I bought a used YS 120 engine. The guy I bought it from said it was running well, just hadn't used it for awhile. When I received it, I immediately disassembled it and ordered new gaskets, o-rings, bearings and piston ring. None of the old parts looked bad but I did not want to try my luck on them. I got my parts from Central Hobby (nice folks to deal with). They got me the right parts as well as identified my engine as an YS 120-NC. When I finished assembling the engine it turned over real smoth. I put it on the test stand with a 15-10 wood prop prop. As per instructions, I turned the prop over ten times with the throttle wide-open. Closed the throttle to an idle, put a few drops of fuel in the carb and proceeded to crank the engine by hand. It would only pop a few times so I backed the low-speed adjustment out a quarter turn (regulator) and the engine started promptly. After running for about 10-15 seconds, I heard a noise and the engine quit. Upon inspection, the prop had come loose. Retightened the prop and started it up again with no problem. Ran for about 10-15 seconds, sounded really nice, then again the noise and the prop is once again loose. I know it did not run in reverse as it blew my rag off of my bench, etc. Did this same scenario about 4 times and then decided to check on a few things. It still turns over real smoth. I noticed the ridges on the aluminum part that the prop goes up against were very smooth. Although this is my first YS, I have had many glow-engines, two strokes and four strokes, and the ridges were much more prominent on my other engines. Also when I received the engine, I got two of the same kind of prop nuts instead of the one with the indention and one with the protrusion to lock up against it, I got two of the prop nuts with the indention and was just locking these together after tightening the one against the prop.

Any suggestions would be appreciated.
My guess is that the ridges on the aluminum part are not digging into the prop like they should as the back side of the prop didn't have the normal pattern that my other props on other engines have had after tightening them down . I am also guessing that I need to get the other kind of lock nut that will go into the one that I have in order for it to truly lock intself down.
Thanks in advance for your help.

Roy Seals