Edmunds Answers



  • karjunkie 11/08/08 6:09 pm PST

    Using a vacuum gauge and setting for the highest vacuum can cause hesitation and backfire. You cannot adjust the idle unless the engine is fully warm. When the engine is warm enough to properly set the idle, the choke butterfly will be in the vertical or wide-open position. It is important to understand the idle circuit to properly adjust the idle. Contrary to popular belief, the idle mixture control screws DO NOT adjust the mixture. The mixture delivered by the carburetor is controlled by the idle tubes (gasoline jets), and the idle air bleeds (air jets) in the carburetor. The idle mixture control screws control the VOLUME of the preset mixture. For BEST results, the clearance from the throttle plates to the throttle body will be about 0.020 (20 thousandths) at idle. To enrich the idle mixture, you must first measure the inside diameter of the idle tubes. One can then drill these tubes oversize but NO MORE than 0.005-inch increase in the diameter. The first attempt may be made at plus 0.002 inch. If this is not sufficient, then subsequent attempts should be made in 0.001-inch increments not to exceed 0.005 inch total. To increase the idle air supply, you may drill small holes in each of the throttle plates of the center carburetor. The throttle plate will appear as two hemispheres. For best results, the holes should be drilled in the center of the hemisphere AWAY from the idle mixture control screws. One should start with a 0.060-inch hole. If necessary, the holes may be increased in size, not to exceed 0.125 inch. This modification does not change the idle mixture, rather this modification is done to control the clearance of the throttle plate to throttle bore. This clearance is important to minimize or eliminate bog from a stop.

  • ozkey23 07/15/09 11:57 pm PST

    You better be prepared to buy another carb if you follow the previous answer . Try blowing 

    all the passages with air via  a blow gun before you start drilling into anything 

  • moonman15 12/04/10 4:32 pm PST

    I have been out of the business for a very long time. It is rare to see a question relating to older vehicles. I am a former class A repairman at new car dealers, I was certified in all 8 categories by NIASE in 1974, one of 4 % that passed all 8 tests. Because of the low number of passing grades, they made the test much simpler, now anyone that can lift a wrench is "certified technician." You don't see the old "General Automotive Repairman" patches with the 8 (or however many tests you passed), under it, they were blu and yellow. But I digress by bragging.

    The best method to adjust a carb i to use a tachometer or the very common, (back then), tach and dwell meter, they are inexpensive and vital with the year of your car. Engine warm, hook up meter, this is very, vsry simple with the clip on wires attached to the meter. Use a screwdriver to turn the mixture screw until the highest reading is obtained, carb is now adjusted.

    The other use for the meter is the Dwell Angle reading, this allows you to insure the ignition point gap is correctly set on you engine. This adjustment should be done before the carb adjustment. The meter is connected the same way. The dwell angle is determined by the number of cylinders in your engine; an 8 cyl. engine requires a 30 degree reading, a 6 cyl. engine requires a 45 degree reading on the meter. This adjustment is tricky to the novice. It is accomplished by setting the air gap between the contacts on the ignition points under the distributor cap, you need the specs for your engine on the air gap. The only break is on 8 cyl. GM made cars, they have a lttle door on the distributor cap that you can use to access the point adjustment setting with an allen wrench. For this type of set up, slide the allen wrench in, start the engine, adjust to 30 degrees. On all other engines you need to set the gap to the spec with a feeler gauge of the proper size, then put distributor cap back on and start engine to confirm proper dwell angle on the meter. If you are handy, I am going to give you a short cut that works perfectly; remove distributor cap, replace points if they are pitted or worn, you must change the condenser also, you must, it is a small silver cylinder shaped object that is wired to the points. Install the new points and condenser if needed. With the distributor cap off slightly loosen the 1 screw that hold the points in place, and, I mean slightly, you should have some resistance if you try to move the points adjustment gap, have someone crank the engine while you adjust the air gap until the dwell meter tells reads the angle you are trying to set. This, to me is the best method, it kind of lets you adjust any car like the 8 cyl GM products with that little door. Don't forget to check the distributor cap, remove it and look inside for pitting or wear on the contacts, check the rotor for wear, you may want to check the ignition wires as well since you mention backfiring. If you have an OHM meter, you can connect one of the two probes to the end of the ignition wire where it hooks up to the spark plug, the other probe should touch the inside of the distributor cap contact for the same wire, check for a reading on the meter, if no reading is found the cylinder that wire connects to is not getting the spark it requires. If the backfiring is happening all the time, check the firing order as well, that is found on the intake manifold mos of the tme. Good luck.


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