I've had the same problem with the GloBee tach. To test it, point it at any household light and it should read 3600 rpms. If it does, then the tach is ok, and the irratic readings are just the "nature of the beast". You can usually move the tach around behind the prop until you get a believeable reading. A stock Homelite 25cc should turn a 16 x 8 prop at around 7000 to 8000 rpms, depending on lots of variables. If your tach reads 10,000, or perhaps 4800; those aren't reliable readings. When you get a reading that is in the range you expect, that's the one to "trust".

You might have a little more luck if you can get the sun in front of the engine while you take tach readings from behind. Another (semi-desperate) way is to do it at night with a light shining through the prop arc. (Note that it has to be a battery powered light to work right.)

If you want a really good tach, spend the bucks and get a "TNC" tach. They cost around 80 bucks or so, but really work great. They are not sensitive to position, lighting conditions, etc., and I have verified their accuracy as being excellent.

Let me know if you find any way to make the Glo Bee function better. I even put a tube shield around the photo sensor on mine but it didn't seem to help much.... It is too bad, because except for the erratic performance, I like the tach. []

AV8TOR