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Carburetor Tune Up Guide

previous page: Tune-Up of the Gasoline Engine


For good compression, we must have pistons and rings that seal properly with a minimum of frictional drag.  Good lubrication by the proper oil will maintain this condition for many thousands of miles. Both intake and exhaust valves and their seats must be perfectly free of carbon, and have even seating surfaces. Valve stems should be free of carbon and gum formations to prevent them from sticking in their guides, and should not have too much clearance between the stem and guides, as this causes oil to be drawn thru intake guides and generally upsets carburetor mixture.
Valve tappet clearance is also most important and should be checked very carefully when the engine is hot. The intake manifold and manifold gaskets must also be kept tight because a loose manifold or leaking manifold gasket will cause excessive air to be drawn into cylinders, thereby causing the mixture to be too lean. This makes the engine hard to start.


To have good ignition in a car, we must have a battery that is built to take care of the load put on it by all of the units and accessories, and it must maintain reserve energy. Battery cables should never be less than size one (1) or preferably size nought (0) and the battery post and cable terminals should be perfectly clean and shiny on their contacting surfaces, otherwise, they do not conduct the current as intended, and cause many baffling ailments. The starting motor must be in good general condition, as otherwise it will not turn the engine over fast enough, and will often require more current than necessary under normal conditions, thereby robbing the coil of its primary current, and making it almost impossible to start the engine. All primary ignition cable from starter post terminals to ammeter, to ignition switch, to coil primary, and to distributor primary connections, should be of the right gauge so that it con­ducts the proper amount of current through the primary circuit. Wire size - Gauge 14 to 16.

Distributor points, or contacts as they are commonly called, should be installed in the distributor while the distributor is clamped in a suitable testing stand or vise, and should be checked for approximate clearance in thousandths of an inch before they are properly aligned and then checked for final clearance after proper alignments. Contacts must be aligned with faces parallel when contacts are closed and outer circumferences of contact points must be in line one with the other. This is most important because of the fact that when contacts are not properly adjusted and aligned, correct coil saturation cannot be attained, and the resulting coil secondary output will be weak. It is a well-known fact that contacts which are not carefully and properly aligned do not perform for as long a period of time as they should, therefore, we will run into trouble immediately after doing the tune-up job. Distributor rotors and distributor caps should always be checked for corrosion and wear because if they have excessive wear it tends to add too much resistance to the secondary circuit, which is definitely a load on the coil and of course helps to rob the spark plugs of the amount of spark they should receive.

Spark plug cables should be in good general condition in order to deliver the spark to the spark plugs uninterrupted. Replace cracked or frayed wires.

Spark plugs are an item which have had many mechanics baffled with new modern high-speed, high-compression engines. For this reason, you should fol­low spark plug manufacturer's specifications for heat range and type very closely as well as follow their specifications just as closely on gap clearance.


Spark plugs operating under normal carburetor mixture should last approximately 10,000 miles, if cleaned and gapped every 2,500 miles.


To have good carburetion, we must of necessity have a good fuel; one that will give us the best possible performance in the particular climate in which the car is being operated. Naturally, we must also have a fuel pump that will deliver fuel to the carburetor under the various operating conditions.

We must also have a carburetor that is perfectly clean and free of carbon and gum deposits as well as having all its component parts in a condition which will enable them to mix the proper amount of gasoline with the air intake so that the mixture will vaporize properly and burn smoothly and slowly. This will give the best possible results from the amount of fuel used.

An air-fuel ratio greater than fourteen and one-half (14 ½) to one (1) will burn spark plugs and valves prematurely, causing the engine to operate in­efficiently before it has had a chance to break-in properly.


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Table of Contents

The Carburetor and Its Purpose

Tune-Up of the Gasoline Engine