As title says, I'm considering buying a 1971 Evinrude 25hp outboard from Craigslist. The post says "Runs good". I'm assuming the only testing I can do on the motor before buying it will be on ear muffs. What are the limitations of ear muffs? Should I avoid putting it in gear? How much throttle is safe to use on ear muffs? It being a nearly 50 year old engine makes me nervous, but the price is intriguing.
A compression test would tell you a lot. You can get a compression tester from autozone/orielly's for 30-40 dollars. If it's a pull start, pull the plugs out, and make sure the plug wires are folded out of the way, so you don't accidentally ignite gas fumes coming out of cylinders, If it has a kill switch, have that engaged too. Do several (5-8) brisk pulls. You should see atleast 100 psi, and make sure there is no more than 10% difference betwwen the readings for each cylinder (10 psi). An exceptional motor will read 120-130 psi, average is around 110-115 psi, good/acceptable readings are 100 psi. However, be careful that the screw in adaptor on the compression gauge that you thread into the spark plug hole doesn't stick in so far that it hits your piston, I had this happen once, and had to shorten the adapter slightly. The pistons come very close to the plug hole opening at top of stroke. If it's electric start, do the same, only run the starter maybe 3 seconds, and check it. Make sure it shifts in and out of gear by putting it in neutral, the prop you should be able to turn by hand both directions freely. Put in forward gear, it shouldn't turn either direction, same with reverse, except a little play both ways. No need to have it running for this. Be careful with muffs, sometimes they don't seal good, you should see water coming out of "pee hole" within 2-3 seconds of initial start, if not, shut off immediately, and readjust the muffs, or better yet, run it in a large barrel of some sort, just make sure the water level is high enough in barrel. There are a myriad of diagnostic tests, to address many issues, but this is a bare minimum before I buy a motor. If i'm satisfied after doing these tests, most other tests i'll do after a purchase to address fine tuning, and optimizing performance/reliability.