idle issues

hi,

i have had a 1.8 clipper for 3 years now and it has always had idling issues. i recently changed it to weber carb (manual choke) to try and fix this problem. at first it ran like a dream idling smoothly at 850 rpm idle with the choke in, but now a couple of weeks later it is become very temperamental. we have adjusted the mixture as the guide said. sometimes it will start up and run smoothly and sit at idle then randomly seems to stop idling or just refuses from the start.

any idea what could be causing this?

cheers