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  1. #1
    Race, wreck, repeat delagem's Avatar
    Join Date
    Aug 2006
    Homer, NY

    Trouble with PET2100 Tach? Not PWC related...

    I'm having some trouble using a PET tachometer on a small motorcycle, and nobody but PWC enthusiasts seem to know anything about these tachs.

    Anyone ever used a PET and got roughly double the expected reading? The bike I'm using it on is a 2 stroke single cyl. Used to be a 50cc, now has a YZ80 dirtbike motor in it. It should be showing approx 10,000 at peak. Setting it in the 21:42 setting should be correct, but it shows 18,000 rpm. Raising it to 22:44 ought to cut that in half, but it doesn't, it brings it to 11,200 rpm. Not exactly in half, so something is definitely not right.

    I spoke with the experts on this bike, a race shop in CA. They confirm it fires just like all 2 stroke singles with a magneto ignition, once per rpm. They had never used a PET, and went off on some tangent about the "Left Handed Rule of Magnetism" or something and told me to flip the bike upside down or something.... (they actually told me to flip the alligator clip upside down, but that seems just about as useful)

  2. #2
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada

    Arrow Pet-2100dxr

    I don't use a PET tach, but it sounds like the spark pulse signal into the tach is not clear and strong.

    What the PET input is sensing is either the electric voltage field spike during spark or the magnetic field surge around the plug wire when the spark fires.

    According to this instruction manual, there are two methods of using the PET2100DX.

    Have you tried the non-contact method, which says something along the lines of "...Without using a wire, locate the PET2100 between 1 and 30cm away from the plug wire with the arrow pointing towards the plug wire, until the reading stabilizes..."

    The alternate method when using the sense wire is to remove the clip and wind the sense wire around the plug wire between 3 and 5 times. Again, you may need to experiment with this until the RPM reading becomes stable.

    I suspect you may be sensing electrical signals not just from the plug wire, but also from the ignition coil or other wiring.

    There is a note near the bottom that says "...When the engine is stopped, the mode is automatically switched to the mode set while non-measuring time and the display is cleared about one minute later..." Could that mean that the measuring mode is changing after you have set it?

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