There are a lot of things it could be. If the bottom is clean and the prop is not damaged, and trim tabs are functioning, then start with the fuel filter, make sure spark arrestor is clean (I've seen rubber from the belts clog them), if it's carbureted make sure the choke is going fully open when warm, (Mercruiser electric chokes are notorius problems), and the throttle linkage is going fully open. Beyond that there could be issues that a good mechanic needs to look at. (ignition timing, compression, spark plugs, distributor, plug wires, fuel pressure, etc)
I'm not an MPI person but they have diagnostic readers available that can pinpoint problems quickly.
Good Luck!
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