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Sporadic/unstable idle speed is a condition of idle mixture either being too lean or too rich. As ARH mentioned, if the throttle operates correctly, then this is most likely your culprit. These bikes don't come tuned very well from the factory unless you live at high altitude, and tend to be on the lean side of things. This will cause the idle to "hover" at higher RPM's when coming off the throttle, and then it will eventually drop, and then proceed to hunt trying to equilize.
The standard approach to setting your idle mixture screw is to turn the screw in, and count the number of turns or half turns until it just lightly seats/stops. Make a note of that somewhere just in case you need a known "running" starting point.
From there,
1. turn the idle screw out 1 1/2 turns. This is the middle ground between 0 turns and 3. Install the carb back on the bike.
2. start the bike, then ride it around for say 10-ish minutes and get it up to temp. bring it back home.
3. set your idle speed, preferably with a tach to 1500 rpm
4. This is the tricky part because of where our idle mixture screws are located, but it is doable.
a) turn the pilot screw out a half turn at a time until you hear the idle
drop. During this procedure you may hear the RPM's rise, this is a
good thing. Keep turning half a turn at a time until you notice the
rpms drop.
b) Turn the mixture screw back in a half a turn at a time until you find
the position with the highest and most stable RPM.
c) Re-adjust the idle if necessary back to where you started. If the idle
is stable at this point you can call it good. If it is still a bit unsteady
you can repeat this process by turning the mixture screw a quarter of
a turn in or out at a time (giving the idle a chance to stabilize) until
you again find the highest and most stable rpm.
d) At this point you should have a good stable idle. You can go into even
finer tuning if you desire, giving the screw 1/8th turns, but this
usually isn't necessary on most bikes/carb setups.
The key in this entire process is to always keep a note of how many turns you have on that mixture screw. I.E.: If you start at 1 1/2 turns, and turn it out 1/2 a turn and the idle drops, you are at 2 turns out. Back it off half a turn, you are back to 1 1/2 turns.
Also, this is just one method, and there are variations of this method out there, but the end results are very similar.
2 things to keep in mind if you want to go through all of this.
1, it's a good idea to have a fan flowing on the engine to help keep it cool. If you feel the engine may be getting too hot, shut it off for a few minutes or so and let it cool. Just don't let it get cold, or you will have to wait for it to warm up again.
2. Before you dive in head first with all of this, see how it simply runs at 1 1/2 turns out. You may find that it holds a steady idle right where it is. There are many Hawk owners that have simply set the stock carb to this position and it has worked for them with no need for further adjustment.
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