Stock 429 with Pretronix 1281 set timing at 8% btdc machanical 28%btdc vacum 5% total 41% . Problem after long drive check it and vacum is at 10% . Car was at operating temperature when timing was set vacum canister is turned out as far as can be. Wondering what is going on here. 46% is way to much advance from what I have read .

 

I am not that familiar with some of the items you mention.
I believe this engine is carbureted, correct? Yes/No.
If it is, there may be , depending on where the throttle butterfly is located (idle position) a slot in the venturi that if the butterfly is open past the slot, it will allow engine vacuum to be sent to the vacuun advance..
There are some folks on the forum that would have more background with what you are attempting to determine.

 

Yes it is Carb. Was hoping not to have to do to much to correct this issue. The less i have to do the better