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Posted by . on July 9, 2008, 3:43 pm
> Hey guys, does anyone have the specs on the pilot screw setting? Just
> cleaned carbs and was using 2 1/2 turns out. Was wondering if that is
> right.
That's a good place to start, considering that your carbs came with
#35 pilot jets. #35's are fairly small idle jets.
It used to be so easy to adjust the idle mixture on a 2-barrel
carburetor on a car. We would just set the idle speed, then turn each
idle mixture screw in a bit. When the engine ran a little rough, we
would back the screw out 1/4 of a turn, rev up the engine twice, close
the hood, shut off the engine and go have a cold beer.
Why do shade tree mechanics make such a hard job out of adjusting
motorcycle carbs?
The procedure used to adjust the idle mixture on your constant vacuum
carbs is to set the master idle speed knob so the engine idles at the
specified RPM, then turn each idle mixture screw in about 1/4 of a
turn, and if the idle RPM increases, turn the idle speed down.
Then turn each idle mixture screw clockwise another 1/4 turn and turn
the idle speed down again.
You want to get the engine to idle smoothly with the smallest amount
of throttle opening.
But most shade tree mechanics go the wrong way. They don't realize
that the idle mixture screws are intended to RESTRICT the flow of fuel
to the idle circuits.
They turn the idle mixture screws too far counterclockwise, and find
that screwing them even further doesn't increase the idle speed, it
slows the idle speed down.
Then they turn the idle speed knob up, and when they blip the throttle
to test idle mixture, the idle speed hangs up and takes a long time to
idle back down to the specified RPM.
What happens is that turning the idle speed up too far uncovers three
acceleration transition ports next to the throttle butterflies.
The transition ports are there for accelerating, not for idling. You
don't want the transition ports uncovered at idle, so keep the idle
speed knob turned down, and while you're adjusting idle mixture,
generally turn the idle mixture screws clockwise, restricting fuel
flow.
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