1/2 turns out. I have never seen a reference to only 3/4 turn. Also, what is your initial spark advance?
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Old 02-25-2019, 06:15 PM   #6
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Default Re: Idle Screw - Running Issue on a '33 BB

Is the idle speed screw you are turning, touching the throttle lever ? In other words is the screw too short, and only contacting the throttle lever when it is turned in all the way ?


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Old 02-26-2019, 12:15 AM   #7
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Default Re: Idle Screw - Running Issue on a '33 BB

is it a Detroit lubricator or Stromberg carb? You can run the Stromberg with the top off to watch the fuel level in the bowl could be sticking needle and seat, crud around the emulsion tubes or more likely a leak in the fuel line or gasket on the pump sucking air.
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Old 02-26-2019, 08:39 AM   #8
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Default Re: Idle Screw - Running Issue on a '33 BB

Carb is a Ford 59A (essentially the same as a Holley 94).

Idle adjust screw touched the throttle lever at each point - screw is standard length of all adjusting screws I have seen. Truck only stays running when fully screwed in and idle is high. As I originally posted, will eventually stall at this setting as well after 30-45 minutes of running. Truck immediately stalls when accelerating if idle adjust screw is not fully screwed in to full length.

Service Bulletin from March-April of '34 (I think, doing this from memory as the book is elsewhere right now) advises that the mixture adjust screws be adjusted to each set of cylinders by sound of the engine, but goes on to say
that they are usually 3/8 to 3/4 turn off full seat.

I have no idea what the initial spark advance is set at - simply installed the distributor as it came back from the rebuilder.
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Old 02-26-2019, 09:40 AM   #9
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Default Re: Idle Screw - Running Issue on a '33 BB

I don't believe 3/8 to 3/4 turn out is right. 1 1/4 to 1 1/2 is normal. Obviously if it doesn't run at your settings, it needs to be changed.
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Old 02-26-2019, 10:57 AM   #10
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Default Re: Idle Screw - Running Issue on a '33 BB

Speaking with limited experience .... The position of the idle adjustment screw should not cause your engine to stall when you try to accelerate. Is it doing that under load, no-load or both ?

Beyond the setting screw idle and air mix screws, I can't tell from your description if the idle circuit in the carb is at fault or the accelerator circuit. An idea of RPM's at failure point would help. There are reports of problems having to do with the materials used on the power pumps of carb's.

I use a vacuum gauge to adjust my air mixture screws (as said, the starting point for air-mix adjustment screws varies with different make/model carb's - I've been told up to one turn pending but my gauge works best for me)

Having a Bubba distributor set-up I'm disregarding ignition thoughts, and your carb an pump are rebuilt. Assuming (1) you have the correct length idle adjustment screw for that specific carb, (2) the linkage arm lever on the carb is proper and not bent or binding (3) there are no "restrictions in the fuel lines - metallic and flex parts - check your flex line for interior deterioration that could restrict fuel flow, and (4) your rebuilt fuel pump is working properly - say 1.5 to 2.5 psi for single carb, then IMHO its sounding like your running short on fuel into the engine on acceleration and something's wrong inside the rebuilt carb.

Last edited by Drbrown; 02-26-2019 at 12:22 PM.
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Old 02-26-2019, 11:41 AM   #11
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Default Re: Idle Screw - Running Issue on a '33 BB

Having to run the idle adjust screw all the way in to a high idle speed suggests that the idle fuel circuit, adjusted by the two idle mixture screws, is not providing fuel. At idle the air flow through the venture is not high enough do draw through the main jets. The idle circuit is designed to provide fuel for that very low rpm.

Open each idle mixture screw 2 turns from full closed, regardless of whatever other reference you have found. Get it running at the slowest rpm you can with the idle speed adjustment on the linkage. Then start closing the idle mixture screws 1/4 turn at a time to see if the idle speed picks up. if it does, back off the idle speed adjust to lowest idle, and go back to the mixture screws, this time, one at a time, to find the idle mixture adjustment that gives highest rpm, again backing down on the idle speed adjustment as you progress. If this fails, we need to look elsewhere in the fuel system.

Low fuel level in the carb float bowl can cause your problem. Start with checking fuel pump output. Disconnect the fuel line from the carb, and crank the engine WITH THE IGNITION OFF. You should get a strong spurt of fuel from the line of about a tablespoon for each cycle of the pump. If that fails, check the stroke of the fuel pump push rod at about .2 inches. If that is bad, the cam is worn and you need an electric fuel pump. If the stroke is good, try the pump with the input line a hose into a jar of gas. If the pump works okay with no obstruction to its' input, check the flexible fuel line for cracked or swollen, and fuel line from the tank unobstructed. On a '33 you can blow into the fuel tank to see if a stream of fuel emerges from the fuel line to the pump.

If the pump was good, remove the needle and seat from the side of the carb and look for a sticking needle. The rubber tipped ones get sticky in alcohol. I recommend replacing with the Grosse type that uses a steel ball.

If that doesn't fix it, check the float level adjustment to be sure it is not too low.

If that does not do it, the carb comes off for a cleaning and/or rebuild.

Good luck.
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Old 02-26-2019, 11:41 AM   #12
Bob C
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Default Re: Idle Screw - Running Issue on a '33 BB

Page 141 of the Jan-Feb 1934 Service Bulletins does state 5/8 to 3/4 turns open
as does page 126 of the 1932-1941 Service Handbook. I always thought it was more than that also.


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Old 02-26-2019, 12:10 PM   #13
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Default Re: Idle Screw - Running Issue on a '33 BB

Drbrown and Drolston - thank you both for giving me a rundown of additional things to check. The fuel pump and flex line are brand new, but will run those checks anyways (will also proof the metal line going into the tank which is original). Will also try the mixture procedure Drolston described. Carb is a Mac's rebuild - probably wouldn't hurt removing the needles and checking the tips.

Bob C. thank you for finding the exact Service Bulletin references. I didn't question the 3/4s off full seat because that is roughly what the Zenith on a Model A and an NH on a Model T run best at - anything else is usually too rich.
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Old 02-26-2019, 01:38 PM   #14
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Default Re: Idle Screw - Running Issue on a '33 BB

I have built a lot of engines and never had a carb the would idle decent at 3/4 off the seat. Go ahead and believe the bulletin if that makes you feel good. Have you tried turning the idle screws out?
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Old 02-26-2019, 02:00 PM   #15
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Default Re: Idle Screw - Running Issue on a '33 BB

Not really a matter of what makes me "feels good" as much as it is a matter of what makes the truck "run good." As I said, am new to V8s - but not new to early Fords (so I went with what a Service Bulletin recommended and that happened to be validated by my previous experience with As and Ts). I did try turning the mixture screws out and mentioned that in my original post ("have played with the mixture adjustments endlessly"). That being said, I didn't try it while simultaneously adjusting the idle speed as Drolston suggested below - will try that next.
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Old 02-26-2019, 03:16 PM   #16
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Default Re: Idle Screw - Running Issue on a '33 BB

They need to be adjusted together, if the idle gets too high it gets off the idle ports and on to the main jets. Each time the rpms increase it needs to be idled down so you are sure it is running on the idle fuel only.
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Old 02-26-2019, 07:34 PM   #17
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Default Re: Idle Screw - Running Issue on a '33 BB

Actually you will get into the transition ports. The main jets will still not be flowing.
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Old 02-26-2019, 08:04 PM   #18
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Default Re: Idle Screw - Running Issue on a '33 BB

I was trying to keep it simple.
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Old 02-26-2019, 11:02 PM   #19
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Default Re: Idle Screw - Running Issue on a '33 BB

New fuel pump from where? Hve you checked the pressure from the pump? Might be too much fuel going down the carb and causing the need for more air. Mike
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Old 02-27-2019, 03:01 PM   #20
Terry,OH
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Default Re: Idle Screw - Running Issue on a '33 BB

Does the 59 have have a vacuum port on the base? If so it has to be plugged or blocked. Your engine does not use this vacuum. Have you put a vacuum gauge on the engine manifold?
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