I have been chasing down ignition problems all summer long after getting my boat back from a bunch of work; transom, drives, motors, etc.... including the starboard side tach being erratic.
In another thread I posted about getting no spark and turned out a distributor came apart...
So, replaced both distributors. still had a spastic tach.
I replaced the MSD box on that side as well (now both are new) and the tach is still spazzing out....
by spazzing out I mean it is twitching around a lot at idle, any switch you push or press the tach pegs, and while cruising the tach is reading about 2k rpm's faster than actual....

I pulled the tach and sent it back to Livorsi for testing and Livorsi confirmed the tach is fine.
the only thing left on that side I have not changed is the Blaster SS coil.... I have a spare one to change it but is there any logic in the coil causing this kind of interference?

I'm not sure where else to look...
what color wire is the tach signal wire in a MSD set up? (im running the 6m-2l boxes, blaster ss coils, and MSD distributors)
and what is the next thing I should consider - besides ripping out this MSD crap... thanks.

supposedly that was already done, the shop that initially did all the work said they checked all grounds and connections and everything was good there but I am still skeptical.
any possibility the coil could casue this? I always thought the coil either works or it doesnt..

are those msd coils ?

The tach wire is the gray wire coming out of the MSD box.

Yes, MSD Blaster SS coils

Does anyone think there is any good logic in the Blaster coil effecting the tach, or somehow causing some interference?

As,mentioned earlier - make sure tach is hooked up to the box ,and not the coil.

If that is okay, try turning the cyl select switch on back of tach back and forth a bunch of times, Put back to 8cyl and retest,

Swap tachs if above doesn't work and see if problem stays or switches sides.

I tried running some new msd coils on stock Mercury ignitions, they started to over heat the timing modules to the point that the distributor was double firing the engine @ 10* and again @ 270*................ they are a POS....

^^^^^^^^^^^what Steve (SB) said................and check the ground wire on the tach and make sure it's secure then trace it to it's grounding point and check that connection. Sounds like a ground issue since actuating any switch causes it to peg out.