Try this,,,got this off other thread on XROC.

Turning the CO pot screw will either richen or weaken the mixture at idle and really needs to be set up using a CO meter.

Follow this procedure if you DO NOT have the Weber ISCV in the airbox lid:

Sounds like your base idle is out...this is an idle that is hidden behind the idle you see on your rev counter.

Get the engine up to operating temperature and then leaving the engine running, disconnect the iscv. Revs should drop....may even stall.

From the front of the engine, look at the throttle body..in the 9 0'clock position, you should see a recessed screw. Turning this screw anticlockwise will speed up the idle, clockwise will slow it down. Set the idle at around 700 to 750 rpm. Once done, reconnect the ISCV. Revs will jump up (may jump up) and may "surge" up and down.....do not rev the engine........leave all alone for the revs to settle normally. Turn off engine once surging has stopped and then restart after a minute. If enegine stalls when ISCV is first disconnected, put 1/2 turn on the idle screw, reconnect ISCV and then start engine and go through above procedure again.

Blip the throttle up to 3,000 rpm and release........needle should pause at 1800 rpm for a fraction of a second and then drop to 1350rpm and then slowly drop to normal idle of 950rpm. If the needle "hangs" when release.....base idle is too high. If the needle drops like a stone to normal idle, then base idle is set too low. Have the CO checked after carrying out this procedure and set it to 1.10% with the fan running.