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Slant Six Tuneup Tips

Also see vintage Plymouth, Chrysler, and Dodge car repairs (at allpar) and Valiant repair tips.

First, hit the parts shop if you need to, to stock up on supplies and instruments (mainly, vacuum hose, T, and gauge; screwdriver; tachometer.) 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. Then connect a vacuum gauge to intake vacuum, using that T and vacuum hose.

Make sure that the car is running on 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.

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...! We're only going to deal with the slant six in this article.

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.

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. Then there's the fast idle, or the screw that hits the cam which we talked about earlier. If the cold idle is too fast or slow, that's your culprit. Adjustment is fairly obvious for that.

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!). (We’ve been told by numerous owners that advancing the timing a few degrees usually increases gas mileage — move it back again if you get pinging.)

Replacing Chrysler slant six spark plugs and spark plug tube gaskets (at allpar).

Bob Lincoln wrote:

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.

Also see slant six upgrade tips

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