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Tuning Carburetors: A Guide for People as Inexperienced as Me

Cars built in the 1970s (at least, after 1972) and 1980s tend to have numerous vacuum hoses, and over the years, shadetree and “professional” mechanics have usually left some off and re-routed others. Before you start work, replace any cracked or missing vacuum hoses, and check vacuum hose routing against your service manual diagrams (or, in most cars, a color sticker under the hood somewhere). Make sure you get the right grade of vacuum hose when you replace them — some carry gasoline fumes, and require a special formulation. When in doubt, get the pricier but safer gasoline-safe hoses.

Vacuum hose, gauges, and Ts are sold by most auto parts stores. Vacuum gauges are usually large dials with a narrow rubber hose or metal hose grip leading out; most come with various plastic tips and fittings.

Warm the car up by driving it around for a bit, then 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. 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 [on cars that have them]. 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.”

cold idle cam

Make sure that the car is running on the regular idle — there’s a cam (a roundy thing), and a screw that rests on 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 #1: on my 1974 Valiant, the illustrations in the Hayne’s manual were wrong. Caution #2: the Holley 1945 shown here was the wrong one for the car; the main difference is the lack of an evaporative emissions hose.

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 often has a spring between it and the carburetor to increase friction and prevent movement.

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 needs cleaning and rebuilding. This, and float adjustment/replacement, are another story.

idle mixture screw

warmupOnce the idle mixture is producing the highest vacuum it can, and turn the screw in by a quarter of a turn, and 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 body bolt, the negative terminal of the battery, etc. If it doesn't work, you haven't found a good ground.

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. On my two barrel, I 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 idles smoothly. Then set fast and curb idles.”

The carburetor pictured is not correct for the car and has been replaced by a NOS Holley 1945.

I generally set the fast idle speed, which is controlled by the screw that lands on a cam (as described earlier), a few times a year to match temperature changes; the colder it gets, the faster I want the fast idle, within reason.

The cam is connected by a straight piece of heavy wire to a very basic thermometer — a wound piece of metal that has one type of metal on one side and another type of metal on the other, so that the difference in contraction between the two metals twists the screw as it gets cold, and pulls the cam. Thus, as the car warms up, the cam turns, so that the screw lands on one of several different positions — or misses the cam entirely. (The cam only turns when you hit the gas a little, which moves the screw off the cam, which is why, after you start the car and let it race for a minute, goosing the gas pedal slightly cuts the idle speed). 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, and quite a few vintage cars already have it. Electronic ignition does not need to be adjusted, but often the ballast resistor on Chrysler systems needs to be replaced or the car won’t start.

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 (it was two degrees over when we retuned the next time, and we didn’t have to touch anything).

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