Chevrolet Repair: Wont stay running, vacuum leaks, chilton manual

Question
I have recently been given a 1972 GMC 1/2 ton with a 350 4 barrel column shift 3 speed. I would like to gradually restore this truck but i will need to use it as well. The engine seems to be very strong, clean plugs and the oil level has yet to drop since i have been driving it. However when this engine is cold it runs its best; and then as soon as the choke is fully open it will not hold a idle without force; In fact when you do force it to keep running it is very power deficient, jugging and tugging like it is completely out of time. When it is cold and running well high idle set at about 1000 rpm the timing seems to be 25btdc then when the choke opens it jumps another 5 degrees ahead. Yet if I adjust to bring it back it is even worse then if I could move it ahead more which the distributor doesn't allow due to turning room. Is this the timing chain chain and if so why does it run so well when cold. lots of power etc.? Or should i be looking at something else first? vacuum Adv? carburator?

Thanks


Answer
It may run well because it is getting a lot of gas, or not as much air.
Check for vacuum leaks.

There is a timing chain. A Motors or Chilton manual will give a good description of how to check it for being worn out. Basically, you remove the dist cap, and rotate the engine in the normal direction till you are sure the rotor is turning. Then rotate the engine backwards till the rotor moves in the opposite direction.
if you need to rotate the crank more than just a few degrees befort the rotor moves, the chain and gears could be worn.

I'm thinking someone might have dropped the distributor in a tooth off.

Or the vacuum advance is connected directly to manifold vacuum, rather than to ported vacuum. There should be NO vacuum to the vacuum advance at idle.

Van