try pulling the cyl. head if it proves to the a PH issue.
Soak the pistons with wd-40 wd … PB blaster and like products let it sit for a few days to soak in then start tapping the pistons with short firm taps alternating the cyl and keep an eye on FW mouvement ( mark a referrence mark with a marker). use a 1 inch round wood dowel)
if no mouvement …. try to resoak a few more days try again … took me a week and half to free up a badly stuck 9.5hp and to this day it works great as my kicker .
if no success then proceed with investigation of the innards
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