This is long shot but have had same issue. If it bench test fine it might be the lower unit locking up. Check your battery with meter. I'm working on a boat now with the same issues. Good luck John
I'm posting this for my father in law. This has been in storage for last 20yrs. 1982 Sylvan Alaskan 16ft with 1981 25hp Johnson electric start tiller steer. The trailer is a 1982 tilt highlander with 12 wheels with bearing buddies. Included will be anchor, gas tank, battery box no battery, a set of oars, and a fishing net. I just put a new impeller and changed the gear oil. $2500
This boat is very clean. firm on price. Contact me if any questions. John 585-455-9030