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Installing Automatic Choke on a Holley Carburetor

36852 Views 2 Replies 2 Participants Last post by  Ray III
Due to the lack of information out there on properly setting up a Holley automatic choke, I wanted to write a tech article so the next guy has the reference that I sorely needed. Holley's instructions that come with a choke kit are an absolute joke and makes an otherwise fine product look like a piece of junk because no one has a clue what to do with it. Hopefully this helps everyone out there who have been mistaken into believing that their carbureted engine can't be made to run well when cold.

This writeup will detail the adjustment of the current self-contained choke housing with internal pulloff and electric heated bimetallic spring, which looks like:

However, the general principles of operation are the same for older style chokes that have a separate pulloff and air-heated spring, which looks like:

picture credit

For these, and for certain older electric chokes, there's some adjustment instructions on the last page of this document: There is also another article that covers these as well, in the link posted below.

Some truck carburetors have a governor with a built-in manual choke lever. They don't look like an automatic choke can be installed on them, but I could be wrong. Either way, the chokes on these setups work way better than "dumb" automotive manual chokes so there is not much of a reason to change them.

For further reading, here's another writeup with lots of pics and great info. Apparently it has only been up a few months and I did not realize this was out there until I was halfway through my own article, so between the two of us you should be all set.

And a thread on that site on this subject:

Theory of Operation

Why we have a choke

Modern gasoline engines are designed to run at around 200° F, and the parts are sized to fit perfectly when the coolant is at that temperature. When the engine is cold, however, the pistons and rings have a looser fit. Until they are heated by combustion and expand to seal the bore, there will be a loss in the vacuum they are able to draw on the intake tract. In addition to this, there is a loss in power from fuel vapor condensing in the cold manifold and burning poorly in the cold chamber. Finally, we have increased drag from engine and transmission oil as well as the alternator charging the battery.

The idle circuit of a carburetor keeps the engine running when vacuum is created below the closed throttle blades, drawing fuel through holes in the side of the carburetor bore. With a loss in vacuum with the throttle closed, the fuel mixture leans out to the point that the engine will not run. This can be corrected by adding a further restriction above the throttle blades, bringing manifold vacuum closer to the level of a warm engine. This is the purpose of the choke plate.

To compensate for increased drag and poor combustion, the throttle can also be slightly propped open. This has two purposes. The obvious thing is that it increases engine speed. But it also exposes part of the transfer slots. Since fuel is drawn through the transfer circuit by a combination of vacuum and the Bernoulli effect of air rushing through the gap between the throttle blade and carburetor bore, the mixture is less dependent on a good vacuum signal and slightly enrichened due to the added restriction of the choke plate.

When the throttle is opened beyond the transfer slots, fuel is drawn in purely by air velocity with the Bernoulli effect, and the choke plate serves no purpose regardless of how cold the engine is. (This is why some people can get away without a choke during the summer by holding the throttle open long enough to warm the pistons and rings.) If the choke is closed too much at an advanced throttle position, the engine will bog down from vacuum that is not supposed to be present at the main jet.

How Holley's choke works

With the engine off, the cold choke spring puts tension against the choke plate towards the closed position. Since the fast idle cam must move with it, before cranking the engine the driver presses the accelerator slightly to lift the fast idle lever off the cam and allow it and the choke plate to spring into position. This position will depend on the temperature of the engine. In warm weather or when the engine has been previously warmed up the choke may only close a little, but in cold weather it will snap shut completely. The fast idle cam holds the throttle open slightly when the choke is closed.

When the engine is cranked, the choke plate makes the engine suck enough fuel from the carburetor for the engine to fire. As soon as the engine starts and vacuum increases, the vacuum pulls on a small piston inside the choke housing which forces the choke plate open to a specific gap against the tension of the spring. Without this, the engine would immediately go pig rich and die. At the same time, an electric heater begins to warm up the choke spring.

As the spring warms up from the heater and close proximity to the manifold, it becomes longer and slowly begins to push the choke plate open. Since the pull-off piston is bottomed out, the small connecting rod passes through it as the choke opens, uncovering the hole where the rod seats. The air that pulled the piston down is then drawn through this hole, causing air to be drawn through a vent into the choke housing and across the choke spring, preventing it from overheating. After an amount of time depending on ambient temperature and the position the choke spring was rotated and set at, the choke plate is fully open. As the choke opens, it allows the fast idle cam to be rotated by gravity, decreasing the idle setting.

If the throttle is pushed wide open while cranking, a projection on the fast idle lever trips the fast idle cam so that it pops the choke plate open a little. This is supposed to help clear a flooded engine.

For part to full throttle where the choke is not needed, Holley seems to expect increased air velocity to pull the choke plate farther open due to the offset hinge, to prevent swallowing gasoline and bogging down. This does not seem to work properly, especially at low speed and high load when little intake velocity is present. Further down this post I will show you a way to modify the fast idle lever to mechanically open the choke.


Everything you'll need to convert most modern Holley carburetors to automatic choke is in one of the kits listed on this page:
Most people will want Holley part number 45-223S for internal vacuum source (seals to a port on the carb) or 45-224S for external vacuum (hose connection to vac port). The "S" suffix indicates "shiny" finish as opposed to the standard drab finish. Click on "Application" in the info for each one to determine which one is yours.

However, if you have a model 2300, 500 CFM, list number 4412 2-barrel carburetor like the one used for illustration purposes below, you will also need part number 45-465 which is not included in the kit that includes all needed parts (huh?). Holley makes no mention of this at all in the instructions, thanks guys! The part is a replacement fast idle lever that includes the tang to de-choke at WOT and is found here:

An optional part is the manifold heat sensor, part number 45-267. In cold weather it slows down the heating of the choke spring. For most engines this should not be needed and may actually make things worse, since the cold ambient air already slows down the heating of the spring. Try the choke as-is before spending the money on this:

When you get the kit, you'll find these instructions in it.
External vacuum source -
Internal vacuum source -
I'll clarify some of it and fill in some missing information.

First they want you to (after removing the old choke) replace the fast idle lever located on the throttle shaft under where the choke mounts. 2-barrel carbs use the secret lever that's sold separately. 4-barrels have an extra lever that's just added on. I'm not sure why there's a spring involved here; it seems to do nothing but hold the extra lever in place. The purpose of this change is to add a de-choke capability where at WOT the throttle shaft trips the choke open via the red cam. For marine engines there is no fast idle lever, just the de-choke. The reason for this is that boats don't have brakes and if they are put in gear while idling fast they will take off and hit something.

Figure 6 shows the installed fast idle lever next to the manual choke bracket and cam, even though this was removed in the first step!

It is possible to install the choke housing that uses external vacuum on a carburetor with an internal vacuum provision. This is why the external choke kit includes a ball plug and instructs you to make sure no vacuum port is present behind where the choke will mount.

There are 2 sets of screws to install the choke housing. One set is longer for if you need to use the spacers. I think this is only for the 2-barrels. Hold the choke housing with the red cam over the screw in the fast idle lever; if there is a large gap where it will mount, you need to use the spacers, longest screws, and also an offset choke rod.

The "index" position on the choke housing for the cap is the one with a small arrow, in the middle of the row of marks. In the picture posted earlier, you can see the mark on the choke cap at this setting.

When you connect the positive lead to the choke cap, it should be routed through an oil-pressure switch or connected to the alternator so it is only on when the engine is running. If you connect it directly to a switched power source, make sure anyone who drives the vehicle knows that the key must be off if the engine is off! If you use an oil-pressure switch, you want one with 2 terminals, not one that grounds. For the alternator connection, I don't yet know how that is wired but will update this when I figure it out. In all cases, the choke cap must draw power through a 10 amp fuse.

In the section "for list 4412 carburetor," you are instructed to remove the screws holding the choke plate. My carburetor did not have these screws! Instead, you can bend the tab that the choke rod hooks on just enough to swap the rod for the replacement offset one, then bend it back.

Step 5 in "for list 4412 carburetor" is really vague. Just ignore it and install the secret fast idle lever I told you about earlier, pictured below on the right.

Step 6 in "for list 4412 carburetor" suggests installing the timed spark hose first because it's a pain in the ass to get to. If you're still using this port, now is a good time to cap it and move the distributor hose to full vacuum. The engine will idle much better this way; the ported vacuum hookup was only used as a band-aid for emissions.


1) Verify Correct Idle Mixture

With the choke housing installed, the engine will draw a small amount of additional air. This should not be significant, but either way you should double-check the adjustment of the mixture screws. If the engine is idling lean, it will require too much choke when cold, which will screw up part-throttle drivability.

Start the engine and let it warm up to 200 degrees. Don't worry about how it runs when cold right now; just get it warmed up. Make sure the choke plate is fully open.

Shut the engine off and hook up a vacuum gauge without disconnecting anything that is normally connected. Turn all the idle mixture screws in till they touch the seat, then back out the appropriate amount (usually 1 turn) so they are all the same.

Restart the engine and start turning the screws in (if it runs worse, turn the screws out instead), no more than 1/4 turn at a time for each screw to keep them all in the same position, until you find the point at which the vacuum gauge has the highest reading. From there, back each screw out (richen) 1/4 turn or until the engine just barely begins to lope.

If the idle speed has changed, adjust the throttle stop screw. If you make an adjustment of more than a couple hundred RPM, fiddle with the mixture screws again to double-check (covering or uncovering the transition slots changes the mixture slightly).

2) Fast Idle

While the engine is hot, open the throttle just enough that you can lift the red lever all the way up, then release the throttle. This puts the fast idle screw on the top step of the fast idle cam. Check the engine speed at the tachometer; you want it to be running at 1600 RPM for now. Adjust the fast idle screw to get this speed. If there is a secondary diaphragm housing behind the choke housing, you will have to make the adjustment with the engine off.

The next time the engine is started when it's cold enough to completely close the choke plate (tap the accelerator to "set the choke" first), you should see the engine rev up to somewhere a little over 1,000 RPM like 1,200 or so, assuming the choke pull-off was set correctly as described in the next section. Tapping the accelerator again should drop the idle speed somewhat, but it should stay in the neighborhood of 1,000 RPM. If this is not enough to make the engine run smoothly (again, assuming correct pull-off adjustment) and not stall immediately after starting or during sudden deceleration while driving with a cold engine, you can increase the setting. 1,800 RPM (with fast idle screw on the top step of the cam and engine warmed up) works for me. You shouldn't need to go over 2,000 RPM with this unless you have a wild-ass cam or something.

If you're cursed with an automatic transmission, these things LOVE to drag a carbureted engine to its death while coming to a stop. This condition is only worsened with a cold engine. To fully correct this, you should look into installing a dashpot under the throttle lever, which slows the closing of the throttle blades so the engine does not stall. This is not the fault of the carburetor; it's because automatic transmissions suck. Manual transmissions will not stall a correctly tuned carburetor, assuming you remember to step on the clutch pedal!

3) Choke Pull-Off

The pull-off is the most critical part of the automatic choke, so of course Holley decided not to include any adjustment instructions whatsoever! Anyways, this mechanism sets the choke gap at the best position for a COLD engine, and I don't mean 60 degrees cold. Therefore, the following check MUST be made during the winter or the coldest time of year you will run the engine at. If you ended up installing this in July, your best bet is to set the gap at 1/4" as explained later and leave it alone for a few months. You will be reminded to set the pull-off when the engine starts to run like ass in colder weather.

When the engine has been run, it can retain heat for many hours that will improve its next start on that same day. So the best thing to do is to bring your box of drill bits home with you in preparation for first thing in the morning, when the engine will be at its coldest. Before starting, open the hood and remove the air cleaner or intake elbow to expose the choke plate. Start the engine and allow it to run for 5-10 seconds, but don't let it get warm. Slowly open and close the choke plate until you find the SMALLEST gap (between the front of the plate and the aluminum air horn) that the engine runs SMOOTHLY at. Take your drill bits and find the one that matches this gap.

Later, remove the carburetor from the engine again. Trust me, it's far easier to work on it this way as well as be able to see the tiny adjustments you're making. Before messing with the pull-off, first check the relationship between the fast idle lever and the choke plate. Loosen the choke cap screws and rotate the cap counter-clockwise until the choke plate is firmly shut (open the throttle to let the fast idle cam clear). The lever on the fast idle cam should be just touching the choke housing; in other words, it should be at the maximum extent of its travel--look at the picture at the top of this page. If not, bend the choke rod so that the red lever and choke plate hit aluminum at the same time. This will ensure that the engine stays at fast idle for as long as possible. We want to make this adjustment first because it would change the choke-pull off setting if done afterward.

Remove the choke cap. Find the adjusting screw for the pull-off piston; if you look at the picture at the top of this page again you can see the screwdriver pointing at it. Back this screw out until it is flush with the top of its hole. Make a tool out of heavy wire that has the last 1" bent 90 degrees for the purpose of holding the pull-off piston down in its bore. Hold the piston down so it is bottomed out, while pulling the tang that the spring attaches to to simulate the spring tension against the pull-off. You should be able to do this with one hand.

Find the drill bit that's 1/8" larger than the drill bit that matched the gap you found earlier. Match this larger drill bit to the gap under the choke plate. If it is dead on or you have less than a 1/16" air gap between the drill bit and edge of the choke plate, you're fine. If not, you'll want to bend the lever inside the housing at the part where it's necked down. To do this, use strong needle-nose pliers and block the pliers against the choke housing with a small screwdriver handle, as shown:

Bend towards the left as shown to increase choke plate gap. In the unlikely event that you need to decrease choke plate gap, bend it to the right. It may occur to you that it would be easier to bend the little connecting rod into an S shape, but this offers very little adjustment before the rod will not slide through the piston properly.

Now--and this is very important--make sure you didn't get that little arm all screwed up. Pull the lever to pull the piston up where you can see the hole where the little connecting rod goes through, and wiggle the connecting rod with small needle-nose pliers or tweezers. The rod should have enough slack at the pivot to touch all sides of the hole in the piston. If not, carefully bend the arm in the required direction. If necessary, remove the nut so you can take the lever out and make sure the arm isn't twisted relative to it (the whole thing should be flat). If there is any binding whatsoever, the end of the rod will not seat in the piston correctly, vacuum will leak through that point, and the choke will not be pulled open to the prescribed setting.

Another thing. You might be looking at that vacuum piston sliding around in there, and thinking, "this would be a good place to put some light grease." DO NOT DO THIS! The piston relies on gravity to pull it down against the rod before vacuum will grab it, and because it's so lightweight, greasing it will retard its free movement, resulting in failure to seal to the connecting rod at the moment of truth and causing you to pull your hair out when the engine starts fine one day and acts crazy the next. Even a drop of oil on the connecting rod pivot point will cause unwanted stickiness.

If at any point you want to remove the piston to straighten something out (or clean out grease!), you can knock out the rivet above the piston with a hammer and small punch and work the piston out with a small screwdriver. Don't forget to remove the choke housing and hammer the rivet back in afterwards.

Once you get a snug fit of that drill bit (gap found experimentally + 1/8") between the choke plate and air horn while holding the piston to the bottom of the bore AND holding back on the arm inside the housing, try it out with air. A vacuum pump will not work here due to the volume of air needed to overcome the normal leakage around the piston. Instead, put a clean vacuum hose on the fitting, and while holding the spring tang to the left with light finger pressure, suck like Kanye West at a children's charity auction. The choke plate (closed) should quickly open to exactly the right gap, which you can check with that drill bit if you're quick enough.

Finally, turn that adjustment screw back in one turn. This should put your pull-off close to the right place. Each turn of the screw results in roughly 1/16" change to the gap, so you can adjust it that much one way or the other. Turn the screw in to close the gap and make the engine run richer, turn out to open the gap and make it leaner. The screw is pointed and the piston has a bevel on the bottom, so you can even adjust it while the engine is running without damaging anything.

Reinstall the carb without turning it upside down, or the piston may get stuck at the top of the bore. For this reason, any time the vehicle is turned upside down and will not start afterwards, you can try rapping on the choke housing with a screwdriver or tapping the choke plate to knock the piston back down.

Now when you go to start the engine, in any temperature down to freezing, it should promptly fire up and idle smoothly with nothing more than a tap of the accelerator to set the choke. If it runs smooth for a few seconds and begins to stumble, or if the engine stumbles while driving, pull-off needs to be leaner. If it starts and dies, pull-off needs to be richer or fast idle increased. When the temperature gets below freezing, you will need to start adding fuel with the accelerator pump to supplement the choke before cranking. The amount will vary from engine to engine and based on temperature, but by the time you get down to -20° F, you should be adding 2 full pumps. If it gets colder than that, put a block heater on the damn thing before you shatter a piston.

4) Electric Heated Choke Cap

This is pretty straightforward. Rotate the cap counter-clockwise to make the choke come on at a warmer temperature and stay on longer; rotate the cap clockwise to make the choke come on at a colder temperature and come off faster. Basically, make the choke as lean as possible (clockwise) and still have the engine start and run well. It is better to have the choke too lean than too rich; if the engine runs rich the cylinder walls will be washed with gasoline, plugs will foul, fuel economy will suck, and you will be unpopular with anyone who walks by your exhaust.

In any event, below about 70 degrees the choke plate should be fully shut. If the engine is being choked at an unwanted time ABOVE this temperature, rotating the cap clockwise may take care of it. However, if the engine does not start properly below this temperature, THE CHOKE CAP HAS NOTHING TO DO WITH IT! The pull-off is responsible for the initial opening and the cap does nothing but determine how long it takes for the choke to OPEN beyond the gap set by the pull-off. If you are rotating the cap with the engine running and find that it alters the amount of pull-off, either something is screwed up in the pull-off or your engine makes piss-poor vacuum.

There should not be any "summer" setting or "winter" setting or any such foolishness. If you can't find a setting that you're happy with, where the choke does not close completely and opens fast in the summer yet stays on long enough to maintain a good idle in the winter, then install the heat sensor mentioned earlier. It bolts to the manifold and replaces the ground wire to the choke cap. When the manifold is cold, it restricts the current that heats the spring, thus amplifying the difference in summer and winter operation. In the picture below you can see the small black box to the left of the distributor. Pardon the ghetto wiring on the other side, I haven't gotten around to straightening that out yet.

5) Off-Idle Drivability

The only purpose of a choke is to make the engine idle. If the carburetor is choked while the engine is not operating on the idle circuit, a pig-rich mixture results that prevents the vehicle from being driven normally because it stumbles. As mentioned earlier, Holley's design seems to be for air velocity to pull the offset choke plate open, which does not work too well when velocity is killed at low speed and high load. Increasing the pull-off alleviates this somewhat, but the choke can only be leaned out so far before hurting the ability of a cold engine to stay running with the throttle closed.

As a solution, I removed the fast idle lever, cut a strap out of 16-gauge steel and brazed it to the de-choke tang. Then I curved it to form an arc around the throttle shaft, and trimmed it to a total extension of about 3/4". The result:

Here, the choke housing has been removed, leaving the fast idle cam, to show its relationship to the modified fast idle lever.

As you can see, when the throttle is opened, the throttle shaft rotates counter-clockwise, the protrusion rotates the red cam clockwise and holds it there, and the red lever pulls the choke rod down, opening the choke plate. Because there has to be enough slack to allow the choke plate to shut completely for starting while having the throttle at fast idle at the same time, there is still a small window just off idle where the engine MIGHT run rough, but once you lay into the throttle the choke is popped open and rubber is burned.

If you have any drivability issue after installing one of these chokes, I highly recommend this modification. Even doing it just for the hell of it has no disadvantage that I can see. If you don't know how to braze, find someone who does. I wouldn't use anything thicker than shown, since it might rub against the round part of the fast idle cam and get the throttle stuck open. Getting the arc and length just right is a trial-and-error process; start a little long and gradually shorten it, installing it and the choke housing to see if it rubs and whether it is short enough to let the choke plate shut completely. Any shorter than absolutely necessary is detrimental!

Aside from this change to allow the choke to be mechanically opened by the throttle shaft, you will have to keep increasing your pull-off setting by turning the adjustment screw out a little at a time until you find a good compromise between drivability and reliable idling. If you notice an increased tendency to stumble as the engine warms, turn the choke cap leaner as well (or remove that damn temperature sensor if you installed it just for the hell of it). You may not be able to completely eliminate a light stumbling at low throttle, but you should be able to reduce it to a non-issue. It can be completely eliminated by driving one gear lower than usual while the engine is cold.

6) Verify Cold Performance

This is a typical scenario when a correctly tuned engine is started at a temperature of around 20 degrees.

After pressing the accelerator halfway, the key is briefly turned and the engine immediately fires and revs to a smooth idle of 1,200 RPM. A tap of the accelerator kicks the idle down to 1,000 RPM. The engine can be revved by pressing the accelerator with no problems; punching the throttle wide open may result in a hesitation but under no circumstances will the engine stall.

Putting the transmission in 1st gear and very slowly engaging the clutch, the engine should idle strong enough to pull the vehicle into motion without any shuddering. When the vehicle is stopped at the end of the driveway, the engine does not stall. After pulling out onto the road the engine briskly accelerates without any jarring motion being felt during gear changes. Part-throttle stumbling under load should be very minor if not nonexistent.

The engine can be shut off at any time during warmup to go into a store, and when restarted without pressing the accelerator it resumes a smooth idle. After five minutes of driving the needle on the coolant temperature gauge is beginning to come into range, and if the carburetor is checked at this time the choke plate is almost open or fully open. Idle will have dropped to 700 RPM.

Bottom line, if it "runs like it's carbureted" IT'S NOT PROPERLY ADJUSTED! There should be no difference from an identical fuel-injected engine; either system of fuel delivery is well tuned or it's not.

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pretty good.

the thing for those newbys to remember is that not all of the castings have the bleed hole drilled in the casting....that gets covered and ducted into the round canister...

I need to stress that you DO NOT use the little black box that bolts to the manifold until you've tried using the properly adjusted choke and you're absolutely certain the choke needs to stay on longer. I removed this heat sensor and the minor stumbling at part throttle was completely eliminated. So that was a waste of $40 or whatever the thing cost. I don't know if the modification to pop the choke open with the throttle was really necessary, but it can't hurt anything, so it's still there.

Again, install the choke kit AS IS (with the separate fast idle lever for a 4412) and do your best to read everything I wrote and set it up correctly before changing anything. It's very easy to have it adjusted incorrectly and think there is something wrong with it.

Anyways, I haven't monkeyed with the choke since last winter. The engine starts and idles perfectly no matter the temperature with only an initial tap of the accelerator.

I didn't realize there was a Tech Articles section on this site. If a moderator wants to move this over there, feel free.
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