1989 Mariner loss of power and shaking/ sputtering

I have a 1989 Mercury Mariner 50 hp motor. The motor starts on first turn and runs strong except that it sputters/shakes when i accelerate slowly. If i accelerate or decelerate while its shaking it stops. Also the top end speed has slowed, i used to hit about 35 going full throttle and now i can only get 30. I can tell by the sound that its not quite hitting its highest gear. Ive replaced the spark plugs, gas tank, and fuel pump so i dont think its fuel related. Any help would be appreciated. Thanks

Check compression and spark....

Try adding a 1/2 turn OUT on the idle mixture screws.

Jeff

Thanks for the replies, im gonna try and test spark tomorrow. The compression is good. Before i mess with the mixture screws, would that be something that changes over time? Because ive had the motor for about a year and the problem just started. Basically im asking if the idle mixture screw could over time need adjustment?

The tiny passages get smaller with corrosion and gunk build up. In addition, ethanol gas needs more fuel to make the same power.

Jeff

Once you are sure that your spark is fine you might want to pick up a couple of "snake oil" products.

If you have a Merc dealer nearby grab a can of "PowerTune" and just follow the instructions on the can (sell for less than 10 bucks). It cleans out any cooties in the powerhead itself (everything behind the intake).

Second, try some SeaFoam in the gas tank - it will clean up everything from the tank to the intake (and beyond).

Between the two products you will get rid of carbon, varnish, gum etc in the whole system. In your case it may do the trick. No magic in a can but both products work half decently if you catch a minor problem before it becomes a mechanical problem.

Good advice (as usual). One needs to ALWAYS close the vent on the tank when not running the motor (to prevent moisture getting into the gas) and running the motor dry after each use (if the motor will sit for a few days).

Jeff