Hi
What carb are you running?
If it is a solonex carb, they have a little air bleed scree in the side that blocks up, causing it not to idle properley. You can get to it from the drivers side of the car, remove it, clean it and replace. If it works for a bit then goes back to poor running, you'll have to remove, strip, clean and re-build the carb.
Probably won't hurt to do the above any way, I its been done for a while.
If it is another brand of carb... I dunno, sorry.
Hope this helps
Cheers
kraM