Hard starting Mercury 50hp

I have an early 90's Mercury 50hp 3 cylinder 2 stroke that is giving me fits. It's always been a bit hard starting but now something is off enough that I need to hook up one of those portable jump starter packs to get it started. 

For example yesterday after a few cranks I get her running and cruise around for a few minutes. Stop several times and restart with no issues. No jump pack used yet. Back at the ramp I turn it off. Now it will not start but cranks like hell. Hook up the jump pack and she fires right over. Does this several times and this is consistent with what it's been doing the last year or so. 

Battery tested good and cranks starter fast. Electrical connections are all tight and clear of corrosion. I'm thinking electrical part(s) have failed but I'm tired of guessing. Have replaced a good number of things over the last couple years. 

I did notice the voltage on my fish finder read 16.4 at times the last couple days. I don't recall ever seeing it near that high in the past. From what I've read regulator and/or rectifier are likely shot. But if so how would either of these cause the motor to only start on a jump pack? 

Has anybody ever seen something like this before?

Been meaning to give an update but never got the boat back out last fall. I've been out twice so far this spring and the motor starts and runs great. Problem seems to be resolved with the stator replacement as it provides charging and ignition on this motor. 

Good deal – let us know how the story ends!

New stator, rectifier, and battery installed. Starts right up with ease now like it has never in the years it's been mine. Didn't even have to use the high idle lever that was a must before. Cautiously optimistic, but this was only in the driveway. The real test will be a couple weeks from now when I have the chance to get it out on the water. I'll update again afterwards.

My battery was tested to be good and cranked the motor fast but I replaced it because it was a "sealed" flooded cell. Turns out with these old non-regulated Mercury's, maintenance free batteries should NOT be used. Only flooded cells with the removable caps to add distilled water should be used. The flooded cell battery acts as a regulator of sorts boiling off any excess charge through the vents. The sealed batteries cannot do this. They won't last on this type of system and they can cause damage to the charging system itself.

The power pack stores voltage from the stator to release it to your ignition coil when the trigger tells it to. Often a faulty Power unit will see intermittent or no spark on cylinders.

Also, they are notorious for intermittently failing. They usually start to act squirrelly when the engine warms up. They can be really tricky to diagnose, as they are working half the time, and you can’t diagnose it has failed when it’s actually working...

Get it fixed before you start shorting out other things, locators are like any other computerized device,  they like a certain voltage, 16 volts will cook a unit if left unchecked long enough, 

The rectifier and stator both failed the ohms test. Replacement CDI parts are ordered. Hopefully that's it and the power pack is ok. 

Definitely the first thing to fix! 

Three terminals, top two have yellow wires coming off. That is the rectifier. 

I just don't get how the starter can crank like heck off the battery and not start the motor, but first crank with the jump pack and it starts. I'm thinking damaged stator, switch box, or coils that won't fire correctly off the battery alone, only with the voltage boost from the jump pack. Expensive parts to be guessing though. Looks like a rectifier and voltage regulator (if I have one) can be found cheap. 

I see the rectifier, but I do not see a voltage regulator. Parts list/diagram online shows one and it appears to be somewhere above the rectifier. 

But if I'm getting over 16v at the battery the alternator must be charging the battery?