Originally Posted by ill_will
Annoyingly, I don't think the various carb stripdowns/cleans have worked - the idle mixture adjuster on cylinder 4 is still a fair way further out than the others in order to get the cylinder firing at idle at the correct mixture on every stroke (seen using a colourtune.)
It is often the rubber carb mounts, check by flexing the carbs up and down a little to open any potential cracks. A common air leak for no. 4 is the vacuum pipe for the brakes. Check it all the way back to the check valve on booster.
Colortunes are great things but I found that a set of vacuum gauges really helps to balance a pair of carbs and pinpoint air leaks.
My distributor is still not advancing properly too - suspect the springs. It wasn't advancing enough so I tried reducing the inertia weight spring preload, but now it's still not advancing enough and is all over the shop at idle. Stripdown time, and new springs.
If you feel like you deserve a present then the latest thing is an all electronic distributor with 16 different advance curves. Saves all that messing around with springs.