Mikuni fuel pumps, designed to be used with carburettor-type engines, use a vacuum principle to function. As the engine works, the suction it creates to draw in fuel and air through the intake port creates a draw which causes the fuel pump to work. This vacuum pulse approach works fine when the entire system is sealed. However, when gaskets deteriorate or hoses are weak, the vacuum pulse can be disrupted. Troubleshooters should look for these problems in addition to general maintenance.
Examine the current location of the fuel pump in relation to the intake valve of the engine. Measure the length of the hose line from the fuel pump to the valve. Confirm the length is less than 19 inches total. Feel the hose to make sure it is stiff and rigid and not flexible.
Examine the fuel pump in relation to the engine. Confirm if it is far enough away from the engine surface to not be affected by engine vibration (it should be mounted on the motorcycle or scooter frame).
Confirm the last time the pump has been rebuilt. Begin a rebuild process if the last check has been more than 300 hours or riding or two years' time. Purchase a Mikuni rebuild kit for your fuel pump. Use a crescent wrench and socket wrench to loosen the bolts securing the fuel pump to the vehicle. Disconnect the hoses attaching it to the carburettor and intake valve of the engine.
Use a Phillips screwdriver to loosen the pump screws and open the fuel pump unit. Remove the old gasket. Replace it with a new one after cleaning out the unit with carb cleaner spray. Insert and retighten the unit screws with the screwdriver. Reinstall the fuel pump back on the vehicle with the socket wrench and crescent wrench.
Replace the hoses to the intake valve and the carburettor with new hose line. Throw away the old hose line. Secure the new hose pieces with banjo clamps, tightening them with the screwdriver.
Regardless of how well maintained the fuel pump is, its vacuum strength will fail as the engine itself begins to develop leaks in the engine gaskets and oil seals. This will then require the engine to be rebuilt to solve the problem.
Fuel pumps rely on vacuum to work, but the fuel systems they are attached to rely on gravity for fuel flow. The pump should not be higher than 39 inches from the engine to remain effective.