why does my ignition timing retard from 10 degrees to 0 when i rev the engine to appprox 2000 rpm,i have tried 3 different ignition modules and 2 distrbutors , it is a carburated 502 Mercruiser

If it's a thunderbolt V ignition, they have ignition retard function built in. It is controlled by the knock deterctor system. If that malfunctions or the connections between it and the ignition module are loose, it will pull timing.

Putting a thunderbolt V module on a IV system will cause the same thing.

I dont think my engine got a knock detector,i have a thunderbolt IV, can i use a thunderbolt V if the engine does not have the knock sensor?

No, they are not interchangeable, the V will retard all the time w/o the knock module present.

Ok thanks i will try a complete MSD system and hope that will work

put a HEI in it, run no vacume advance, set your total timing, simple works great...