Coughing or backfiring through your Yamaha Road Star 40mm Mikuni carburetor? Have a read of Mr Tidy’s article on how to adjust the accelerator pump and see if it helps you out.
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How to Adjust the Accelerator Pump on the Stock 40mm Mikuni Carburetor
by Mr Tidy
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.
Click for a larger image
Look in the lower right-hand portion of the image, this is the area of the accelerator pump.
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.
Turn the adjustment screw fully clockwise or IN. 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.