Pao idle problems

Hello all, I hope some one has come across this problem before and can point me in the right direction?
My 1990 Pao developed a flat spot when the throttle was pressed down, and if you didn't catch it right, would cut out, usually while pulling out of a junction. Very annoying. I gave it to my friendly retired engineer friend to play with. He seems to have got rid of this, but now, when the choke has cut out, it won't idle at all. Cuts out constantly and therefor impossible to drive. Friend thinks it may be a vacuum leak, but it seems quite severe for this. All the various pipes have been checked and a few replaced. I suspect he has done something he shouldn't, but I have no idea. Any help, most appreciated.