In the water, as soon as it fires off, give it a bit of throttle, not much but enough to keep it running till it warms up a bit, I think I take mine to a bout 2200 RPM, I go by sound though.
A carbed engine can be cold natured on cold starts, normal hot idle throttle is usually too low for a cold idle.
But, if it's not making power properly after a few minutes warmup then there could be a problem that needs immediate attention.
Don't run the engine hard until after it's warmed up a few minutes at an elevated (no wake) throttle, the cylinders need to expand and stabilize.
It's not fuel injection like so many are spoiled by, FI can start and idle like it's already warm b/c the computer controls fuel mixture based on a map and sensor input, computer calculates how much fuel is necessary for the conditions of throttle position and engine rpm, air temperature and engine vacuum, so it knows and reacts automatically if engine rpm's drop or air temps are low.
It's carbed, so expect to "nurse" the throttle open slightly when engine is cold, but beware the jet pump continuously pumps water and propulsion is a direct function of engine speed, along with the rear bucket position (reverse, "neutral", forward) Neutral is a balance between forward and reverse such that the boat almost doesn't move.
And yes, the engine bay needs cleaning. I use bilge cleaner and hot water, some guys use simple green or some other products names I can't recall at the moment, Super Clean, I think it's called.