[pegasus1457]

I am trying to check on my tachometer's reading, using a non-contacting optical tachometer. I attached a reflective spot to one point on the lower belt pulley, below and to the right of the water pump housing.

The reading I am getting is about 2X what my tachometer reads. So at idle the tach reads about 800 RPM and the optical device reads 1600 RPM.
I don't think my idle speed is that fast.

I realize I have no idea what is attached to this pulley, inside the engine. Is it the crankshaft? Or a camshaft that might be running at 2X the engine speed?

Added info: the pulley I am looking at with the optical tach has 3 flat "feet" each of which has a sturdy threaded hole in the flat part. Was this meant for a power take off in its previous life as a tractor engine before marinizing? If so, this must be the crankshaft?

Does anyone know? Is there a better place to stick a reflective spot?

thanks,
pegasus