Doing some looking around and came across this, you may be on the right track w/ignition problem. Stay with the T-Bolt, once you get your new ride sorted out you will never need to worry about ignition again. If you start putting vacuum controlled advance on I would anticipate possible problems in respect to anticipated vacuum levels under loads between a HEAVY Boat and an automobile, which I an certain the vacuum one is designed for. Just because it says Coast Guard Approved does not mean good to use on all boats, for example, if I were looking for a replacement for a 16-18' Donzi Classic it would be fine as the load and vacuum would more in line with a heavy car.
Anyways here is what I found while looking at dist. modules:


Sterndrives.Com
Marine Parts & Accessories

Distributor Sensors and Pick-up Coils
Sterndrives.com! stocks most of the Mercruiser electronic
ignition sensor coils and ThunderBolt Sensors.

All sensor coils must be properly installed or they will fail.
Over tightening of the mounting screws is a common mistake.
The Delco Distributor Module requires the use of transfer adhesive.
Most Mercruiser Distributors have a ground wire attached to the housing.
Replacement of a Thunderbolt Style Sensor will require the destruction of
the sensor wheel. Always replace the sensor wheel with NEW!

Sensors DO go bad from time to time. Symptoms can range from not
getting over 2500 RPM's to the engine not being able to start at all.

Well I'll be darned if that doesn't sound EXACTLY like what mine is doing. It was hard to start 3 times. Then today for giggle I tried to crank it. It busted off for about a half a second and died. Then wouldn't start.

I have run 5 thunderbolt iv engines never an issue with anything. I have heard people say the old style distributor pickup with the studs and external wire connections fail due to corrosion, but all mine are the newer sealed type.

A vacuum advance distributor isn't used in a boat because it is never under light load high vacuum cruise like a car. It is always under heavy load cruise. A vacuum advance distributor still has mechanical advance.

So why will dropping an hei distributor in a boat and capping the vacuum pot not physically work ? Not saying I am doing this I understand that would be dangerous and illegal by the coast guard rules. Just trying to understand WHY it won't work.

"Sensors DO go bad from time to time. Symptoms can range from not
getting over 2500 RPM's to the engine not being able to start at all.

Yea, when I read that I saw your boat! That was in reference to dist. module/pickup rotor combo.

Now if you are Hell Bent on changing the ignition my money would be on the "Daytona Sensors" box. Simply graph out your desired curves on the laptop and download to module. If your gas varies you could store several curves for gas grades adding a cable to dash would make it very smart.
Now if you are a geek (and motor-head) with this box you could add a throttle position sensor to your carb and a vacuum sensor to the manifold and program your ignition w a 3-D map. Now a vacuum makes sense.

That does sound cool. Surely I could figure it out. I'm fine with keeping the thunderbolt that you guys all are so fond of. If I can get it to work that is lol.

I'm not bent on changing the ignition just want to know why the hei wouldn't be sufficient. It advances timing with rpm manually. Kind of goes hand in hand with this carburetor for simplicity theory doesn't it?

It does work? That is exactly what a T-Bolt dissy IS, only they completely omitted the vacuum as it (vacuum port) is totally useless (constant low vacuum) instead of blocking it off.

It will work fine unhooked or plugged. It will essentially be working the same way as a non vacuum advance distributor since the vacuum isn't hooked up. Most marine carbs have the vacuum ports plugged anyway, although the quickfuel carbs I have do have the ports with rubber plugs on them.

Thank you guys for clearing that up for me. That is what I thought is that it would work just fine. Now to get the parts ordered for this thunderbolt to make it work again.

" It advances timing with rpm manually"
No it advances mechanically with mechanical tolerances and inaccuracies, does it not? Look at some pics here on OSO of the better known name brand distributors after a couple years marine use. Yes they were marine version all rusted and corroded. Like I posted earlier I personally can not recall seeing a corroded/rusted T-Bolt dissy. I have seen a lot of crusty Merc motors although.