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.
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.
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.
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|Accelerator duration screw adjustment|
Written by LuckyDon, on 01-02-2015 16:33
ABF284... I found the same thing. The adjustment screw seems too far away from the plastic cam to be able to come into play.
Written by gold03, on 05-02-2014 21:05
Just one thing regarding pump timing. The pump must start to spray fuel before the throttle plate moves. If it des't you get a sudden lean spot and it will pop back through the carb. You need to slowly turn the throttle while looking at the throttle plate and adjust the top screw out to make it come in sooner and in to delay it.
Much carb popping and coughing will be cured this way.
Mine was set late from the factory and needed top screw adjusted.
|Pump plunger binding.|
Written by compman, on 07-01-2013 17:25
On my 03 model I have had a problem with the accelerator pump plunger binding against the pump cam arm. The cam is spring loaded to push the plunger down as the throttle opens, either the spring is too weak to do the job or as I have found periodic cleaning of the mechanism and lube the sliding face of the pump cam arm and pump plunger plastic cap with a dry slide graphite spray will do the job for a while.
|Written by n8sroadstar, on 05-03-2012 23:39 |
thanks for the info
Written by blackmark, on 04-27-2010 17:11
Im confused on which jet kit to go with Dynojet or Mikuni?
|Has not helped|
Written by afb284, on 09-15-2008 19:02
I had a question. Even though I adjust the accelerator adjustment screw it does not appear to make a difference as the accelorator pump cam seems too far away from the screw anyway, for its adjustment to make a difference. Perhaps my cam is not aligned properly?
|Thanks for this valuable info|
Written by mrdantzler, on 09-29-2006 20:28
Thanks so much!! I just bought a brand new 2006 silverado about a week ago and I had the exact same problem right of the showroom floor and it was very frustrating. I was beginning to think this bike was a piece of junk. But, I followed your suggestions and turned up the idle speed the problem has greatly improved. It still does the occasional pop or cough but I guess I will have to live with it. Thanks again!
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