I don't think this is piston slap, beside not sounding like it at all, the cadence is too fast. It sounds more like double crank speed. IDK, I can't recall ever having heard that before.
Can you hear it in the tailpipes or in the carb?
No Sir. I mean, I have not stuck my ear on the pipes when it's running. But, ambient, no. It does not seem to come out of carb throat. But I've never used a stethoscope before, and do not know exact points to pull from....
