1996 Merc 90 hp, low power and only 4000 RPM

[Texasmark]

Very possibly there were some finer black particles that made their way to your carbs and are clogging up your venturi jets. Dropping the bowl on one should answer your question. If you see anything in the bowl you can figure the carbs are clogged.

Also, be sure to put something under the bowl when you remove it to catch the fuel that drains out when you loosen the screw. That fuel could contain vital clues also.

Mark