'89 100hp Merc quits

[beckoning]

1989 100hp Merc 4cyl. Wonderful motor, historically trouble free. Now, it falters and frequently stalls when moving throttle from idle to forward or reverse, or creeping forward at idle speed. It starts fine, runs at 2000rpm and above very well. Below 2000rpm, 2 cylinders stop as is normal, so I'm running on 2 cylinders, and sometimes the faltering worsens as the motor goes slower driving the propeller, until it completely quits.

To summarize: starts good, runs good above 2000rpm, falters below 1500rpm, and frequently quits below 1200rpm or when shifting from neutral (idle speed) to forward or reverse. Case in point: when approach the dock and slowing to creep speed, the motor quits. When idling without load, it idles fine. Shift into forward and the motor quits. Starts right back up, and it happens again.

I'm open to your suggestions and thoughts.

 

[Dave1027]

Could be the top two carbs need to be cleaned out paying careful attention to the low-speed circuit. You could try re-adjusting the low-speed mixture screws first. What is the idle RPM in gear? Maybe that is set too low? Should be 650-700 rpm.

 

[beckoning]

Below 2000rpm, the motor runs on WHICH TWO cylinders? Can someone confirm that?

 

[Dave1027]

Below 2000rpm, the motor runs on WHICH TWO cylinders? Can someone confirm that?

Top two. Look at the carbs. You'll see only the top two have idle (low speed) mixture screws. That's how the whole 2+2 system works.