Yes, your problem is likely varnishing in the slow-speed passages of the carb, but other factors, such as over-full oil can also show similar symptoms. What method was used to clean the carb? Did this problem just start? Do you always run the carb out at the end of each day?
The only Proper way to clean these carbs is by a complete disassembly (including jets and emulsion tube nozzle), followed by a 4-hour room-temperature soak of all non-rubber parts in real carb dip (available in 1-gallon cans at auto parts houses), and then a liberal and aggressive blow-out with generic carb spray. The spray alone in not adequate to dissolve the organic varnishing in the small passages.
Never ever insert any kind of wire through the jets or passages, as scratches can cause weird flow characteristics, turning the carb into scrap.
Always set the idle speed by tachometer, and verify that it's OK by warm idling both in and out of gear. If too slow, the motor will stall when engaging gear.
Not only is it illegal (in the US) to drill out the plug covering the idle mixture screw, they are set by instrument at the factory, and it is extremely difficult to improve on that, let alone get it as good as it was originally. If you richen the low-speed adjustment to cover up a varnishing condition, the mixture will be wrong at other speeds.
You must run the carb out at the end of every day to reduce varnishing.
The carbs for the MFS 4/5/6 are interchangeable, so if your carb is beyond cleaning (due to corrosion, etc), you can go with the carb for the MFS6A2, (as long as your boat can accept the 6 hp rating) for less than $200.