THE CARBURETOR SHOP / Early carburetor float adjustment

The Carburetor Shop LLC

204 East 15^th Street

Eldon, Missouri 65026

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Adjustment early updraft carbs (where no specifications exist)

There are many early carburetors, many of them brass, for which there are no surviving adjustment specifications (if any ever existed) as to setting the fuel level in the bowl.

The following procedure has been documented for other carburetors from the same period, and in the absence of original data, provides a good initial adjustment:

(1) The tuner needs what was originally referred to as a burette (picture), but if the tuner does not have one of these rare and expensive tools, a workable substitute is possible.


Side view	End view

The glass tube is open at the top.

A "poor man's workaround" may be constructed with the use of a couple of brass fittings, some clear plastic tubing, a hose clamp, and some wire to keep the tube upright.

(2) Once the special tool has been fabricated, measure the distance from the top of the carburetor flange, through the venturi to the top of the main discharge nozzle.

(3) Add 1/8 (0.125) inch to the above dimension.

(4) On the outside of the carburetor body, measure from the top of the flange and scribe (or pencil) a horizontal line using the dimension calculated in step (3). It should be noted that the horizontal line should be 1/8 (0.125) inch BELOW the top of the main discharge nozzle. This is the "set point" for the FUEL LEVEL, NOT THE FLOAT.

(5) With the carburetor assembled, remove the fuel bowl drain plug and attach the special tool fabricated in step 1 above. If you are bench testing, the "head pressure" should be the same as what the carburetor will see when mounted in the vehicle. For those unfamiliar with the term "head pressure", it refers to the average distance from the fuel in the fuel tank above the fuel entry point in the carburetor. This is fairly critical, as the fuel inlet orifice specification depended on the head pressure). In researching the Stromberg original specifications for early brass carburetors, a common size 2 (1 1/4 inch) carburetor might have a 0.113 inch fuel orifice with a head pressure of 18 inches, which was increased to 0.140 if the head pressure were only 8 inches. Failure to consider this issue MIGHT result in either flooding of the carburetor, or starving the engine for fuel.

(6) Add fuel to the carburetor. Note that since the glass tube (or clear plastic for the home-made tool) is open at the top, the fuel level in the tube will be identical to the fuel level in the bowl. Compare this level to the scribed line from (4). You now have a starting point for adjustment.

(7) Many of these older carburetors have a float valve which has a collar at the top into which two prongs from the float (or two float followers) was inserted moving the float valve vertically to open or close. The prescribed method in the old days was to take a measurement, remove the fuel valve, remove the solder from the collar, adjust the collar as needed from step (6), re-solder the collar, put everything back together. One repeated steps (6) and (7) as necessary to obtain the correct adjustment.

(8) HOWEVER (READ THIS BEFORE DOING STEP (7) ), if the measurement is close, it MAY be possible to make the adjustment by changing the thickness of the washer beneath the fuel valve seat. And this is MUCH easier the multiple steps (7).

This same "trick" may be used on newer carburetors instead of bending (and possibly breaking) a possibly rare and/or expensive float.