CARBURETOR MAINTENANCE

The TX750 is equipped with two "constant vacuum" carburetors (design is similar in operation to the SU type carburetor) mounted on rubber intake manifolds. Air flow through the venturi is controlled by a throttle slide (vacuum piston). The slide is lifted and lowered by engine vacuum rather than a cable directly linked to the throttle grip.

FLOAT LEVEL ADJUSTMENT

	1.	The float level may change because of a worn needle valve or a bent float arm. If the fuel level rises above a specified level, the air/fuel mixture becomes rich. If the fuel level drops, the mixture becomes lean.
	2.	Remove the float chamber body and float bowl gasket. Invert the mixing chamber body. Slowly lower the float with your fingers until the float just contacts the top of the float needle. Do not compress the spring in the float needle.
	3.	Measure height "A" from the top of the float chamber gasket sear (gasket removed). Standard measurement: 25mm (0.98in.)
	4.	Bend the tang (which contacts the float needle) so that a correct measurement is obtained.

DISASSEMBLY AND ASSEMBLY PROCEDURES

The vacuum chamber cover on top is held in place by four screws. Remove these screws and lift off the chamber cover.

Note:
When installing the set screws, be sure to correctly position the throttle cable anchor bracket.

2. Once the cover has been removed, the spring, jet needle retainer, jet needle and vacuum piston can be removed.

3. Installation of the vacuum piston is accomplished by inserting it into the carburetor body and lining up the small projection on the outer edge of the rubber diaphragm with the corresponding notch in the outer edge of the carburetor top mating surface.

4. With the top removed, the inlet passage (#1) and air passage to the starter jet (#2) are visible.

5. The starter jet housing mounts to the left side of the carburetor. It is held by three screws. A gasket fits between the starter jet housing and main housing.
6. The float bowl mounts to the bottom of the main housing. Remove the four retaining screws and then remove the float bowl.
7. Both the pilot jet and the main jet are screwed into orfices in the float bowl. The pilot jet is removed from the inside, the main jet is removed from the bottom, after first removing the cover screw.

8. Turn the carburetor upside down, pull out the float pivot pin, and remove the float.
Caution:
Directly beneath the float adjustment tang is the float valve. Remove this part immediately to prevent its loss.

9. The needle jet fits into the main housing from the bottom. If removal is required, pull it down and out by hand. Reverse this procedure to install the needle jet.

ADJUSTMENTS

Idle mixture: Turn the pilot screw in until it lightly seats, then back it out 1 ½ turns.

2. Idle Speed: Set both idle screws so the engine idles fast. Pull off one spark plug high tension lead. Back out the idle speed screw on the running cylinder until the engine dies. Hook up the spark plug lead, start the engine, and pull off the opposite spark plug lead. Slowly back off the idle speed screw of the running cylinder until the engine dies. Attach the loose spark lead and start the engine. Both cylinders should be idling at the same speed, pulling evenly. If engine rpm is too high, back off both idle speed screws an even amount until idle is 900~1,000 rpm.

3. Jet needle (mid-range): Mid-range fuel supply is controlled by the position of the needle jet. The needle clip comes standard in the 4th groove from the top (4th stage). To lean out the mid-range mixture, move the clip one groove higher (dropping the needle). To richen the mixture, place the clip one groove lower (raising the needle).
4. Main jet (high speed): The carburetor comes equipped with a #130 main jet. Install a one step larger main jet (#135) to richen the high speed mixture. Install a one step smaller main jet (#125) to lean out the mixture.

BUTTERFLY SYNCHRONIZATION

Both butterfly valves must be adjusted to open and close simultaneously. Fully close the throttle grip so that both butterfly valve actuator mechanisms rest against their throttle stop screws. Slowly twist the throttle grip and note whether both butterfly valves start to open at the same time. If this does not occur simultaneously, use the cable adjustor to lengthen or shorten one cable until correction action is achieved. In addition with the throttle fully closed, both throttle cables should have approximately 1mm free play before both butterfly valves start to lift simultaneously.

	5.	The starter jet housing mounts to the left side of the carburetor. It is held by three screws. A gasket fits between the starter jet housing and main housing.
	6.	The float bowl mounts to the bottom of the main housing. Remove the four retaining screws and then remove the float bowl.
	7.	Both the pilot jet and the main jet are screwed into orfices in the float bowl. The pilot jet is removed from the inside, the main jet is removed from the bottom, after first removing the cover screw.

	3.	Jet needle (mid-range): Mid-range fuel supply is controlled by the position of the needle jet. The needle clip comes standard in the 4th groove from the top (4th stage). To lean out the mid-range mixture, move the clip one groove higher (dropping the needle). To richen the mixture, place the clip one groove lower (raising the needle).
	4.	Main jet (high speed): The carburetor comes equipped with a #130 main jet. Install a one step larger main jet (#135) to richen the high speed mixture. Install a one step smaller main jet (#125) to lean out the mixture.