We have a pretty comprehensive Tech Tip on starting a long-dormant engine --
Check here -- "Bringing a dead engine back to life."It may seem like like a long and tedious process when you have a real urge to just put some gas to it, a fresh battery and ... let'r rip. But .... Why take a chance on ruining a possibly good engine because you skipped over something simple?
Rebuilding a 503 -- horrifically expensive if even possible.
Checking the oil, or for a stuck piston and taking a little time to correct -- easy and cheap.
We put that Tech Tip together from a lot of experience in bringing dead engines back to life. We even field tested it on a truck we (the ODSS members) eventually drove out of a swampy location where it had been for decades. We even got the brakes working fine. BUT ... we took our time and methodically went through the checklist before we just sprayed ether down the carburetor throat, put the 12-volts to her and cranked away. That engine and truck has had a lot of miles put on it since then (it's collectively owned by the members of the Old Dominion Stovebolt Society). You can read about it, and see pictures & video,
here