Digital Tach Tested--Strange Display

[tmcafe]

I got this little gizmo, which is supposed to be pretty accurate and reliable (and made in USA). The digital display shows the rpm in 10 revs increments:

 

ENM Tach

 

To set my idle I wanted to know what my actual RPMs are, since the tach on the V11 is said to be very inaccurate. The little tach is supposed to read the sparks, one every two revs. Okay, so I wrapped the red wire around the left plug wire and grounded the black one. The bike was warm and the idle sounded perfectly constant. The bike tach showed 1k. The little tach instead kept changing and showed all kinds of numbers, up and down from around 1240. On the right plug wire the revs appeared to be lower, still over 1100. The idle (helped by the hi-idle lever, otherwise it would be too low) didn't really sound that fast, at least from what I can tell. Also, I noticed that if I placed the little tach on the tank, it would show in the 1500s. I don't know exactly how it works, but it seems to be influenced by its position, also it seems that the revs shown are fluctuating too much, and they're also much higher than estimated.

 

Any thoughts? Thanks!

[Tom M]

My guess is it's picking up some pulses from the other cylinder because the sensor wire isn't insulated well enough.

[tmcafe]

Thanks Tom M. Makes sense, I'll try to stand back as far as I can from the bike on the side I'm measuring from. The wires are 3.5 ft, not too long.