How did it get there?

[Rob43]

Yesterday, we were messing with my YS 60sr, trying to tune it. I just wasn't responding predictably to needle adjustments, nor was it running consistently from one tank to the next. Good pressure, no leaks, etc.

I removed the carb and looked it over. I removed the cap to the regulator assembly...there was a small metal shard under the diaphragm. Just barely into the space under the diaphragm coming from the tiny port on the edge. I removed it, and all was good.

Previously, I experienced detonation, and presume it came from the hole in the piston, but how did it get to where I found it? There is a check valve, two sintered brass clunks, and two filters before it could reach the carb in the fuel delivery path. However, it would have a direct path "backwards" through the crank passage back to the carb. Since the piston downstroke compresses gas pressure into the crank case, there is pressure in all directions inside the crank case. I guess it's possible to also push the particle back into carb...? What am I missing? I think it was too large to pass through the sintered brass clunks, and frankly, believe that it should have hung in the check valve had it gone through the backplate port. I never lost any pressure to my knowledge. Always had to burp the tank after each run.

Any thoughts?

[Rob43]

Looking at the engine diagram, I think it had to come from the crankcase into the carb body in a "backwards" manner. If it had come from fuel delivery, it should have been trapped on "top" of the diaphragm, and not under it where I found it.