I've put the boat away for the winter, but I just thought I'd throw out what's happening for some ideas. It's a 1988 motor I've had since March of last year.

 

1st problem:  At WOT, the boat would go, but not get up on plane until it had run about 10-15 minutes. Found out cylinders 3 and 4 weren't firing until it warmed up.  Once it picked up, it was fine the rest of the trip.  Replaced the trigger to no avail.  Screwed around with switching coils, and the problem resolved. Probably a bad ground.

 

2nd problem: Motor would start and idle fine, but would dog down as I throttled up, but usually wouldn't stall.  Frantically squeezing the primer bulb would usually pick it up enough to get to WOT and plane.  But once I idled down the motor or shut it off, it would do the same thing again.  Rebuilt fuel pump, replaced primer bulb and fuel lines to no avail. Finally replaced needles, seats, and floats in all three carbs, and that took care of the problem.

 

3rd and current problem:  Motor starts and idles fine, but will dog down as I throttle up. If I don't throttle down to a crawl, it will stall.  Squeezing primer (like problem 2) does nothing.  If I go through the process of throttling up and down 5 or 6 times, shutting it off in between, it will take off and be fine the rest of the trip.  All cylinders are firing.

 

Plugs were new last year.  I use only ethanol free gas and pre-mix with oil.

Any thoughts on my latest problem?

Sounds like the low speed jets to too lean, open each one about a 1/4 turn and see what it does.  Understand, the carbs on that motor are a mother bear.  I went through a set three times and adjusted them an 1/2 dozen before I ever got it to run right.

If it does ok there, you can try leaning them back a little at the time if it idles too bad.

I'm looking at a schematic of the carb but the jets aren't labeled.

Low speed are the idle mixture.  Those are the only ones that are adjustable.  Normally, if you adjust a two stroke motor so it idles the best and smoothest, it bogs on acceleration and sometimes lean spits.  Mercury even put and idle control module on that motor to help make it idle smoother.

Lets also state what I have said many times, I am not a Mercury person.  The only reason I know much of anything about your motor is my dad had one, and I wound up with it and let my brother keep the boat it's on at the lake where he lives.  So, I've had to work on it a few times and had to buy a factory manual for it, which I gave to him and he says I didn't.

Gotcha. This might seem like a silly question, but should all three carb jets be set the same?  I believe I set all 3 to 1 and 3/4 turns out when I replaced the needles and seats, so perhaps I caused problem 3 when I was trying to fix problem 2.

And the problem definitely progressed.  After I did the carb work, it worked fine for a few outings.

Then for a few outings, when throttling up, it would just pause for 5-10 seconds, not throttling up or bogging down, just chugging away at less than full power, and then it would take off and be fine.

Then, the next few outings (until now) it starting doing what I described above.

Adjust all three evenly for now. going out about a 1/4 turn each time to see if that helps.

Now, you could have some sort of ignition problem where it's loosing a couple of cylinders causing the problem, but I would start with the carbs.

 

Will do.  Thanks.