Chevrolet Repair: Wont stay running, vacuum leaks, timing variations

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
Make sure when you are setting the timing, the vacuum hose on distributor vacuum advance is disconnected and plugged.
I would then set the timing to specs and re-adjust the idle to keep the engine running. If it just does not run or stay running when the timing is at specs it's very possible the timing chain has jumped. Also the distributor can also be bad...worn shaft causing timing variations.
If all that is ok, I would look at vacuum leaks and carburation next. Carb could be shot.