Hi
a common problem if the engine was used in salt water then put into storage. Often there is no indication from the tell tale, come the new season.
Take the hood/cover off the engine. There will be an outlet from the head that relates and corresponds so the tell tale outlet underneath. Get some electrical wire and poke it up the tell tale with engine running. It should clear. If able to, remove the hose at outlet from heads of engine. Sometimes its hard piped, sometimes a rubber hose you can remove and clear.
Failing all this, it’s the impeller needing done.
But if salt water, try the old wire up the tell tale trick whilst running in the water.