The best I can do and its not Thunderbolt IV but is V using MEFI 3 and ther was plenty of them in the field

http://www.boats.net/parts/search/Me...nts/parts.html

and even a ton more 4.3 EFI look at the serial number runs of how many. Also the 6,2 MPI and the 5.0 all used this same set up thru out certain engine serial runs. 4.3 below and of all the engine serial runs, since many know boats.net.

http://www.boats.net/parts/search/Me...%29/parts.html

http://www.boats.net/parts/search/Me...tor/parts.html

Well consider me wrong ! LOL. Worked on many Merc BBC EFI/MPI, but I guess not enough Merc SBC EFI/MPI.

I must have made an ASS U ME. LOL.

Anyway, if one can post that wiring diagram, it may help, as it shows many things worth checking. That's for sure.

+1 on the lanyard, didn't some one have this issue last year and jump around it on the ignition switch to test?

That's why I preach like a SOB up here about engine serial #'s I can careless what year the hull is.

Now boat owners can see why you need your engine serial #'s and report if your app is stock and a stock tune or not. You can't fix it nor get the correct help if no one knows exactly what they are dealing with nor order the correct parts for it. Point made enough on this one by me.

the voyager ext coil HEI setup. is there a stand alone spark controller that can run it for a carb set-up, or could it be adapted to a thunderbolt-IV?

The stock module has a great advance curve in it. You can run it as is for Carb. The advance is only 22* - so it's great for many performance engine's.

Okay, checked my notes, here's the advance curve in the module for the Voyager / GM EST.
Note: since the advance starts so early (700rpm) make sure to set by the total as idle rpm may already have advance started.

RPM = Advance
700 = 11°
750 = 12.5°
800 = 13°
1000 = 15°
1250 = 16°
1500 = 16.5°
1750 = 17.5°
2000 = 18.5°
2250 = 19.5°
2500 = 20.5°
2750 = 21.5°
2800 = 22°
3000+ = 22°

Thanks for so much for all the input. Lanyard checked out. I will get pictures and post. I bought a camera snake this evening to try to get to get the serial number.

what year is your app and do you have a boat title or a non neg title ? Most states with boat tiltles list the engine and outdrive serial #'s right on the title or paperwork. Do you have a service records a lot of times engine serial # is listed on the service records .

If your boat is around 1999 to very early 2001 the parts list for the V-8 would be correct - unless your engine and ignition has been changed out to older or newer engine app.