I have an early model YS 120SF that has been sitting around for a few years and now needs help to be revived. It ran strong when I put it away years ago, but now it won't start unless I force feed fuel to it, and then only for a couple of seconds. I replaced the fuel pump diaphragm. The check valve and fuel filter seem to be working OK. Is there someplace I could send it for a thorough checkout and repair if needed?[]

Chances are the passages in the regulator have gummed up fuel from sitting in them. Use raw fuel and to clean them. Sometimes a pipe cleaner helps. If you would rather send it in for a good check up, bearing could need help from sitting, you can always send it in to YS Parts and Service. Richard and his guys will give it a good going over and they will test run it before sending it back.

I recently had the same problem with a YS 120AC and yes the passages in the regulator were plugged with sludge.
Also the YS engines need pressure in the tank to feed the pump.
If the setup is in the original airplane I would certainly check out the tank and lines to ensure there are no pressure leaks.