99 mercury 75 hp. Is driving me nuts

[Fiberglass and more]

Hello, I have a 1999 mercury 75 hp. That's on a pontoon. And it will not run properly at full throttle.
it acts like a bad coil, or a miss I have went thru the carbs I can cruise at low speeds but when you throttle up it will start to gain rpm then subside.
Between 3000 rpm to 4000 you can not get top end or full consistent power. I have tried to adjust mix screw. On each carb. Till I'm blue then red .
Can some one help me ??

 

[ThomW]

Have you checked the timing?

 

[flyingscott]

leave the timing alone. Check for spark and compression

 

[dadofemily]

Mine was doing the same thing. I pulled one spark plug wire off at a time. two of them made the engine die. One made no difference at all when unplugged. That cylinder had broken reed valves.

 

[Texasmark]

Mine was doing the same thing. I pulled one spark plug wire off at a time. two of them made the engine die. One made no difference at all when unplugged. That cylinder had broken reed valves.

That's most unusual. I expected you to say that your CDI/CDM was triggering intermittently.