1992 Mercury 115 occasional rough running

[userreeducationtool]

I just picked up a new-to-us boat that has a 1992 or thereabouts Mercury 115 4-cylinder 2-stroke engine. From the previous owner it sounds like was stored a lot and used very little, and the outward condition of the motor reflects that. It starts and runs fine, very smooth at idle and up to 1800 RPMs when the 2nd set of carbs start opening up. From 1800-2000 RPMs or so it's kind of rough and choppy, then smooths out above that. WOT is fine, there might be a little bit of a bog on the holeshot but not much. Coming back down to idle / a slow-no-wake speed from WOT though it idles very rough, and has stalled once or twice on our test run. Starting it back up though, once the idle smooths out it runs well at low speed again.

I'm not sweating it too much right now under the assumption that there's probably still old gas in the tank and I haven't gone through and done any carb cleaning or major adjustments on the motor yet but is there anything else that sounds likely? We already have the recommended plugs installed per the service manual to improve running between 1800-2000 RPMs. Really I'm mostly concerned about the stalling coming down from WOT / cruise speeds. Anytime I've had major carburetor issues it's manifested itself as idle issues first and foremost, not this type of issue.

 

[QBhoy]

Is it not more likely that, this is just the usual 2+2 behavioural antics…and you maybe just need a carb clean/tinker ? They do run very rough before and whilst transitioning from 2 cylinders to 4…and vice versa. Infamously so even. The engine stalling isn’t normal though, obviously…maybe that can be dealt with by a fine adjustment.