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Tuning Carburetors: A Guide for People as Inexperienced as Me

Also see vintage car repairs. By David Zatz, Ph.D.

What you will need: golf tee, vacuum gauge, vacuum hoses, vacuum hose T for the same size, hand-held tachometer, slot-type screwdriver, maybe Phillips screwdriver, patience, courage.

Before you start, replace any cracked vacuum hoses, check hose routing against your service manual diagrams, and warm the engine up so the temp gauge is in its usual place. Make sure you get the right grade of hose — some can carry gasoline fumes, and require a special formulation.

Connect the vacuum gauge to intake vacuum; you can put in a T and share vacuum with another function, if you plan to use a vacuum hose inside the cabin, or use a port that supplies vacuum at all times. Ts and vacuum hose are sold by just about all auto parts stores, as are vacuum gauges. Vacuum gauges are usually large round dials with a narrow rubber hose leading out; most come with various plastic tips and fittings. Bob Lincoln wrote, “I’d hook the gauge up to the vacuum choke pulloff, which is a round metal case on the driver’s side of the carburetor. It’s a good place to measure, since it always has vacuum, and it’s okay to unplug it and plug in the vacuum gauge with engine warm, without disrupting anything. I wouldn’t plug in at the brakes, if it has power brakes.”

Make sure that the car is running on the regular idle — there’s a cam, that is a roundy thing, and a screw that hits it when the car is cold. When it’s warm, the screw misses the cam entirely. The screw that hits the cam is the cold idle adjustment, and it’s only used when the engine is cold, to keep it running faster; using a cam lets the engine slowly “idle down” as it warms up. The best way to warm up is by going for a drive; when you get back, the cam will have moved to the warm position, and the car will be running on the “warm idle” screw.

Caution: on my 1974 Valiant, the illustrations in the Hayne’s manual were wrong...!

On cars with two-barrel carburetors, there are two idle mixture screws. On cars with single-barrel carbs, there is a single idle mixture screw. It’s often the only screw that you can’t see both ends of (not including the bolts that hold the carburetor onto the intake manifold).

Bob Lincoln pointed out that if the idle mixture screw does little or nothing, the carburetor is probably clogged with deposits of dried gasoline and sludge, and it needs cleaning and rebuilding. You may be able to do this yourself, but it’s beyond the scope of this page.

Once the idle mixture is producing the highest vacuum it can, and you’ve turned the screw back in again a quarter of a turn after getting it that high, connect the tachometer. Mine is a two-wire type where one, where the positive/red one connects to the negative pole of the coil (the black cylinder that has a thick wire going to the distributor cap, which is the thing that has the thick wires that go out to all the spark plugs). The other lead goes onto any ground - an exposed screw, etc.

Once the idle mixture is set, set the idle speed set up. For the moment, we are ignoring the fast idle screw (the one that hits the cam), we want to set the one that’s being used now that the engine is warm. Set it exactly to the speed on the sticker under the hood by turning the screw in and out; one way is faster, the other way is slower.

John Dahlin wrote: "Use a vacuum gauge to adjust your carb idle mixture and you will nail it dead on. You should get about 21" of vacuum. I have a 273 V8 and adjust each side until I get the highest vacuum reading I can. My Valiant doesn’t run rich and I get over 20 mpg on the highway and about 15 around town."

Bob Lincoln wrote: “Mixture should be set with a vacuum gauge, not a tachometer. Take the screw out 1 1/2 turns (after warmup and with timing set correctly [and the vacuum timing advance, if you have it, blocked]), and set for the highest, steady vacuum that can be obtained with a smooth idle. Turn the screw in in 1/16 turn increments until it just starts to stumble slightly, then back off about 1/16 turn until it just idles smoothly. Then set fast and curb idles.”

“Scamp71” added that one must also set the fast idle speed, which is controlled by the screw that lands on a cam (a round thing with several flat spaces cut into it). As the car warms up, this cam turns so that the screw has several different positions. (The cam turns when you hit the gas a little to move the screw off the cam, which is why one way to slow down your idle speed is to goose the gas slightly). Cars that stall or race when warm may need the warm idle speed adjusted, while cars that stall or race when cold may need the cold idle speed adjusted. The fast idle speed is also specified on the underhood sticker or in various reference sources.

There is also an adjustment which can be made to the float level inside the carb, but that requires it to be taken apart (for most cars). Cars made before the mid-1970s will need to have the points checked or replaced; many people prefer to retrofit electronic ignition.

Setting timing

Now that the idle mixture and speed have been set, it is time to set the timing. This has to be done with the engine warm.

Unplug the vacuum hose that goes into the distributor (the thing with all the wires on top), and plug up that vacuum hose with a golf tee. (I have been known to cheat and pull off the other end of that hose and cap the vacuum source instead. Same effect, sometimes easier to reach. If you have a slant six, the distributor’s down there, and it’s a bit hard to reach the vacuum hose.)

Bob Lincoln added: A strong exhaust smell could come from a leak that no one has spotted, such as a cracked exhaust manifold or a rotted section. These cars pollute more and you can normally smell a little gas.

If the hose absolutely won’t come off, slice the end of it a little with a razor blade, carefully so you don’t hurt yourself; that’ll make it easier. You’ll need a replacement hose unless you’ve got at least an inch of slack, and can cut off the damaged end.

Now, follow the instructions on the timing light or in your car manual (for me, surround spark plug wire #1 with an inductive grip, and connect power leads to the battery). Top Dead Center is the middle of the metal plate, so if the legend and stripes are rusted out, hope the specifications call for Top Dead Center; otherwise you may need to figure out where the degrees are marked out by using pictures from the manual (and hoping they are accurate for your car), comparing to a similar car from the same year, or buying a replacement marker. On the slant six, the cylinder closest to the radiator is #1.

Our 1974 slant-six Valiant was a full 12 degrees off, resulting in severe pinging before we reset the timing, so this is important even if the car has been under the car of a mechanic. On this car, the bolt holding down the distributor clamp (you will almost certainly need a socket extension to reach it) was 7/16 inches. Several people suggested the slant six from this year would run better with two degrees of advance over the factory recommendation, and we implemented this suggestion without problems.

For 1963-69 cars, also see: Tuning the 1963-69 vacuum advance control valve for better power and economy

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