Shown is the left side of Tecumseh Carburetor 640342 (#1). The only feature here is a pressed in plastic external atmospheric vent. There is no idle mixture screw or fixed idle jet.
The right side view (#2) shows the primer tube, fuel hose, and behind that is a vent for the nozzle.
Throttle / governor linkage (#3) typical for fixed speed applications.
View of governor arm, spring, adjustment tab and a bottom view of float bowl (#4). To adjust engine speed faster, bend the tab upward, putting more tension on the spring. To set the governor, position the throttle at wide open, loosen the pinch clamp and rotate the slotted governor shaft clockwise. Tighten the clamp screw.
Bottom view of float bowl and bowl (#5). Note the ball plug in the bottom of the bowl nut. This is where the hole for the main jet was drilled through the nut and the entry point sealed. Also, there is a bowl drain, used to drain stale fuel or water that may have accumulated.
The step in the bowl should be aligned parallel to the float hinge pin, with the deeper part toward the free end of the float. Compare #5 and #6.
After removal of the bowl nut and float bowl (fuel bowl) the float, float hinge rod, and bowl gasket can be seen (#6). Remove the bowl gasket, then the hinge rod. Take precaution not to let the damping spring jump away, as it is likely to do. The float can now be lifted out with the inlet needle attached to it by a clip. The needle can easily slip off the float and get lost.
The main fuel jet is drilled through the bowl nut via a passage that is closed with a ball plug (#7).
#8 shows the other holes in the bowl nut. The larger hole in the side of the nut is the main fuel pickup. The smaller hole is the idle fuel transfer port.
Gum, residue, or debris clogging the holes in the bowl nut are the most common source of trouble in Tecumseh carbs.
A wire shows the location of the idle fuel pickup orifice within the center leg of the carb (#9). It is directly opposite a ball plug on the outside wall of the center leg.
With the carb inverted, the free end of the float should rest float should rest 11/64 inch from the body of the carb (#10). The tang on the float can be bent to adjust the level.
#11 and #12 show the float, hinge rod and damping spring with and without the bowl gasket.
To remove the float simply slide out the hinge rod (#13). The damping spring will try to jump as soon as the rod is removed. When you lift the float from the body, the float needle (inlet needle) will come with it and may slide off. Be careful not to lose it.
A closer view of the damping spring and hinge rod (#14). See separate page on installing Tecumseh float damping spring.
The inlet needle is attached to the float with a spring clip (#15).
The inlet seat is made of rubber (#16). It has a groove on one side and the center hole is chamfered on the other side. The seat is installed with the grooved side facing into the recess. The inlet needle mates with the outward facing chamfered hole.
The main nozzle / emulsion tube can be remove by pressing it out from inside the venturi with the flat blade of a screwdriver (#17).
The nozzle in this carb has two o-rings, one of which might remain in the center leg (#18). You can fish it out with a paper clip bent to form a small hook.
The external atmospheric vent is a plastic part pressed into the left side of the carburetor (#19). Ensure that the orifice is clear with a 0.012" wire.
The wire in #20 shows that the external vent leads into the large recess behind the welch plug (welch plug has been removed) above the float.
The primer tube fitting (#21) connects to the area above the welch plug (#22).
In #23 and #24 a wire indicates where the external brass fitting leads to the area between the o-rings of the main nozzle inside the center leg, thus providing air to emulsify the fuel passing through the main nozzle, i.e. to break up the fuel into small droplets and mix it with air.