Reprinted by permission from Mr Tidy's tech tips

On the 40mm Mikuni Carburetor

One of the problems I've had with the Road Star is a coughing or backfiring out the carburetor through the breather. One of the cures is to decrease amount of fuel injected by the accelerator pump when first opening the throttle.

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Look in the lower right-hand portion of the image, this is the area of the accelerator pump.

Duration adjustment
  1. The "blue" colored screw in the image is the duration adjustment screw. Underneath the head of the screw is a lock nut and the bracket arm that the screw is threaded into. To decrease the amount of fuel squirted the adjustment screw is turned "clockwise" or in. To increase the amount of fuel squirted the adjustment screw is turned "counter clockwise" or out. To get the max, amount of decrease remove the screw and take the lock nut off of the screw. Then replace the screw into the cam arm, then replace the lock nut underneath the arm on the screw. this gives you another 3/32" of an inch in which the screw can be screwed in.
  2. Start your engine, Blip your throttle open, if the engine stutters, (hesitates), it is not getting enough fuel so turn the screw counter clockwise to increase the amount of fuel squirted from the accelerator pump into the carburetor. Keep blipping the throttle and adjusting the screw till the carburetor starts to cough. Stop here and
    turn the screw back in till the coughing stops. Tighten down the lock nut. This should give you the best throttle response with the least amount of coughing and backfiring out the carburetor.

Timing Adjustment Screw
This is the "Green" colored screw just above the Blue duration adjustment screw. The" Mucker" said it best,,,,, This upper screw adjusts the "timing" of the squirt. By altering it's setting, you can advance or delay the onset of the fuel squirt. But probably, it won't have to be touched.