> One way I thought about doing the "calibration" was to pick a
> RPM, say 2,500. With the tach out of it's case and the cable
> out of the firewall, have someone hold the throttle at 2,500
> RPM and paint a new mark on the drag cup. Stop the engine and
> move the drag cup to the new mark and install the needle
> pointing at 2,500 RPM. At least it would be accurate at mid-range.
> What do you guys think??
What if you just note how far the needle needs to move (eg in 100's of rpm)
then take it apart and move the needle that far? For
example, hold the throttle at 2500 rpm (as indicated by the digital tach) and
note the original tach reading, perhaps 2000 rpm.
Then disassemble, hold the cup so the needle reads 2000 rpm; make a reference
mark there, then move the needle to indicate 2500 rpm.
You can use the new reference mark to double-check the reading after moving the
needle. I would probably use pencil though, and
erase it afterwards to avoid confusion for the next technician :)
I wanted something a bit more sophisticated, so I could adjust the strength of
the magnets as well as the position of the needle, so
I rigged up a cable to the lathe and used it as a variable speed source.
http://i258.photobucket.com/albums/hh260/TR3driver/Speedometer%20repair/DSCF0050.jpg~original
Thought of doing something similar with an old sewing machine motor, but the
lathe was easier to link to the cable.
I measured lathe shaft rpm in various gears (with the HF tach), then used a
simple spreadsheet to translate that to mph
http://i258.photobucket.com/albums/hh260/TR3driver/Speedometer%20repair/Atlas%20Lathe%20speeds%20rpm%20vs%20mph.jpg~original
Randall
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