Setting the Carburetor

How do I set the carburetor on my TO-35?

Harry Ferguson considered setting the carburetor an important maintenance task. He constantly preached the need to maintain Ferguson tractors and equipment in the highest state of repair to obtain the maximum level of performance. Failure to perform as desired was usually attributed to failure to maintain.

There are three adjustments on the carburetor. The small needle valve screw is the idle mixture screw,and the initial setting is 7/8 turn open from the fully seated or closed position. The large needle valve screw is the main jet, and the initial setting is 1 1/4 turns open from the fully seated or closed position. There is a third screw or wing headed screw on the back side of the carburetor. This is the idle speed screw which is normally set so the idle speed is 450 RPM. The above settings are correct for both the Marvel Schebler and Carter carburetors.

Start the tractor and let it warm up. Set the idle speed to the 450 RPM. The Idle Mixture screw can now be adjusted slightly to obtain the highest, smoothest idle speed. If you have a vacuum gauge, set the screw to obtain the highest steady vacuum reading. Reset the idle speed to 450 RPM.

The Main Jet should be set while the tractor is working under load. With the tractor under normal load, quickly move the throttle from half open to full open position. If the engine coughs or stall, the mixture is too lean. Open the main jet a little at a time (1/8 turn) and repeat the test. When the engine responds immediately to increase acceleration and runs smoothly and evenly. Never set the Main Jet to less than one full turn open as it will cause the valves to burn.

Before performing these adjustments make sure the screws hold the upper and lower carburetor halves together are tight, and the nuts holding the carburetor to the manifold are tight. Also make sure the manifold nuts are tight. Failure to keep these fasteners tight causes air leaks which will not allow the carburetor to operate correctly.