I purchased a 25hp Mercury with manual start. The engine only has 8 hours on it, The problem is the previous owner stored it horizonally and now the pull rope is seized, Thinking the cylinders were full of oil, I pulled the sparkplugs and still no luck. I removed the pull cord and I can rotate the flywheel with medium pressure with a socket. I replaced the pull cord and it is still locked up. I have done some research and it appears putting some lubricant in the cylinders and let it set for awhile is the next step. I just wanted to run this by folks who know more than me before trying this. The engine is 4 stroke if that matters.

get some fogging oil, spray a decent amount and with plugs out rotate flywheel. put a pencil in cyl. and watch for it to move.

you may need to pull the pull start and clean/lube the parts that engage the flywheel. watch out for the spring unwinding.

wal mart has the fogging oil.

https://www.walmart.com/ip/STA-BIL-...lg9xEAQYASABEgJkXfD_BwE&gclsrc=aw.ds

Not to sound stupid but make sure it isn't in gear.

Thank you sir.

Not a stupid question at all but it is in neutral.

There is a plastic part that tends to go bad that keeps the cord from being pulled unless the motor is in neutral.
You would have to find a schematic of your motor and look for the part and order a new one. The part is broken/stuck, and it's telling the motor it's not in neutral even though it may be.
Crowley Marine is a good source and shows the schematic and parts number and you order online, quick shipping if in their warehouse.
Sometimes you can turn the tiller handle slightly like you are going to put it in gear, and pull on the rope at the same time and it will release.
Some people have removed the part completely but that can be dangerous if you forget to put the motor in neutral before starting.