1990 3.0 OMC Misfires under load when hot

So here is the issue. I have a 1990 four winns 170 freedom with a 3.0L OMC Cobra. It sat for a year and was never run . I actually had some time to use it this year so I took it out and had a great time . . . . . for a little while. I have probably put 20 hours on it since the start of summer. It was running great and all of the sudden it started to sputter and die out on me when ever i had anything electrical going. So i took the negative cable off and it died. Replaced the alternator and things were good, for about 45 minutes. Then next thing i knew it was sputering when under any load, but mostly when i trim the motor . I read on here that someone else had the same type of issue that just so happened to be the coil. I figured what the heck , why not try it , its only 25 bucks. Fixed the problem for about 1 hour, then it has been doing it again. I am really at a loss. Here is a list of everything i have done so far :

Changed oil
Plugs
Ignition coil
points
condenser
alternator
added a 2nd battery and a 2nd ground to the motor
fuel filter
checked the tank itself , no debris at all


any ideas???? Thanks so much
Dave

You could have a bad main electrical connection. Take off your original battery cables, clean and tighten and the same at the other end. Sounds like when using other high current loads your ignition is starving for voltage. How did you wire in the 2nd battery, in parallel or using an isolator of some type?

Ok lets get back to basics:

1- One battery one set of cables. Cable off the battery make sure that the cable is good, don't just say they are good make sure. Follow the neg cable to the motor mount ground, take off clean the connection for sure shiny and clean.

2-Pos cable good, follow to connections of course disconnected battery. Every connection cleaned. Barrel plug take apart and clean inside, green inside ? Maybe one wire going into the barrel or coming out bad ? Look, wiggle, look some more.

3- Run it fixed? if not. We got to go to the dist. shaft tight no play, got points? they clean gapped right ? Run sandpaper on the points, they new do it anyway.

4- Wires good connecting the coil, all wires not grounding out? Rotor good? Cap good, no moisture in there. Listen don't say you checked. say you checked again chief after you asked me too.

5 - all plug wires good, you checked it all right?

Heres what i would do if i was you, ready:

Run the boat in pitch black darkness.

If you see sparks you missed something.

Fix what you missed.

Post back after the dark test. Then we will talk about bypassing stuff to see if we can narrow it down to the actual problem. That is if it's not fixed by you after you run it down.

Thank you for your replies! I will go over everything this week and hopefully get it out this weekend to test it . Thanks again for the help I truly appreciate it!
As for the 2nd battery, I wired it with a marine grade battery switch , that has the option for battery 1, battery 2 or both . I am going to trace all ignition wires and all related wires and clean them very well. Im sure it will be something simple , once i figure it out lol

Please do us both a favor go back to one battery no switch, cable straight on the battery.

In the future, for not much more than the price of a 1,2,BOTH,OFF switch you could install a Yandina combiner that will eliminate the switch while keeping both batteries isolated and fully charged.
Another option is a Blue Sea Systems VSR (voltage sensing relay) which does the same thing. The 1,2,BOTH,OFF switch allows only one battery to be on line. The second battery just sits there until the user changes the switch. With the automatic electronic devices noted above, this operation is nullified.
The point is that you want to be sure that both batteries are always fully charged at all times. It's a safety thing. If for some reason your start battery goes flat you will need that second battery to be there, ready.
The 1,2,BOTH,OFF switch is so old school.

I have not gotten a chance to get to the boat but i want to say thanks again for all the help, i look forward to up****** you as soon as i get a chance.

I got a 2.3 which dies after 15 or 20 ninutes . Look at thread "2.3 solenoid sticks". I have always thought my voltage is too high 14.8 to 15 or more at battery when running. Today I noticed it fluctuates continually. The volt meter at the helm bounces up and down between maybe 13.5 - 16v. I looked at the navigation lights and they were pulsating. At WOT I get 13.6 or more at the + coil. Could this be a bad regulator? I have meticulously cleaned all grounds and checked and cleaned all connections. I looked up the alternator on Doug Russel. They show a replacement regulator available and other sites show one too around $70.

not for nothing whats this got to do with the original post?

No wonder i'm cutting back on posting. Guy's don't use any common sense. Who is gonna reply to you when you tag onto another thread ?

Sorry... didn't mean to upset you Chief. I have the same symptom. "Misfires under load when hot, only mine stalls soon after. I thought I might get some feedack from somebody with the same issue. I had a "history" on the other thread. Rather than rehash I thought i would send them there. For anybody else thats interested, my alternator tests as good. Still don't have a clue whats wrong with this boat and the boat shop had it 3weeks and $660 and it still acts the same way.

start another thread ! it's for your benefit.