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Carburetor Choke Adjustment

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Old 09-04-2014, 01:46 PM
Heesman Heesman is offline
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Carburetor Choke Adjustment

Hello!
I hope it is ok to post my question here...

I am in need of some assistance on choke settings on my carb – see signature for the truck. The carburetor is a Holley model 4150, 4779-C part number with an electric choke. Now that the weather is starting to cool down I am noticing differences in starting up my truck.

I have a couple of questions – I have searched the internet but I can’t seem to find answers to my questions.

#1) If the choke cap is set correctly, regardless of ambient temperature should the fast idle cam always go to the fastest lobe on a cold start once the throttle is opened prior to starting? Right now my carb from a cold start will only go mid-way or so on the fast idle cam. The choke cap is 2 marks lean from the mid-point. At this setting the fast idle cam comes up half way or so and the choke closes some (not even half way). The truck starts, idles so-so at 1000 rpm. Once warm the truck runs great.

#2) If I go just rich enough on the choke cap to get the fast idle cam to go to its fastest lobe, the choke fully closes but doesn’t open at all when the engine first starts. As a result the engine runs rich, bogs and ultimately dies. From my reading the “de-choke” should open the choke enough to allow the truck to run. Mine isn’t doing this or perhaps I am misunderstanding how this is supposed to work.

Lastly, I have checked my fast idle speed (1600 rpm) so I know the idle speed is correct if I can get the choke to set it there.

I would appreciate any help from the carb crowd. All my co-workers are laughing at me because I have a Holley.

Eric
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Old 09-04-2014, 02:13 PM
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First, a Holley?! I'm with your friends.

Seriously though, I'd worry about how far the choke closes instead of the step the throttle gets to on the fast idle. That's because the fast idle speed is adjustable on some Holley's, although I'm not sure about yours. And, because you don't want it going to the highest step when the engine is cold but the outside temp is hot.

So, let's start with the choke itself. There are spec's somewhere for exactly how that carb is supposed to be set up out of the box, meaning the notch the adjustment is on, and how far the de-choke mechanism will pull it open. But, without the spec's I would set the adjustment so the choke just closes when the outside temp is hot but the engine is cold. Then see if the de-choke diaphragm will pull the choke open a bit when the engine starts - or you can usually collapse the diaphragm by hand to see that the choke pulls off some. From there I would drive it for a few days and see what you think of the choke setting. Then, once the choke is set properly play with the fast idle speed.
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Old 09-04-2014, 03:33 PM
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A common misconception is that the cap adjustment applies more or less choke for starting........but this isn't right.

If the cap adjustment is even in the ballpark, the choke should shut fully when you first press the accelerator before starting; the pull-off diaphragm then cracks the choke open the amount it is adjusted to (just enough to run - there will be a spec for your engine), as the engine cranks over.

The cap adjustment, apart from applying tension to close the choke, determines the rate at which the choke opens, as the coil warms - not how much choke is initially applied.

If your pull-off isn't working, you can't go any further without fixing/replacing it. The choke coil also needs to be unwinding when it heats up.
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Old 09-04-2014, 09:02 PM
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On most Holleys, you will not see a external diaphragm for the choke pull-off. Most Holleys have it built into the choke housing. It's a little metal piston made into a bore in the housing, and has a vacuum port made into it from the carb. I will see if I can find a picture and post it. You can then see if it's what you have. It just may be the piston is stuck in your carb.
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Old 09-04-2014, 09:08 PM
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#31 is the housing. I think #24 is the piston/lever assembly.


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Old 09-05-2014, 10:34 AM
Heesman Heesman is offline
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Thanks for all of the replies.

Franklin2 - your diagram is pretty much what I have. I found a PDF from Holley that says using a high tech paper clip as a tool you can adjust the the de-choke. I will look into this this weekend as I am convinced my problem is with this. The one question I have that I seem to be getting varied replies to is what position of the fast idle cam should be engaged when the de-choke operates correctly - I have heard the fastest as well as one step down from the fastest. Have you ever adjusted this style of de-choke?

Gary/Ken - thank for the information. My only experience with carbs was on my 1977 Maverick - it had a Motorcract 2150 that always seemed to work perfectly.

I will report back after the weekend.
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Old 09-05-2014, 04:00 PM
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I have never messed with that part. I like carbs that are pretty much factory set, and just need a clean-up and parts replacement when rebuilding. I try not to mess with all those little settings unless I have to. Make sure to check to see if something is wrong before adjusting anything.

Of course if someone before you has messed with it, you will have to adjust it.

I will tell you most carbs will idle pretty fast when first started. What I do is after it starts and runs a few revolutions I kick the throttle to the floor. That will usually slow it down a notch or so and it won't sound like it's coming apart.

But remember this is old technology, if you go back inside and drink some coffee, you are going to come back out to a roaring engine. They usually won't kick down by themselves, and as it warms up it runs faster and faster. It usually requires you to go out and kick it down at least one other time to bring it down to a somewhat normal idle.

People are spoiled with fuel injection now.
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