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Carburetor Tune Up Guide

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For a normal idle, with throttle stop screw adjusted for a minimum of 7 miles per hour in high gear, the hand of Vacuum Gauge should register between 18" and 21" while engine idles. In setting the carburetor idle point with Vacuum gauge, best results are obtained by first jacking up the rear wheels one inch from the floor, starting the engine and putting the transmission into high gear, then adjusting the throttle stop screw until speedometer indicates 7 miles per hour minimum. DO NOT attempt to adjust IDLE AIR NEEDLE or SCREW until minimum speed has been set. Adjust hand close to one full turn out from seat until highest reading is indicated. When certain that highest reading has been obtained, check speedometer again. If there is a higher or lower reading than 7 miles per hour, reset the throttle stop screw for the 7 M.P.H. minimum. THIS IS VERY IMPORTANT.

SPECIAL NOTE:  On automatic transmissions, the idle stop screw adjustment should be made with an electric Tachometer for specification set up for the individual job.

After proper engine idle speed, and approximately correct idle mixture is attained, accelerate engine by quickly lifting throttle arm and releasing it. The Vacuum Gauge hand should drop to 2" and recoil to 24" or more. If the recoil is not more than 24", there is indication of very well diluted oil in the crankcase or poorly sealing piston rings.

Next, run engine to about 30 M.P.H., and hold throttle steady. The hand should at first fall back, then gradually climb up to its peak. Reading should now be from 1" to 2" higher than idling reading. If it isn't, the automatic advance mechanism is not functioning properly, or at all. If it does advance, the advance mechanism is working satisfactorily.

When the throttle is opened to 30 M.P.H., or more and held steadily, if the hand of Vacuum Gauge fluctuates rapidly between 10" and 21" or 22", it is an indication that the valve springs are weak, and not closing the valves as they should. As the engine speed is increased, the fluctuations become more rapid.



Connect gauge to intake manifold. Start engine after jacking up rear wheels just off floor. Set throttle stop screw until speedometer shows 14 to 15 M.P.H., no more. Hook up Vacuum Gauge on distributor side for convenience, loosen dis­tributor lock plate screw. While watching Gauge, turn distributor body which­ever way it has to go for retard, until hand indicates 16" or 17". Then, turn distributor body in opposite direction to advance until hand reaches its highest point and begins to fluctuate ahead. Hold it at this point for an instant, then turn distributor body back again very easy, just enough to remove the bounce or fluctuation, thus causing the hand to remain perfectly steady. This is the best point of setting. Lock distributor plate lock screw on side. This allows very close ignition timing setting without "ping" or knock. A road test will then definitely prove the setting is correct.

If the motor is in perfect condition, the hand of Vacuum Gauge will remain steady between 15" and 21".

Altitude has a definite effect on Gauge readings.

At sea level, the Gauge will read approximately 19.5".  

For each 1,000 feet above sea level, the Gauge will drop one inch.       

At 2,000 feet the Gauge will read approximately 17.5".          
At 5,000 feet it will be 14.5", and at 10,000 feet it will be 9.5".


next: Instructions for Use and Purpose of Compression Gauge


Table of Contents

The Carburetor and Its Purpose

Tune-Up of the Gasoline Engine