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Old 01-24-2021, 12:51 PM   #1
mcgarrett
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Default Holley 94 jetting question

I have a Holley 94 on my 1940 Tudor. Car runs fine, but I have noticed that when starting cold it needs a lot of choke. Throttle responds normally once warmed up.
I have notice that once warmed up if I hold the accelerator above idle, the engine increases RPMs and smooths out noticeably if I add more choke.

Here's my question: Is this an indication that I need to increase the main jet size? It seems the engine is asking for more fuel.
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Old 01-24-2021, 02:06 PM   #2
ford38v8
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Default Re: Holley 94 jetting question

No. You may verify that you have .050 or .051 jets. Tune your idle screws and do a vacuum test. You may have a vacuum leak.
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Old 01-24-2021, 05:40 PM   #3
mcgarrett
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Default Re: Holley 94 jetting question

Thanks Alan. I'll adjust idle mixture screws and check for vacuum leaks as you suggest.
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Old 01-24-2021, 06:39 PM   #4
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Default Re: Holley 94 jetting question

I second idle screw adjustment. How far are they turned out? Should be about 1 1/4 to 1 1/2 turns.
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Old 01-25-2021, 07:00 PM   #5
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Default Re: Holley 94 jetting question

The carbs have three circuits plus the accelerator pump. Idle screws have to be adjusted for best idle after the engine is fully warmed up. The idle circuit only draws fuel when the throttle plates are at or very near the idle position. Once the plates open passed the idle ports, there is little or no flow there. Under acceleration, the accelerator pump give the initial boost with the power valve to even things out with more fuel under load. At road speed, it's all on the main jets unless pulling a long hill or accelerating. Main jets are generally only changed due to changes in altitude of vehicle operation. Smaller jets would be used for higher altitude and larger ones lower altitude. Ford and Holley manufactured carbs for higher elevations in the late 40s & early 50s on with different dash number codes on the carb but I don't know about prewar or early post war carbs.

It's normal to run with some choke when cold to richen the fuel mix but it shouldn't need further choke when fully warmed up. Pulling the choke will reduce air flow to richen the fuel mixture. If it's not getting fuel to the venturi then there may be a bit of blockage not allowing proper fuel flow. Vacuum leaks will generally lean the mixture due to a lesser quantity of air passing through the venturis as it should so a check for leaks is a good idea.

Checking the function of the ignition system's automatic spark control is not a bad idea if no carb problems pop up.

Last edited by rotorwrench; 01-25-2021 at 07:09 PM.
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Old 01-25-2021, 07:27 PM   #6
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Default Re: Holley 94 jetting question

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Thanks for all the helpful feedback. The carb that came on the 1940 85 hp flathead is the 91-99 model and works like most of the others except for the absence of a vacuum port. I understand the operation of these carbs pretty well, but will be checking mixture screws and checking for vacuum leaks just in case something is out of adjustment. The vacuum brake mechanism on the '40 helmet distributor seems to be functioning correctly, but will confirm.
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Old 01-25-2021, 09:33 PM   #7
G.M.
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Default Re: Holley 94 jetting question

I use an exhaust anolyzer in the exhaust pipe and
change jet sizes until it is close to perfect. G.M.
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Old 01-27-2021, 10:30 AM   #8
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Default Re: Holley 94 jetting question

Yes, back i the day we would run our hand over the carb at a fast idle and see if the engine reved up. if it did we would increase the jet size. However, I think GM idea is more reasonable. Using an AF meter can check the complete range of carb operation. ON a lean mixture, you need more timing advance.
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Old 01-27-2021, 02:02 PM   #9
rotorwrench
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Default Re: Holley 94 jetting question

The automatic spark control has the vacuum brake but it also has flyweights that can get sticky or worn on the rotor. both phases of control have to be in good condition.
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Old 01-27-2021, 07:05 PM   #10
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Default Re: Holley 94 jetting question

Rotorwrench,
You got me to thinking about something else I've noticed recently. I'm hearing a buzzing noise that sound like it's coming from the distributor. Only notice it, at idle. I'm wondering if the flyweights are loose or worn. Sometimes I can quickly rev the engine and it goes away as it settles back to idle. Anyone have ideas about what that might be?
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