The reason i say condensor is because back in the day it was common to have them fail even if they were new. If you have the old one its an easy swap to prove it.
Loosen the dash pot screws off and then lift the piston up and down with your finger to centralise it and tighten the screws evenly as you do it to make sure it stays free and easy.
when its ticking over you should be able to lift the piston a small amount and the revs should rise slightly if the mixture is correct.
also if it was a permanent air leak and running really lean then it would probably cut out as you slow down to stop mate.
If its too rich it will hunt on tick over and get slower and slower.
Happy days, its all coming back to me.........lol