May be carb jet problem ... without being there ... try coaxing it over
the 1800 ... I had some carb jet (I assume) blockage (could have been
water) that went away with a little coaxing. I store my 4 bbl carb dry
(1983 not yet rebuilt) ... I installed a fuel shut off so I can run the
carb dry. In the spring, I top the fuel bowl up through the vent with
carb cleaner, let it sit, then start it up. I keep saying I'll do this
mid-deason but haven't yet. Parking the boat carb is keeping the float
from becoming saturated, jets from gumming up and so on ... well,
that's the plan.

You might try that ... run the carb dry, top up with cleaner, ...

You should pull the plugs and check them. This imo is something to be
done every month or so .. especially with an older engine ... as
mentioned, mine's an '83. Your plugs tell you all about your engine
condition including if your risers etc might be leaking water ... very
useful to know before destroying the engine :-)

Anyway, you asked what would "I" look at first, and those are it.