The clicking sound you are hearing may just be the sound of the valves opening/closing. Hard to say without hearing it, but it might be normal. However, if you have a "knocking" sound, that's more serious, and I agree you might need service. If you really do have a sharp knock, not just a click, coming from the engine, I'd send it in for repair.

When the engine ran the 4 times it did, was it smoking a lot? It sounds like it may be set too rich to run correctly. Once a YS engine gets flooded, it can be hard to clear it, and with fuel coming out when you remove the glow plug, yours is certainly flooded. If you decide to try it again (no "knocking" sound), try disconnecting the short fuel line that goes from the regulator to the carburator. Make sure the plane (or engine) is securely held in place, and open the throttle all the way. Connect your glow ignitor and turn the engine over with an electric starter. Be ready, because as the excess fuel is blown out of the cylinder, the engine may suddenly start, and it will rev up immediately and run fast for a second or two. If that clears it out, reconnect your fuel line and try starting it again. If it starts, you'll probably need to begin the process of setting the high speed, mid-range, and idle mixtures, as well as the regulator setting.

If it won't start, then you'll have to begin to troubleshoot everything, like glow plug, glow plug ignitor, etc. If you need more help, tell us what prop you're using, what fuel mixture (%).

David