I picked up a vibra tach (b&s model 19200) today. The directions have me stumped. Can anyone explain in clear, simple terms how to use this? I put the thing on my engine and it bounces all over the place. The thin rod (with yellow loop at end) that extends out vibrates at any RPM setting on the device. What I tried to do is extend the rod out to 1750 RPM setting and laid the device on the engine block. The device danced all over the engine block, so I sort of held it in place with my finger. The little wire did vibrate back and forth...but it vibrated at any RPM setting I tried. I'm lost. Any help is appreciated.