How To Adjust the Idle Mixture on your Imperial


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Question from Tim (1956):

1956 354 hemi w/4-barrel Someone tell me the correct adjustment on the 2-mixture screws on carb. Is it screw in and back out 3/4 of a turn for each or 1- 1/4? I cant remember. Also, what is correct for timing? Clock wise to retard or counter clockwise.


From Eric:

The procedure for idle mixture screws is to use a vacuum gauge and adjust each to the highest vacuum reading alternating from one screw to the other. Connect the vacuum gauge to the vauum port that supply's the distributor's vacuum advance.

From Bill:

Generally, the port that supplies vacuum to the distributor "taps" in above the throttle plates, meaning that at idle there is no vacuum "signal". I have always plugged the gage to a port that supplies manifold vacuum (always "sucking") while the engine is running.

From Roger:

When checking fuel idle screws take them out and look at them. They should be smooth taper to the point. If they have been over tightened they will develop a ring around them. If it has a ring they need replaced. Blow out the hole in the carb with air pressure, cover carb with rag or you will get gas all over. Then just touch the base when closing, back out 11/4 turns and it will start. Then follow Mr. Roddick's directions and you have completed the task. You should also have a minimum amount of vacuum when done. Check your Motor's Man. for this number. If low check for leaks.

From Rich:

I agree the best way is with a vacuum gauge - - but not hooked up to the distributor line. This is "ported" vacuum - it is low or off at idle, and comes in as you open the throttle. You need to hook to full vacuum. I also like to final-adjust the idle [both speed and mixture] with the car in drive because this is how the car usually idles in use, and sometimes a perfectly smooth idling car in neutral will get a little lumpy when put in gear.

Question from Gary (1965):

Can anyone explain the right way to adjust the fuel mixture screws on my 4 barrel. I have a '65 Imperial with a 413 and always seem to be fouling out the plugs. I've tried turning them both in until it starts to stall them back them both out until I get the max idle, but still doesn't seem to be getting it right.

Reply from Ken:

About your 413 problem, first though, a bit of trivia about the engine to help all listening to understand your problem, which others may also be experiencing without knowing it.  The 413 is in the wedge family of engines that Chrysler built up to the late 70's ( about the time Lee Iacocoa took over), they were dropped from production cars and only available in trucks until about 1975, (this is somewhat questionable due to production model runs, some earlier '75's may have a 413 instead of the newer 400).  In any case, being a wedge engine, there is considerable more compression, requiring a higher octane leaded fuel; factory specifications call for 98 or higher octane.  There are octane boosters on the market (very expensive) but they in and of themselves will not solve the problem.  A lead substitute is also needed.  These substitutes are questionable however due to the fact they are not always of the tetraethyl base as needed. 

Also, once you have put the proper gas in the car, you should acquire a good tachometer and follow the basic rules for carburetor setting as follows:

1.  Start on the right side of your carburetor.  Set idle adjustment to 500 rpm's.  Then back out right to the idle adjustment screw until the engine runs at its fastest setting (watch tachometer) and falls off 50 rpm's.  Turn it in 1/2 turn.

2.  Do the same thing on the left side, remembering to turn down the idle adjustment to 500rpm's before starting.

3.  Once both sides are correct, reset the idle adjustment screw to about 850 rpm's more or less by 50 rpm's, depending on whether or not you have (and use) air conditioning.

Also be sure that your timing is 12 1/2 degrees as is factory specifications.

One of the best methods I know of to tune these older engines is to tune them to themselves; that is to say, only start with the factory specifications and make adjustments as is deemed by your engine for its best performance.  A simple timing method is to advance past factory specifications and then put a distributor wrench in your front seat and take a drive.  Find a safe road and from about 20 mph put the pedal to the floor.  If the car pings, back down the timing just a little.  Do this until she pings no longer.  When this method is used, you are timing the engine to its own wear and tear which is different than to the factory specifications.

Remember, too, that the carburetor must be readjusted at this point.  The carburetor is always last in the tune-up operation.

This page last updated June 27, 2004.  Send us your feedback, and come join the Imperial Mailing List - Online Car Club