Dumb Universal M4-30 question

pegasus1457 · #1 (**permalink**) 07-03-2009

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

JimsCAL · #2 (**permalink**) 07-04-2009

I would strongly suspect that the lower pulley is connected to the crankshaft. Getting a reliable reading with an optical tach is not as easy as it first appears. Any difference in reflectivity can produce a "pulse" and through off the reading. Make sure the reflective tape is on a clear are of the pulley and move the sensor in and out until you get a steady reading.

pegasus1457 · #3 (**permalink**) 07-04-2009

Hello,

Thanks to JimSCal for the response.

Here is a photo (not as sharp as it looked in the viewfinder) of the front of the engine. The "mystery" pulley is at bottom left, water pump at top left, and only partially visible in the top right is the alternator.

Does anyone know what the threaded holes are for?

pegasus1457 · #4 (**permalink**) 07-05-2009

Quote:

Originally Posted by JimsCAL

Getting a reliable reading with an optical tach is not as easy as it first appears. Any difference in reflectivity can produce a "pulse" and through off the reading. Make sure the reflective tape is on a clear are of the pulley and move the sensor in and out until you get a steady reading.

I moved the optical tach around a bit and only observed changes in the 10% range.

If this is really the crankshaft and if I can believe the optical tach ... I have a very fast idle!

JimsCAL · #5 (**permalink**) 07-05-2009

From the picture, you are definitely taking readings in a spot that will give you false output. Try putting reflective tape on each of the three spokes and then divide the readout by three to get the RPM.