Adjusting Automatic Chokes: A Semi-Universal Approach
Automatic chokes on automotive carburetors are not new technology; they’ve been around for decades. Still, they’re a continuing source of frustration for many enthusiasts. Too often, automotive hobbyists end up removing the automatic choke; or defeating at least some of the functionality of the automatic choke because they don’t know how to make the engine run “right” and still have the automatic features of the choke enabled. If there was one “typical” issue, it would be that the engine starts OK, but almost immediately begins to run really bad—rough and blubbering—so much so that the engine may even stall. The most common mistake I see—about three-quarters of all automatic choke problems—relate to people who adjust the bimetallic choke coil, when they should be adjusting or replacing the vacuum-powered choke pulloff instead. I’m going to repeat that general theme several times—and, yes, it IS that important. But of course there are other problems as well; and I’ll deal with most of them in this article. None of what follows is intended to replace the manufacturer’s instructions. Since “official” procedures and specifications may be difficult to find, though, I’m offering a common-sense “universal” procedure.
The first thing you need to know: It’s going to take longer to read this than to walk out to the driveway, lift the hood, and adjust the damn thing—at least to perform the baseline adjustments. It’s not rocket science; and it can be tricky (and sometimes time-consuming) but it’s not hard. You may need some “special tools” depending on your carburetor, but in general a selection of screwdrivers, a batch of drill bits to gauge distances with, a hand-operated vacuum pump with a vacuum gauge, and some kind of linkage-rod bending tool is about all you’ll need.
The worst part of adjusting the choke is that only baseline adjustments can be made in heated comfort. Baseline adjustments will get you close. Maybe even “close enough”. But genuine fine-tuning of the choke will mean getting up an hour early in the dead of winter so you can SEE how the choke is working first thing in the morning when the engine is at it’s coldest. Dress warm, but gloves just make things more difficult!
I will assume several things about your vehicle:
The engine is in good operating condition. You have adequate cranking compression and cranking vacuum; proper fuel supply, the ignition timing is correct, the exhaust is not restricted except by a properly-functioning heat riser valve (if used), your carburetor is adjusted for proper curb idle speed and idle mixture; and in all other ways except for initial starting and warm-up the engine runs acceptably well.
The carburetor has fuel in the fuel bowl before cranking. Many carbs have recognized issues with casting porosity; or improperly sealed well plugs. If you’re leaking fuel into the intake manifold when the engine is shut off, it’ll be no surprise that the engine can be overly-rich when re-started even with the choke functioning properly. Vehicles that “heat soak” after being run may boil all the fuel out of the fuel bowl. An engine with no fuel in the carb will be hard to start even with a perfectly adjusted choke. You MUST assure that the carb has fuel in it if you have an engine that cranks a long time before starting! An electric fuel pump energized with the ignition can alleviate the problem of an empty float bowl before cranking.
The starter motor will crank the engine at a “normal” speed. If the engine is slow-cranking because of an electrical fault, it may be hard-starting even with a perfectly adjusted choke. Remember that slow cranking develops less cranking vacuum, which pulls less fuel from the carburetor; it develops less cranking compression, and low battery voltage during cranking may weaken the ignition spark somewhat.
Your engine does not have an extremely radical camshaft. Stock, mild, or moderate camshafts will work fine with an automatic choke. The issue here is that the vacuum-powered choke pulloff requires some minimal amount of vacuum to operate; and a wicked camshaft may not generate enough vacuum at idle. If you can run a power brake booster you can certainly run an automatic choke. Even if you don’t have enough vacuum for power brakes—you may still have enough vacuum for an automatic choke. Keep in mind that a cold engine will have a faster idle speed; and therefore generates more vacuum than at curb idle. If you have enough vacuum to operate the choke pull-off, you should be able to use an automatic choke on your engine.
You’re using winter-blended fuel if you’re in a climate cold enough to require it. The gasoline sales distributors do this automatically in any climate cold enough to justify it. It also means that if you’re still nursing a tankfull of “summer” gasoline in Coldest January, the gasoline may be the problem rather than the carburetor. “Winter” gasoline vaporizes easier than “Summer” gasoline; this is desirable in the cold but undesirable in warm weather because it can create vapor-lock problems.
You know how to start a carbureted engine. Not so far fetched; at this point there are a LOT of younger folks who’ve never driven anything but fuel-injected cars—so the morning ritual of tickling or pumping the gas pedal BEFORE cranking the engine is totally outside of their experience. I’ll deal with this some more at the end of this article.
“Cold” is a relative term. I say “God Bless Global Warming” ‘cause it’s made my hometown much more livable. Around here, -30 in January was simply no big deal thirty or forty years ago. If I say “cold” without a reference to a temperature, it means the ENGINE is about “room temperature”; about 65—to—70 degrees Fahrenheit, so a “cold” engine is just one that hasn’t been run for awhile. All temperatures listed will be in the Fahrenheit scale.
And now we’re ready to dive into the heart of the matter!
An automatic choke requires FOUR adjustments to be correct. And they are somewhat interrelated.
The choke coil must be adjusted so that it OPENS the choke blade at the proper speed as the engine warms up. The choke blade must open slowly enough to prevent lean stumbles; yet it must open fast enough to prevent over-rich mixtures that accelerate cylinder wall wear, oil contamination, and increase harmful exhaust emissions. Please take special note—the choke coil must be adjusted based on the speed of the choke OPENING. Do NOT adjust the coil based on how you think the choke blade should CLOSE! If the choke blade opening speed is tuned to the rest of the engine combination, the engine should run beautifully whether cold, warming up, or fully warm.
The vacuum-powered choke pulloff must pop the choke blade open a little bit as soon as the engine starts. The pulloff adjustment is CRITICAL to engine operation immediately upon starting; and for perhaps thirty seconds or a minute afterwards. If the pulloff is defective or incorrectly adjusted, the engine will usually gargle on gasoline—“blubbering”—even to the point where it will stall from being too rich. Depending on the adjustment it could hesitate, pop back through the carb, or even lean-stall from having too little choke function. Demon carbs do not have a choke pulloff; instead Demon uses openings cut into the choke blade. In effect, the choke blade cannot shut completely. I consider this a penny-pinching shortcoming, and it limits the adjustability of the choke mechanism.
The fast idle cam must be synchronized to the choke blade position. There is a linkage rod from choke blade to fast idle cam; and the critical issue is that when the pulloff pops the choke blade open a little bit, tickling the gas pedal allows the cam to rotate so that the fast idle screw will drop from the highest (fastest) step and align with the second highest (second fastest) step of the fast idle cam. Because the fast idle cam is synchronized to the choke blade position when the choke blade has been opened by the pulloff—changing one adjustment may mean you have to re-adjust the other.
Of course, this doesn’t apply to Demon’s electric chokes! With a Demon, you just have to wait for the choke to heat before it will drop the idle speed. With no choke pulloff, there’s no mechanism to allow the fast idle cam to drop to the second step except to have the choke coil heat up and relax a little bit.
When the throttle is Wide F’n Open (WFO) the choke blade will be forced partway open; and usually just a bit more open that what the choke pulloff would open it. This allows you to clear a cold but flooded engine—you whack the gas pedal to the floor, and it mechanically opens the choke blade. As you crank the engine the turbulence of the lean incoming fuel/air mixture may blow the liquid fuel off of the plugs so the engine can start.
Get those four adjustments correct and your automatic choke problems are almost certainly behind you.
There are two main locations for choke coils, “Divorced”, and “Integral”. There are also several ways to heat the choke coil.
“Divorced” chokes have the choke coil mounted on the intake manifold; a linkage rod connects it to the rest of the carburetor.
In some cases the coil tension is adjustable; in others
bending the linkage rod makes the adjustment.
The lever on the carburetor may have two holes for the linkage rod to
fit into, a “summer” and a “winter” position.
In any case, the exhaust crossover in the intake manifold heats the
coil. The hot exhaust gas passing across the intake manifold heats the
crossover; and that in turn is modulated by the heat riser valve (sometimes
called the Early Fuel Evaporation (EFE) valve.)
Divorced choke coils seem to be the hardest to work with on modified
engines, mostly because so many things that are done to improve performance
involve removing heat from the intake manifold—and then the choke coil
operation suffers. If you have a
carbon-plugged (or deliberately plugged) exhaust crossover in the intake
manifold, the divorced choke won’t work.
You must chisel the carbon out or start looking for an
electrically-heated choke coil. There is
a company selling divorced choke coil kits that have an electric heater— http://www.carburetion.com/quadrajet.asp
(link verified 07 April 09—the electric conversions for divorced chokes are
near the bottom of the page.)
An “Integral” choke coil is attached to a plastic or phenolic cap that sits on the side of the choke housing right on the carburetor.
They can be heated with hot air, an electric element, or engine coolant. In general, adjustment is simple and easy—you rotate the coil cap on the choke housing. One direction will increase the choke coil tension making the choke “richer”—it stays engaged longer. The other direction relieves coil tension making the choke “leaner”—the choke comes off sooner. There is a series of marks on the metal choke housing, and a line or an arrow molded into the choke coil cap. The middle mark on the housing is the “index” mark; usually it’s bigger than the others. The choke below is adjusted about 2 ½ notches “rich” causing the choke blade to take longer to open. Rivets instead of screws attach some integral choke coils; you’d have to drill the heads off the rivets and then knock out the rest of the rivets to service the coil, and then re-attach with either new rivets or tap threads into the housing for screws.
Hot air chokes have a controlled vacuum leak into the choke housing that pulls clean, heated air from a “stove” either in the exhaust crossover of the intake manifold or (sometimes) in the cast iron exhaust manifold. If the heat stove tubing attaches to the choke housing (the hot side of the tube) with a piece of hose instead of a flare and metal tube nut, you MUST have special high-temperature silicone hose or the hose will cook!
Again, a blocked exhaust crossover in the intake manifold
can cause the choke coil to never warm up; and a missing or defective heat
riser valve will slow down the choke opening.
Another common problem is that the heat stove in the manifold may
corrode and perforate. If this happens
not only do you have some exhaust gas diluting the fuel/air mixture; it will
also carbon up the inside of the choke housing.
Replacing the hot-air choke coil with an electrically heated one is
simple and fairly inexpensive as long as you also block the hot-air vacuum
passages, and route power to the choke heater.
Some carbs (
An electric choke is similar to the hot-air choke; but there is no vacuum passage drawing hot air around the choke coil, and no hot-air stove plumbing. An electric element heats the choke coil. The electric element receives power through the vehicle wiring harness. Most folks who convert to an electric choke just apply power any time the ignition is powered, and that can work OK. It’s best if that power is NOT applied while cranking the engine; and I prefer to also route the power through an oil-pressure switch so it’s interrupted if the engine stalls.
One wire operates the oil pressure light on the dash, the other two are the pass-through for the choke coil heater. No oil pressure—no electric power to the choke coil.
There’s no point to heating the choke before the engine starts, and that could happen if you have an extended cranking time; you also don’t want the choke to continue to heat if the engine stalls while still cold. An electric choke won’t work properly if the battery isn’t being properly charged. Be aware that there are thermostatically controlled voltage-reducing switches that can be mounted on the engine; they regulate choke coil heater voltage based on engine temperature. In general, they are NOT needed, but Chrysler has used them on some engines; and Holley sells an aftermarket unit as part number 45-267 “Electro-Dyn Heat Sensor” for about $45. DO NOT buy one until you’re CERTAIN that the four items I’ve outlined above are adjusted properly. You will probably NOT need the outboard voltage reducer.
A few carburetors were built with a water passage in the choke housing; heated engine coolant warms the choke coil. The choke won’t work properly if the coolant is low or you have a low-temperature thermostat in the engine—but you may be able to adjust the coil to compensate for a cool-running engine.
WHAT YOU NEED TO KNOW TO ADJUST THE CHOKE COIL
The choke blade will shut tightly at 65—70 degrees. It will shut forcefully at 50 degrees. This is NOT a problem; and if you’re adjusting the choke coil (or the linkage on a divorced coil) to prevent it from fully shutting the choke blade because the engine gargles on gasoline when FIRST STARTED, you’re adjusting the WRONG THING. This is the biggest, most common problem I see with automatic chokes that “just don’t work right”. People adjust the choke coil when the problem is with the choke pulloff.
With a hot-air, electric, or water-heated choke, start the choke coil adjustment by turning the mark on the choke coil to the factory-specified position; or if you don’t have a service manual, use the “index” position on the housing.
A divorced choke coil may or may not be adjustable; check yours to see. If it’s not adjustable, take your best guess on the factory bends on the linkage rod, keeping in mind that the choke blade should be closed at ~70 degrees.
You may need to go a few marks in either direction (modified “performance” engines will usually but not always be turned in the “lean” direction) as “fine tuning”; but the factory specification or the index position is the starting point. The choke blade WILL snap shut like an alligator eating frozen chicken. That’s NOT a problem as long as the choke pulloff works and its adjustment is correct. Turn the choke coil richer if the engine hesitates, lean-pops, or lean-stalls AFTER it’s been running for thirty seconds to a minute. Turn the choke coil more lean if the engine is over-rich AFTER it’s been running for thirty seconds to a minute. If your engine is too rich or too lean during that first thirty seconds to one minute, (especially in the first ten seconds) adjust the choke pulloff NOT the choke coil!
If the problem you have is that the choke blade won’t open all the way, so the choke blade is still partly closed long after the engine has warmed up—the issue is almost certainly a lack of heat to the choke coil. You need to figure out how the choke coil is heated (electric, water, hot air, or hot intake manifold) and then determine why it isn’t being warmed like it should be. Blocked exhaust crossovers—either deliberately blocked for “extra power” or blocked by carbon deposits—are the biggest problems for divorced or hot-air chokes; but missing or defective heat riser valves account for some troubles, too. An intake manifold exhaust crossover that is warmed the way the OEM expects it to be—heat riser valve properly operating, and no carbon clogging—will not be merely “hot” or even “extremely hot”. It will be SCREAMING HOT. Blown fuses or other wiring problems affect the electric chokes although a defective heater element sometimes causes problems.
As soon as the choke coil has heated enough to relax so that the choke is opened more than what the pulloff opens it—the pulloff adjustment no longer affects how the engine runs. So, the pulloff only controls how the engine runs for the first thirty seconds or a minute—somewhat depending on just how cold the engine is, and how “fast and powerful” the choke coil heating mechanism is.
There are two main kinds of choke pulloffs. The easiest ones to see are the kind with a rubber diaphragm inside a metal or plastic housing, mounted to the outside of the carburetor. These are sometimes called a “Vacuum Break Diaphragm”. This version is very common on many different kinds of carburetors.
This external pulloff has a very convenient screw-type adjustment. Note also that the choke coil is held on with rivets! Clearly, the manufacturer doesn’t expect that you will need to adjust the choke coil, but adjusting the pulloff couldn’t be easier.
Another kind of pulloff uses a metal piston inside a
cylinder machined into the metal choke housing.
Both types rely on manifold vacuum to operate them; the vacuum pulls on the diaphragm or the piston to move the linkage connecting it to the choke blade. The diaphragm or the piston is powerful enough so that a reasonable amount of vacuum will provide enough force to overpower the choke coil and thus pull the choke blade open. The amount the choke blade is opened is adjustable, often by bending a linkage rod or lever, but sometimes you get lucky and there’s a nice, convenient adjustment screw.
A few carburetors use two choke pulloffs; one is a primary that goes to work as soon as the engine starts just like a carb with only one pulloff. The other is a secondary pulloff that opens the choke a little further than the primary; it’s often time-delayed or has a tiny, controlled vacuum leak so the engine has to run a little longer before it activates. The choke pulloff is then a two-stage deal, opening “some” right away and “more” a little later.
WHAT YOU NEED TO KNOW TO ADJUST THE CHOKE PULLOFF(S)
If the engine runs bad during that first thirty seconds or a minute, you’re probably dealing with a pulloff problem not a choke coil problem. Mis-adjusted or defective choke pulloffs are by far the biggest single problem I see with automatic chokes, and too many people adjust—and re-adjust—the choke coil trying in vain to fix the problem with the pulloff.
First, test the pulloff (where possible) by applying an outside source of vacuum. A diaphragm-style primary pulloff usually should not leak vacuum. Some secondary pulloffs will have a controlled vacuum leak; often you can find and plug that calibrated leak with your fingertip or a small piece of electrical tape, and test the pulloff. Metal pulloff pistons will leak vacuum; but they must move freely—they must not be seized or binding.
This is a very “busy” photo. Note that the small thermometer is reading a chilly 20 degrees; the vacuum pulloff (diaphragm type) is holding vacuum—but it only takes about 4 inches of vacuum (as seen on the large vacuum gauge) to fully seat the pulloff against the tension of the choke coil. I’m gauging the amount it opens the choke blade with a drill bit (but for accuracy in measuring, the bit should be on the LOWER edge of the choke blade on a Quadrajet—it shows up more clearly in the photograph when it’s on the upper edge, though.) Use the upper edge of the choke blade for Holleys and Carters.
Some Holley carbs have diaphragm-style pulloffs; the common
“performance” or “aftermarket” Holley carbs (and some older
You need to figure out how the pulloff adjustment is supposed to be made—do you bend a linkage rod or lever; or are you supposed to turn an adjustment screw somewhere? A reasonable baseline adjustment would have the edge of the choke blade about ¼ inch from the wall of the air horn. A common drill bit can be used to gauge the distance—although—some carburetor manufacturers suggest the use of a special protractor (angle-finding) tool.
If the engine gargles on gasoline or rich-stalls during that first minute, adjust the pulloff to open the choke blade a tiny bit more. If it hesitates, lean-pops, or lean-stalls during that first minute, adjust the pulloff so it doesn’t open the choke blade as much. Be careful, a tiny change makes a big difference!
Special note for Holley Carbs:
SOME Holley integral choke pulloffs are adjusted (the “Choke Qualifying adjustment”, in Holley terminology) using a “Special Tool”—a paper clip with one end bent; as detailed on page four, figure 15 of this .pdf link:
http://holley.com/data/Products/Technical/199R8339.pdf (Link verified 07 April 09)
The common integral electric chokes on the Holley performance carbs and the Holley electric choke conversion kits use an adjustment screw having a tapered end. The tapered end contacts the similarly tapered bottom edge of the pulloff piston. Adjusting the screw changes the maximum travel of the piston. The screw is factory-sealed with some kind of “caulking”, but that is easily dug out with a tiny screwdriver and a pick. Turning the adjuster screw counter-clockwise (out) will allow the choke blade to be pulled open farther; turning the screw clockwise (in) will keep the choke blade more closed.
Brass Holley pulloff piston showing taper on bottom edge; and tapered-end adjusting screw.
Holley seals the pulloff piston adjusting screw under the caulking.
Caulking removed; and the adjuster screw turned way too far in, as seen with the end seal pried off of the choke housing.
Once you have gotten the choke pulloff adjusted, re-seal the screw with your choice of sealer. RTV silicone may work if it doesn’t get gasoline on it. Heck, you could probably use a little bit of Teflon-paste pipe-thread sealer, bubble gum or household caulking blorted over the top of the adjuster screw. I would not use a thread-locking compound as sealer or you’ll never be able to adjust it again.
Most carburetors use a curb-idle screw; and a separate fast-idle screw and fast idle cam. Some carbs use a single screw, when the cam is rotated to it’s lowest (slowest) position, the carb is at the curb-idle speed. In either case, as the choke blade opens, the fast idle cam also rotates to provide progressively slower idle speeds.
The fast idle cam must be synchronized to the choke blade position so that when the choke blade is popped open by the pulloff, you can tickle the gas pedal and the fast idle speed is reduced. It won’t reduce it all the way down to curb idle when the engine is first started; but you will go from the highest (fastest) step of the fast idle cam to the second highest step. If the fast idle cam isn’t properly synchronized, you may not be able to kick the fast idle down until after the choke has warmed (annoying, and hard on an automatic transmission when you drop it into gear at the highest fast idle speed); or you may kick down past the second step to the third or fourth step; and then the engine stalls when put into gear.
WHAT YOU NEED TO KNOW TO ADJUST THE FAST IDLE
Once you have the pulloff adjustment reasonably close, determine how far the choke blade is opened by the pulloff using your drill bit set. Then with the proper sized drill bit in place, you can hold the choke blade steady while you verify the fast idle cam position. Adjustment is nearly always by bending the linkage rod or by bending a tab. Some idle cams don’t have discrete “steps”; instead they have a smoothly tapering “ramp”. In that case, there’s usually an index mark or arrow (>) stamped into the cam to indicate where the screw should touch when the choke blade is popped open by the pulloff.
Since the fast idle cam adjustment is based off of the choke blade position when the pulloff opens the choke that little bit—adjusting either of these items can cause the other to be out-of-adjustment. You may have to go back-and-forth to get them both correct.
The choke unloader provides a means to clear a flooded engine by mechanically forcing the choke blade partway open. In general, nobody screws with the factory adjustment and therefore it almost always works just fine. Most folks don’t know the unloader exists. Doesn’t hurt to verify that it works, though.
With the choke blade gently pushed all the way closed and the fast idle cam on the highest step, open the throttle fully. You should feel the choke blade being forced partially open as the throttle is opened. With the throttle WFO, the choke blade should open at least as much as the pulloff would open it (about ¼”) but probably it will open a little more than that. This is not a real critical adjustment; if it’s close—leave it alone. Adjustment usually involves bending the tab on the throttle shaft bracket.
Left: Choke housing removed. The fast idle screw is on the left side of this throttle shaft lever; the unloader tang is on the right side. Right: With choke housing attached, the unloader tang presses against the bottom of the red plastic fast idle cam when the throttle is opened.
HOW TO START AN ENGINE HAVING AN AUTOMATIC CHOKE
Starting a cold engine mostly depends on the actual temperature of the engine and the volume of the carburetor’s accelerator pump squirt! Clearly, a Holley double-pumper with the big “50 cc” accelerator pumps is going to squirt a lot more fuel than a single-accelerator-pump Quadrajet or AFB. What follows is a generalization, “your mileage may vary”.
Anytime you start the engine, you should at least “tickle” the gas pedal BEFORE cranking the engine. You need to open the throttle enough so that the fast idle screw is lifted off the fast idle cam; the fast idle cam can then rotate and the choke blade can close. If you don’t lift the fast idle screw high enough, the choke blade cannot fully close. “Setting the choke” typically will take less than ¼ of the gas pedal travel. Even if it’s a very warm day; or if the engine is still hot from having been run, tickle the gas pedal—the choke may not close, but you are still squirting a little fuel into the manifold via the accelerator pump, which will greatly aid starting.
If the engine is at ambient temperature of 50 degrees, ¼ pump is probably all that’s needed. At 32 degrees, try ½ pump. At 0 degrees, give it a full pump; and at –20 you may need two pumps before cranking. An engine in good condition, having oil viscosity suited to the temperature, with winter-blended fuel, being turned by a big enough battery and cranking motor (starter) WILL START AND RUN at –20. Put another way—Twenty Below Zero is NOT cold enough for an engine to fail to start. If it won’t start—there’s something wrong and you just haven’t found the problem yet. For an engine at –40, things are different. Some engines will start, some—maybe most—won’t. That’s why electric engine heaters are used in extreme cold weather!
As soon as the engine starts, tickle or tap the gas pedal to lower the fast idle cam from the highest step to the second-highest step before putting the transmission in gear and cautiously driving away.
In general, a carbureted engine with a properly-adjusted automatic choke will cold-start and run as good as a fuel-injected engine having a similar grind camshaft—but—the carbureted engine may produce more harmful tailpipe emissions in the process, and perhaps the cold-idle speeds will be higher. In the early years of fuel injection, stock-cam and stock-carburetor engines when properly adjusted were MORE reliable than some of the injected ones in extreme cold weather!
On-line links (verified 07 April 09) to selected “official” carburetor adjustment procedures including choke adjustment:
Demon electric choke installation/adjustment