I am in the process of finishing the re-install of my 502 back into the boat.

I have seen several different viewpoints on how to set the the timing on these motors. I have also seen on this site that a lot of guys are running 87 octane in their 502's as well.

The question I have is where and how do I set the timing on this motor, and can I really run 87 in this thing without the chance of anything bad happening.

I have set the timing on our small block race car engines many times and I just want to know if there is any difference in procedure with the mercruiser thunderbolt ignition setup.

Any help would be greatly appreciated.

Jump pin a to b on the dlc connector. Run engine at idle and set base timing to specified level, usually 8 degrees when warm. Remove jump pin, run engine in neutral over 3000 rpm and confirm timing is at specified number for total timing. Important that you check both.

Excellent! Thank you. Does anyone have any thoughts on the 87 octane?

I believe they are designed to run on 87.

http://www.mercurymarine.com/service...?category=fuel

If it's a Tbolt IV there is nothing to jump.

The boat is a 91 fountain fever. Will it be stamped anywhere whether or not it is a thunderbolt IV?

I'm sorry, you are correct. I thought I read 502 mpi

If its a 1991, then its a TB IV. It should say so on the module also.