"The timing may be on a sticke
"The timing may be on a sticker on one valve cover or the plastic cover on the flame arrestor but I thought it was supposed to be at 6 degrees BTDC. Also, IIRC, there are specific things you need to do in order to set the base timing but I worked with more Indmar motors than Mercruiser and I don't remember the Thunderbolt setup as well.
Regardless, your boat should still run at cold temps and a couple of degrees of timing shouldn't make this much difference. When you do get it to run and punch the throttle, does it make kind of a soft popping sound? If it does, one of the best things you could do is get a manual and look up carb setup. Mercruiser calls it either a "rich pop" or a "lean pop" and if it does it for either reason, it needs setup. I wouldn't do a carb setup or set the timing on a Mercruiser without a manual, just because I don't know the Thunderbolt ignition as well as I know the Delco and Northstar ignitions. A friend of mine just bought a Glastron with the same motor you have and we're trying to find a Mercruiser manual. I haven't read a Seloc manual yet, but since I have seen other aftermarket manuals with errors (Haines and Chilton), I prefer to go to the source.
I don't know if you know anyone at a Mercruiser dealership but a lot of times, once they know you, some dealers are willing to copy sections of service manuals. Mercruiser sends out tons of updates, they constantly change parts and if you need certain parts, be aware that the part number you give them won't necessarily be the current one because when they change a part or spec for that part, the number supercedes to the new one. It's frustrating for you as a customer but it drives people who order parts crazy. Getting to know someone at a dealership is a great resource. This isn't a great time of year to pick their brains, but if you get them at the right time, they'll usually help you out, if they're a decent dealership. ."