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Carburetor Tune Up Guide

previous page: Testing the Ignition Circuit


First, make sure compression and ignition factors are either normal or restored to normal condition, because if a cylinder does not fire its charge the unburned fuel will be discharged into exhaust stream causing it to flow across sampling cell in analyzer which appears on the dial as a rich mixture, when actually the charge of fuel remains unburned for lack of good compression or ignition. In such a case you might be led to believe that the carburetor is adjusted too rich.

After checking all other factors start up the engine and allow it to warm up to operating temperature. While the engine is warming up, adjust the exhaust analyzer as per instructions on the dial. After adjustment is made and the engine temperature is up to normal, speed up the engine a few times to throw moisture out of exhaust pipe then connect sampling tube to exhaust pipe and analyzer.

The air-fuel ratio meter or fuel analyzer as is used today in the automotive service industry has been found to be very accurate in diagnosis and if properly adjusted before being used will help mechanics turn out very excellent work on carburetor adjustment.

When adjusting and balancing the Wheatstone Bridge type of analyzer, always be sure that battery has sufficiently high voltage. Do not use a battery that is connected into a charging circuit of a car, because generator fluctuating voltage will throw bridge off balance and you cannot get accurate analysis.


With carburetor throttle arm stop screw adjusted for engine to idle at seven (7) miles per hour minimum in high gear, adjust idle air screw until analyzer reading is between 12 and 12 ½ to 1 air-fuel ratio. This setting al­lows a perfectly smooth idle.

After adjustment has been allowed to stand for one to two minutes and it remains perfectly steady, then make acceleration test.

Work the throttle arm by hand accelerating the engine twice allowing the throttle to fall back to idle. If the pump discharge stroke is normal, the reading on dial should register to between 9 and 10 to 1 and then fall back to idle reading previously set. There should be no change in idle reading after engine has been idling again for about one minute.

Next, run engine up to between 25 and 30 miles per hour holding throttle steady - needle on gauge dial should move up to not greater than fourteen and one-half (14 1/2 to one (1) for maximum power and efficiency. Advance throttle gradually to approximately forty-five (45) miles per hour. There should be a slight change in dial reading until throttle position is steady at which time reading should not exceed fourteen and one-half (14 1/2) to one (1) after throttle is held steady for one (1) minute or so.

When mixture shows up leaner or richer than above specifications, changes can be made by changing main metering jets or metering rod jets as the case may be.


For best results in making a road test, it is desirable that a separate driver be used to operate vehicle. The analyzer operator and the analyzer should be carried in the body of the vehicle and the analyzer should be cushioned from road shock.

At idling speeds the idle adjustment should be for the leanest possible mixture with satisfactory idling.

At fast idle a somewhat richer mixture will be found necessary for the change over from the low speed to the high speed circuit in the carburetor. A mixture as rich as 11.5 to 1 will not be out of line in this case, the primary consideration being satisfactory acceleration and smooth operation.

At intermediate or cruising speeds, a mixture of 14.5 to 1 will be satis­factory for economical operation with no danger to engine valves, whereas mixtures leaner than this will have an overheating tendency.

At higher speeds, due to the operation of the step-up jet, or to the fact that the smallest section of the metering rod is in the metering rod jet, a richer carburetor condition is realized. A mixture as rich as 13 to 1 is sat­isfactory in this case.

In road testing, operating manifold vacuum conditions are achieved, and the true condition of the carburetor adjustment is brought out. In testing on the service floor, the vacuum or mechanically operated step-up system is never brought into operation, nor is the power section of the metering rod brought into use, therefore, the only true check on these items can be realized by the road test that will bring all of these items, as utilized on the various carburetors, into play.

Remember, it won't take any longer to set a carburetor right than it takes to set it wrong. A little bit of necessary experimental work changing jets to meet the conditions will result in a fine service record with fewer road failures.

If you will think in terms of lives saved because of preventive maintenance rather than in terms of dollars and cents and man hours saved, you cannot help but have the desire to turn out better results.



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Table of Contents

The Carburetor and Its Purpose

Tune-Up of the Gasoline Engine