You ground out a wire on the TB module to enable you to put it into base timing mode.
https://www.perfprotech.com/blog/articles/mercruiser-engine-timing

That is tbolt v. Snd yes, if procedure not done right timing will move around. TboltV moves it around searching for best idle. They call it mean best timing.

Can you verify you have tbolt V ?

No I have TB 4 but I've heard it works the same. This just showing TB 5 because it is latest and greatest tbolt system

Doing some googling and I'm thinking you are right and this is one more thing I found that my mechanic did wrong.

No. They don’t work the same. Nothing to ground out. Just set your advance gun to 0 and aim at the timing tab and dampener with engine running. That module typically means you set base timing to 8*.

You absolutely pos you have a 5 ?

100% positive. So what would make the timing jump all around like it's doing?

pic of the module.

That’s a tbolt iv module.
Oh What the heck….,,,what wire where you guys grounding ?

Think I need a beer or some mushrooms. :wink Hmmm..decisions…..decision's Maybe some advil for now….lol

Calm down buddy..........lol

A really stretched timing chain or bad timing light.