My bike has developed a slight lag between twisting the throttle and
getting power, it also burbles quietly when riding at low throttle positions,
like about 1/4 or less throttle and has lost a bit of power.

This started after I fitted a new clean stock air filter rather than my old
dirty stock air filter.

I figure this means I'm running lean.

Now the question.
Is the mixture screw an air screw (turn out to bleed in more air and
make the mix leaner) or a fuel screw (turn out to bleed in more fuel and
make the mix richer).

The bike seems to run ok with the throttle opened up, so I figure the
mixture screw shoud fix things.
If not, any suggestions?

Its UN Screw for richer.
Its a tapered screw set into a fuel line as an obstruction ... you remove it or back out the screw for more gas to get in.
Cool.
Srinath.

Thanks, hopefully a little richer an things will run nicely again.

I'm stock jetting, but on a UK bike, so I think I'm close to what you recomend to folks.
If this doesn't work I guess It's time to clean the carbs, check float levels and synchronise the slides.

Yea look in the manual ... 40/125 and needle on 3rd clip ... or whatever is 1 richer than stock and pull the needle up as it sits in the carb for richening ... and 3 turns out on air screw ...
Cool.
Srinath.