Harley-Davidson used the Mikuni carburetor prior to the use of the KeiHin constant velocity carburetors. Mikuni produced a side-draft, mechanical carburetor that used the traditional fixed-jet system to control fuel flow. A butterfly valve linked to the rider throttle control with a second butterfly valve acting as a choke plate controls intake air volume, and a float-and-needle valve assembly regulates fuel height in the bowl. Problems with any of the internal parts can lead to poor performance or a non-running condition. Inspect the carburetor for problems once it has been removed from the engine.
1. Remove the float bowl screws using a Phillips-head screwdriver. Pull the float bowl off the carburetor body. Remove and discard the old float O-ring.
2. Remove the float pin screw using a Phillips screwdriver. Remove the float from the carburetor body. Remove the needle valve from the float.
3. Inspect the float for cracks, swelling or signs of damage. Shake the float, listen for the sound of fuel inside, and replace the float if it is damaged or leaking. Push the float down into clean water and look for bubbles. Replace the float if bubbles escape from it.
4. Inspect the rubber tip on the needle valve for damage or wear. Inspect the float needle seat within the carburetor body. Replace the needle and seat as a set if either part is damaged or worn.
5. Remove the accelerator pump housing screws using a Phillips screwdriver. Remove the pump housing, O-ring, diaphragm and spring from the carburetor body. Inspect the O-ring for damage. Hold the diaphragm up to a light source and check it for pinholes or tears. Remove the accelerator pump pushrod and inspect it for damage or gumming. Clean the accelerator pump rod using carburetor cleaner if it is gummed up. Replace any bad parts.
6. Remove the main jet, pilot jet plug and pilot jet, using a jet tool. Inspect the jets for damage or obstructions. Clean the jets using carburetor cleaner and jet picks if they are dirty. Replace the jets if damaged.
7. Inspect the butterfly valves and valve bushings in the carburetor body. Grab the valves and try to wiggle them to determine if the bushings are worn. The valve and bushing assemblies are matched to the carburetor during the manufacturing process, and cannot be serviced. Replace the carburetor as a unit if these parts are damaged or worn.
8. Put on safety glasses. Blow through the fuel and air passages within the carburetor using compressed air and a rubber-tipped blowgun. Feel the outlet ports within the carburetor throat to ensure they are not clogged or restricted. Clean the passages using carburetor cleaner if any of them are clogged, then again blow out the passages.
9. Install the main jet using a jet tool. Drop the pilot jet into the pilot jet tower and install it using a carburetor tool. Install the pilot jet plug and securely tighten it. Take care not to damage the jets during reassembly. The jets are brass and may be deformed or damaged, which will restrict fuel flow, effectively reducing the jet size.
10. Assemble the float and needle valve. Insert the needle valve into its housing, and place the float and float pin into position. Install the float pin screw and securely tighten it using a Phillips-head screwdriver. Position the carburetor on its side with the float pin at the 12 o’clock position. Measure the float height from the float bowl mating surface on the carburetor body to the outer edge of the float, at the 6 o’clock position. The measurement should be between 0.63 to 0.67 inches. Adjust the float tang using a flat-head screwdriver until the measurement falls within this range
11. Install the accelerator pump O-ring. Install the accelerator pump diaphragm into the accelerator pump housing and ensure the diaphragm fits evenly into its groove. Install the accelerator pump housing and tighten the screws securely using a Phillips screwdriver.
12. Install a new float bowl O-ring. Install the float bowl and screws. Tighten the float bowl screws in a cross-pattern.