If it is a Glo Bee tach you happen to be using, it is famous for giving strange readings. The only thing you can do is move the tach around while taking readings, and when you get one that is believeable, you are probably ok. I had one do this, and I even put a shroud around the photocell trying to make it work better. Didn't help. Too bad, because I rather like the tach other than the erroneous readings it gives. The only time it gives good steady readings however, is when it is pointed at a light. Then it gives the magic "3600" just fine...

AV8TOR