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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 (really!), vacuum gauge, vacuum hoses, vacuum hose T for the same size, hand-held tachometer, slot-type screwdriver, maybe Phillips screwdriver, patience, courage.

John Dahlin wrote: "Use a vacuum gauge adjust your carb idle mixture and you will nail it dead on. That's what I do, 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."

Likewise, 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 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.”

Connect the vacuum gauge to intake vacuum - I usually put a T in and share vacuum with the power brakes (actually, I usually just disconnect the power brakes, because they're not being used when I tune; but I am darned sure to reconnect them!). Ts and vacuum hose are sold by just about all auto parts stores, as are vacuum gauges - which are usually inexpensive. Be sure that before you do this, you replace any cracked vacuum hoses, check hose routing against your service manual diagrams, and warm the engine up nicely so the temp gauge is in its usual place.

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 (they use a cam because engine's don't go straight from cold to hot; there are places in between.) Don't warm up the engine by leaving it idle, go for a nice drive, preferably run some errands you'd have to do anyway. Then when you get back, the cam will have moved to the warm position, and the car will be running on the "warm idle" screw. We'll get to that later.

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). Caution: on my 1974 Valiant 225, the illustrations in the Hayne’s manual were wrong...!

Now that the idle mixture is nice so the vacuum is as high as it can be, and you've turned the screw back in again 1/4 of a turn after getting it that high, connect the tachometer. Mine is a two-wire type where one, 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. Your tachometer may be different.

Once the idle mixture is set, get the idle speed set up. For the moment, we don't care about 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 nice and 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.

SCAMP71@aol.com wrote:

In addition to the mixture and idle speed, you must set the fast idle speed, which is controlled by a 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 actually turns when you hit the gas a little to move the screw off the cam, which lets it turn, 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!

There is also an adjustment which can be made to the float level inside the carb, and the timing must be checked and adjusted. Cars made before the mid-1970s will need to have the points checked or replaced - preferably with electronic ignition.

Regarding timing: first, check to make sure that the engine is running at the right speed, and is nice and warm. Second, unplug the vacuum hose that goes into the distributor (remember the thing with all the wires on top?), and plug up that vacuum hose with the 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, but I know you can do it. If the hose absolutely won't come off, slice it open a little with a razor blade, carefully so you don't hurt yourself; that'll make it easier. Of course you'll need to have a replacement hose ready unless you've got at least an inch of slack, and can simply cut off the damaged end.

Now, follow the instructions on the timing light or in your car manual. By the way, Top Dead Center is the middle of the metal plate, so if the legend and stripes are rusted out, you're OK as long as the specifications call for Top Dead Center! Also, on the slant six, the cylinder closest to the front (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. Also, on this car, the bolt holding down the distributor clamp was 7/16" — no metric system on American cars of that time! (Why not 3/8" or 1/2"? We'll never know!).

Good luck!

Bob Lincoln added:

A strong exhaust smell could be a leak, such as a cracked exhaust manifold or a rotted section that no one spotted. But these cars do pollute more and you can normally smell a little gas from their operation.

If the idle mixture screw does little or nothing, probably the carburetor is clogged with deposits of dried gasoline and sludge, and it needs cleaning and rebuilding. Get a good rebuild kit.

A vacuum gauge is a large round dial like a tire gauge, with a narrow rubber hose leading out. It comes with various plastic tips, nozzle and T-shaped fittings to hook up to the vacuum hoses on the car.

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.

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

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