The Ford Barn

hueyhoolihan · 03-28-2026, 03:27 PM

i've read that the two idle adjusting screws on my holly 94 adjust the overall air/fuel mixture. typically the depth a needle in a jet is what sets that, and not an idle circuit. anyone with knowledge as to how that works?

Bored&Stroked · 03-28-2026, 04:50 PM

The mixture screws are only for idle mixture, they do not influence the off-idle mixture --> which is controlled by the main jets, air-bleeds, power-valves, etc..

There are no "needles in the jets" of Holley 94s, 2100, 2110's, ECBs, etc. Same goes for all popular variants of Strombergs -> 81. 48, 97

hueyhoolihan · 03-28-2026, 06:31 PM

thanks for the answer. and it rings true.

i thought so...i didn't believe that an idle circuit would be designed to also control mixture. idle in carbs of many designs, IME, adjust idle by either cracking open the butterfly just a touch (often adjusted with a screw riding on a boss on the throttle shaft) or a narrow tunnel under the butterfly (often called an "idle circuit") and in the middle of it is a screw that adjusts just how much air can get through it. and i'm assuming on the 94 that there is one tunnel and one adjusting screw for each barrel.

Flathead Fever · 03-29-2026, 09:39 AM

You want to run both screws in all the way and "gently" bottom them out. Then back them out 1 1/2 runs to get the engine started. Then on a fully warmed up engine turn them in evenly until the idle starts to drop and then back them out to the maximum idle speed, plus a little bit more for all around operation. Then reset the idle speed and adjust the mixture one more time. If you really want to get it just right, you can take a propane bottle, slightly open it and run it over the top of the carb. If the idle goes up your still lean and need to back out the screws a little more or if the idle goes down, you're too rich and need to turn the screws in. When it's just about perfect the propane won't have any effect on it. I was a smog mechanic and that is how the State of CA made us adjust the idle mixture. It works good except that you might want to richen it up just a little more, maybe an 1/8 of a turn because the atmosphere is never the same. A lot of times you will runs the mixture screws in all the way and the engine won't die, that tells you something is wrong, the carb is able to pull fuel from somewhere it's not supposed to at idle. Time for a carb rebuild. Adjusting the mixture is easy to do, just make sure both screws end up turned out "exactly" the same amount. Also, setting the carb is the "last" thing you do, the dwell and timing need to be done first because it affects the final fuel mixture, whereas the idle mixture does not affect the timing and dwell.

hueyhoolihan · 03-29-2026, 11:18 AM

i have to apologize, it think my title is misleading. i am actually interested in adjusting the air/fuel ratio at all RPMs above idle, and not the idle or the air/fuel ratio while only at idle. (i'll change it)

IME, there is usually a "fuel/air mixture" screw or screws that are accessible from outside a carb to do that, but i don't see any such thing on my carb. and when i use the phrase "..setting the mixture.." or "...setting the fuel/air ratio.." in conjunction with the phrase "holley 94" when searching the net, i get nothing but information on setting what appears to me to be the two idle circuit screws. which is why i mentioned the word "idle" in the title. i want to change the air/fuel ratio entering the intake manifold at all time when above idle RPM is all.

sorry for the confusion.

hueyhoolihan · 03-29-2026, 11:32 AM

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i'm not familiar with the term "power valve", but my research leads me to believe that it functions as an accelerator pump would, and yet my holley has an accelerator pump, so i can only assume it works differently and performs some other function. possibly an fuel economy function? in which case, at this point in time anyway, would be of lessor concern than my fuel/air ratio troubles.

and again, i'm afraid, the term "air bleed" is unfamiliar to me too. and when researching it, it doesn't appear that they can be adjusted either.

is it possible that adjusting the air/fuel ratio on the holley 94 involves a trial and error process with multiple parts?

69a · 03-29-2026, 01:27 PM

It might be better to post the problem you have, rather than the what you think your solution might be.. running rich could be busted power valve diaphragm, or I'll fitting power valve, or jet size too big

ford38v8 · 03-29-2026, 01:43 PM

Huey, of the several “adjustments” to be made on a 94, I think you must be referring to the 2 main jets, accessible from the two plugs on the lower front of the carb. Remove those plugs, then use a skinny screwdriver (grind off the shoulders from a flat blade screwdriver) to remove the 2 jets inside. Observe the numbers stamped on each jet, which will be a 50 or a 51, depending on your local elevation. With multi-carbs, that number may be 45 or 46. Experiment in replacing those jets with larger or smaller jets to increase or decrease your fuel/air ratio.

hueyhoolihan · 03-29-2026, 04:28 PM

Quote:

Originally Posted by ford38v8

Huey, of the several “adjustments” to be made on a 94, I think you must be referring to the 2 main jets, accessible from the two plugs on the lower front of the carb. Remove those plugs, then use a skinny screwdriver (grind off the shoulders from a flat blade screwdriver) to remove the 2 jets inside. Observe the numbers stamped on each jet, which will be a 50 or a 51, depending on your local elevation. With multi-carbs, that number may be 45 or 46. Experiment in replacing those jets with larger or smaller jets to increase or decrease your fuel/air ratio.

so can i assume that that larger the number the richer the mixture? i'm afraid i know nothing about performance modifications like multiple carbs, etc.

Mart · 03-29-2026, 04:43 PM

As stated please say what the problem is, what the car is, what the motor is, where you are located (altitude).
Are you running rich or lean?
That will give us somewhere to start.

hueyhoolihan · 03-29-2026, 05:05 PM

Quote:

Originally Posted by Mart

As stated please say what the problem is, what the car is, what the motor is, where you are located (altitude).
Are you running rich or lean?
That will give us somewhere to start.

sure...

the car is a pretty much standard 1941 ford convertible with the original engine and transmission. i'm at sea level and i'm not sure if it is running rich but i do detect a very strong odor of a rich mixture when my head is in the engine bay, while the engine is running. i do not have to go to the tail pipe to smell it. the exhaust system could be leaking but it is not particularly loud IMO.

the plugs when pulled will be fouled with black soot even when cleaned before starting the car, but i can't say for certain whether it is due to a rich mixture. as i would think that the eventual massive misfiring would also leave a large amount of unburnt fuel in the cylinders and exhaust resulting in fouled plugs.

the original flathead v8 engine readily starts and runs smoothly, but as it warms up it begins to misfire. and it will get so problematic that the engine may stall completely and this is at idle. if i take it around the block it will begin the misfiring more quickly. and putting a load on the engine causes further misfiring.

but if people wish to chime in on an open ended misfiring problem, i'm all for it! fire away.

tubman · 03-29-2026, 05:37 PM

The first thing I would do is check for a leaking power valve, and probably change it for a known good stock unit just to be sure. (Actually, the first thing I would really do is pull the carb and send it to Charlie Schwendler, but I'll play the game.

)

Mart · 03-29-2026, 05:48 PM

I share some insights into 94s here. There are many things that will make it run rich and I touch on some of them in this video.
https://youtu.be/UMiWUQyIkmg
Mart.

hueyhoolihan · 03-29-2026, 05:50 PM

Quote:

Originally Posted by tubman

The first thing I would do is check for a leaking power valve, and probably change it for a known good stock unit just to be sure. (Actually, the first thing I would really do is pull the carb and send it to Charlie Schwendler, but I'll play the game.

)

the car will be dropped off at Vern Tardels this Friday. we spoke on the phone the other day, and he said he'd be happy to take a look at it, as i've just about exhausted my good ideas...i've nothing left but "Hail Mary's", i'm afraid. haha

the plan, like with some of my other "fun" cars, is to travel to ohio with it to visit my surviving family, and i want it to be reliable. i've had good success achieving this with a couple of my other cars, but this one is proving more difficult.

i'm impatient and want to do this, THIS summer or fall not next year! damnit!

tubman · 03-29-2026, 06:12 PM

That's the attitude! Sometimes we are forced to chose alternative methods. My decision to send a carburetor at Charlie is driven in part by his sterling reputation. However, it is mainly due to the fact that, because of the arthritis that has completely taken over my back and shoulders, I can no longer climb the stairs to the carburetor repair area on the mezzanine in my shop. (I am looking for a cheap, used stair lift.)

03-28-2026, 03:27 PM	#1
hueyhoolihan Senior Member Join Date: Dec 2024 Location: central coast california Posts: 593	adjusting the fuel/air mixture, not the idle screws on my 94 i've read that the two idle adjusting screws on my holly 94 adjust the overall air/fuel mixture. typically the depth a needle in a jet is what sets that, and not an idle circuit. anyone with knowledge as to how that works? Last edited by hueyhoolihan; 03-29-2026 at 11:19 AM.

03-28-2026, 04:50 PM	#2
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,906	Re: how can adjusting the idle circuit affec overall mixture The mixture screws are only for idle mixture, they do not influence the off-idle mixture --> which is controlled by the main jets, air-bleeds, power-valves, etc.. There are no "needles in the jets" of Holley 94s, 2100, 2110's, ECBs, etc. Same goes for all popular variants of Strombergs -> 81. 48, 97

03-28-2026, 06:31 PM	#3
hueyhoolihan Senior Member Join Date: Dec 2024 Location: central coast california Posts: 593	Re: how can adjusting the idle circuit affec overall mixture thanks for the answer. and it rings true. i thought so...i didn't believe that an idle circuit would be designed to also control mixture. idle in carbs of many designs, IME, adjust idle by either cracking open the butterfly just a touch (often adjusted with a screw riding on a boss on the throttle shaft) or a narrow tunnel under the butterfly (often called an "idle circuit") and in the middle of it is a screw that adjusts just how much air can get through it. and i'm assuming on the 94 that there is one tunnel and one adjusting screw for each barrel.

03-29-2026, 09:39 AM	#4
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,492	Re: how can adjusting the idle circuit affec overall mixture You want to run both screws in all the way and "gently" bottom them out. Then back them out 1 1/2 runs to get the engine started. Then on a fully warmed up engine turn them in evenly until the idle starts to drop and then back them out to the maximum idle speed, plus a little bit more for all around operation. Then reset the idle speed and adjust the mixture one more time. If you really want to get it just right, you can take a propane bottle, slightly open it and run it over the top of the carb. If the idle goes up your still lean and need to back out the screws a little more or if the idle goes down, you're too rich and need to turn the screws in. When it's just about perfect the propane won't have any effect on it. I was a smog mechanic and that is how the State of CA made us adjust the idle mixture. It works good except that you might want to richen it up just a little more, maybe an 1/8 of a turn because the atmosphere is never the same. A lot of times you will runs the mixture screws in all the way and the engine won't die, that tells you something is wrong, the carb is able to pull fuel from somewhere it's not supposed to at idle. Time for a carb rebuild. Adjusting the mixture is easy to do, just make sure both screws end up turned out "exactly" the same amount. Also, setting the carb is the "last" thing you do, the dwell and timing need to be done first because it affects the final fuel mixture, whereas the idle mixture does not affect the timing and dwell.

03-29-2026, 11:18 AM	#5
hueyhoolihan Senior Member Join Date: Dec 2024 Location: central coast california Posts: 593	Re: how can adjusting the idle circuit affec overall mixture i have to apologize, it think my title is misleading. i am actually interested in adjusting the air/fuel ratio at all RPMs above idle, and not the idle or the air/fuel ratio while only at idle. (i'll change it) IME, there is usually a "fuel/air mixture" screw or screws that are accessible from outside a carb to do that, but i don't see any such thing on my carb. and when i use the phrase "..setting the mixture.." or "...setting the fuel/air ratio.." in conjunction with the phrase "holley 94" when searching the net, i get nothing but information on setting what appears to me to be the two idle circuit screws. which is why i mentioned the word "idle" in the title. i want to change the air/fuel ratio entering the intake manifold at all time when above idle RPM is all. sorry for the confusion. Last edited by hueyhoolihan; 03-29-2026 at 11:36 AM.

03-29-2026, 11:32 AM	#6
hueyhoolihan Senior Member Join Date: Dec 2024 Location: central coast california Posts: 593	Re: adjusting the fuel/air mixture, not the idle screws on my 94 Sponsored Links (Register now to hide all advertisements) i'm not familiar with the term "power valve", but my research leads me to believe that it functions as an accelerator pump would, and yet my holley has an accelerator pump, so i can only assume it works differently and performs some other function. possibly an fuel economy function? in which case, at this point in time anyway, would be of lessor concern than my fuel/air ratio troubles. and again, i'm afraid, the term "air bleed" is unfamiliar to me too. and when researching it, it doesn't appear that they can be adjusted either. is it possible that adjusting the air/fuel ratio on the holley 94 involves a trial and error process with multiple parts? Last edited by hueyhoolihan; 03-29-2026 at 11:37 AM.

03-29-2026, 01:27 PM	#7
69a Senior Member Join Date: Aug 2020 Posts: 323	Re: adjusting the fuel/air mixture, not the idle screws on my 94 It might be better to post the problem you have, rather than the what you think your solution might be.. running rich could be busted power valve diaphragm, or I'll fitting power valve, or jet size too big

03-29-2026, 01:43 PM	#8
ford38v8 Senior Member Join Date: Jul 2011 Location: SF Bay Area Posts: 7,634	Re: adjusting the fuel/air mixture, not the idle screws on my 94 Huey, of the several “adjustments” to be made on a 94, I think you must be referring to the 2 main jets, accessible from the two plugs on the lower front of the carb. Remove those plugs, then use a skinny screwdriver (grind off the shoulders from a flat blade screwdriver) to remove the 2 jets inside. Observe the numbers stamped on each jet, which will be a 50 or a 51, depending on your local elevation. With multi-carbs, that number may be 45 or 46. Experiment in replacing those jets with larger or smaller jets to increase or decrease your fuel/air ratio. __________________ Alan

03-29-2026, 04:43 PM	#10
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 9,239	Re: adjusting the fuel/air mixture, not the idle screws on my 94 As stated please say what the problem is, what the car is, what the motor is, where you are located (altitude). Are you running rich or lean? That will give us somewhere to start.

03-29-2026, 05:37 PM	#12
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 12,132	Re: adjusting the fuel/air mixture, not the idle screws on my 94 The first thing I would do is check for a leaking power valve, and probably change it for a known good stock unit just to be sure. (Actually, the first thing I would really do is pull the carb and send it to Charlie Schwendler, but I'll play the game.)

03-29-2026, 05:48 PM	#13
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 9,239	Re: adjusting the fuel/air mixture, not the idle screws on my 94 I share some insights into 94s here. There are many things that will make it run rich and I touch on some of them in this video. https://youtu.be/UMiWUQyIkmg Mart.

03-29-2026, 06:12 PM	#15
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 12,132	Re: adjusting the fuel/air mixture, not the idle screws on my 94 That's the attitude! Sometimes we are forced to chose alternative methods. My decision to send a carburetor at Charlie is driven in part by his sterling reputation. However, it is mainly due to the fact that, because of the arthritis that has completely taken over my back and shoulders, I can no longer climb the stairs to the carburetor repair area on the mezzanine in my shop. (I am looking for a cheap, used stair lift.)