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Carburetor Tune Up Guide

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Float circuit - Carter W-1

The purpose of the float system is the maintenance of the fuel at a pre­determined level in the carburetor at all times, under any operating conditions. Since the float system controls the flow of gasoline into all the other circuits of the carburetor, its setting and operation must be correct. If the float is set too low, the engine will starve for fuel at higher speeds. On the other hand, should the float be set too high, too much fuel will flow through the carburetor circuits and the engine will load or a mileage complaint will be the result. The float system can only operate properly, if there is little or no wear in the operating parts such as needle valve and seat, float pin or float lever holes.

A positive setting of the float level cannot be made if there is wear in any of these parts. The only way a sure repair job can be done, is to replace all worn parts with new ones.

A loaded float, one that has developed a leak in its air chamber and is partially filled with fuel, will definitely give trouble. This will cause the float to become heavy and lose its buoyancy. When this happens, the float no longer has the power to rise and fall with the fuel in the float bowl and of course cannot control the flow of gasoline, through the needle valve and seat. Replace float with a new one.

Another part of the float circuit that might cause trouble due to wear, is the float pin boss on the bowl cover. On Stromberg BX type carburetors, a round wire retainer is used, to hold the float pin and float against the recess in the casting. This retainer must be in place when float and pin are installed. If not, the float pin will bob out of the casting bosses and the float will not operate the needle valve and seat assembly. This also is true of the Ball and Ball carburetors. Make sure the retainer is in place correctly.

If the serviceman is attempting to set a float without manufacturers' specifications, here is a quick procedure. On floats that are attached to the body casting, set the top of the free end of the float about 4 inch below parallel of the machine surface of the casting, with the needle valve seated.

On carburetors where the float is attached to the bowl cover with the gasket removed and needle valve seated, set float clearance at 3/8 inch. It is best to set the float level a trifle below specifications because as the needle and seat wears the float level rises.

Downdraft carburetors fall into two types, one the unbalanced type and the other a balanced type unit. In the bowl cover of the unbalanced type carburetor, is a hole drilled to a definite dimension. It is through this hole that air flows and exerts pressure, or pushes against the gasoline in the float bowl.

The air going through the air horn has no effect on the gasoline in the float bowl of the unbalanced type of carburetor. As a result, should the air filter on the carburetor be restricted with dirt, oil, or any foreign substance the air flow into the air horn would be slowed down or reduced in volume. When this happens, the carburetor not getting sufficient flow of air runs rich, because the pressure on the fuel in the float bowl remains the same as normal,.
In the balanced type of carburetor this cannot happen. The vent hole in the bowl cover is internal and the float chamber is vented by means of a tube into the air horn. If the flow of air into the air horn is restricted, by this internal venting, the pressure on the gasoline is also restricted. Consequently, the pressure in the air horn and the pressure in the float chamber are balanced.

next: Low Speed Circuit


Table of Contents

The Carburetor and Its Purpose

Tune-Up of the Gasoline Engine