According to Eric in North TX :


There is a large screw and a
smaller screw. They are next to each other.
Does anyone know anything about adjusting these?

One, probably the larger one is the high speed adjustment, the other
the low speed. It gets a little more complicated than that, as they
interact. Changing the high speed effects the low speed. You just have
to get it running good wide open then try for a decent low speed/idle,
then do it all over again till both seem about right. better to end up
a little bit rich than a little bit lean.

The instructor of our small engines course said this:

[Presuming idle set screw et. are reasonably correct.]

Start with both needles opened 1 1/4 turns.[3]

Start the engine and get it warmed up.

Rev the engine to full RPMs [1] and adjust the high screw - 1/8 turn
at a time, and find the upper and lower limit of it running best.
Set the needle halfway between the two points.

Let the engine idle, and do the same adjustment trick with the
idle screw. Now, turn the idle screw 1/8 turn _rich_ from the
center of the best running point[2].

Done.

He went to great pains to say "don't touch 'em again" ;-)

At this point you can adjust the idle set screw for what
you want.

[1] You don't do this with chainsaws. High revs with no
load can cause rather bad things to happen if it goes on
long enough. You're supposed to do your high needle adjustment
by full load cutting on a log, tweak the needle, and retest
cutting again.

[2] The 1/8th turn rich on the idle screw is intended to
give the engine a little bit of extra gas to ensure that
the engine doesn't hesitate on quick throttle increases.

[3] He said that when set this way you virtually always
only have to turn the needle one direction to find the
best position. I forget which direction ;-)
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.