Are you sure that it's the tach that's inaccurate and not the speedo?
Have you compared the mechanical tach to a known electronic one such
as those available as part of inexpensive dwell meters? If you are
sure that the tach is wrong, you can have it calibrated at a
speedometer shop. Not sure how many of those exist nationwide, but I
understand that that the one here in California, Palo Alto Speedometer
(http://www.paspeedo.com/) will restore/repair/calibrate speedos and
tachs via parcel shipment. You send it to them, they'll fix it and
send it back to. And yes, they do work on Veglia products. I have no
idea what it would cost.
George Graves
'86 GTV-6 3.0 'S'
On Dec 21, 2010, at 6:15 AM, George Beston wrote:
I'm trying to figure out the relationship between engine speed and
the drive
speed of a mechanical tachometer.
My Alfetta's tach has been unreasonably inaccurate for some time.
At 70 mph
it reads 4200 rpm when it should read 3500.
I have the tach out of the car to play with as a winter project.
There's no
easy way of making any adjustment to it that I can see.
Before I take it to an instrument technician, I want to know what the
relationship is between the cable rpm and engine rpm.
My best guess is that the cable rotates at one half of engine
speed, because
it's driven off the intermediate sprocket of the timing chain system.
I doesn't look like cable speed is geared up or down at the water pump
fitting, but I don't know for sure.
Has anyone been there / done that and figured this out already?
Your advice could save me some valuable time!
George Beston
Spider Jr.
Alfetta Berlina
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