Acceleration Problems with 1992 Mercury 60hp 2-stroke

Hey guys. My grandfather owns a 1992 Mercury 60 hp 2-stroke outboard motor on his bass boat, and He and I have been having some difficulties regarding acceleration. The motor runs fine while idling and up to half throttle. But when we try to accelerate past half-throttle towards full-throttle, the speed remains the same as if I was still at half-throttle. In other words we can't get the motor to "kick in" when I try accelerating. The motor doesn't really bog down or stall or anything, it just won't accelerate past a certain point. So we took off the cowl and checked the spark plugs and fuel lines and cleaned the carburetor with carb cleaner whilst looking for anything that could be a contributing factor. We even replaced the fuel filter thinking that might do the trick, but the problem persists!

Long story short; We're stumped. I've been exhausting Google for the past few days with no luck. I came across this forum in hopes that we can get some sound advice or even a solution on this issue as to what is causing the problem. We're obviously not professional mechanics and I apologize if my explanation comes off as confusing. My grandfather's a jack-of-all trades type with some mechanic experience, but a bit of a tight-arse when it comes to money... so that's why we're trying to fix it ourselves before we consider a professional shop. We've asked a few local mechanics for advice and got a few different theories on the issue, but I'd like to hear your guys' take on the situation or if anybody here has had a similar problem in the past? Any thoughts or advice is greatly appreciated. Thanks!

Did you check whether the remote throttle control is moving the throttle control in the engine past the midway point. If the throttle control cable is malfunctioning or adjusted/connected incorrectly it might be causing the issue.

Just a quick thought..

Check that all cylinders are firing then check fuel system and carbs..