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Are you sure the '87 had shifted into overdrive?
Here's a dumb, simple, but effective trick you can use to verify your tach.
Paint a bright white spot on the edge of the front pully -- maybe right at the timing index mark.
Illuminate the front pulley with a FLUORESCENT light.
With a helper watching your tach, run the engine and manipulate the throttle (by hand, under the hood) while watching the front pulley.
The white dot will appear to stop (stroboscopic effect) at multiples of the 60 Hz frequency (120 Hz, actually). 900 RPM, 1800 RPM, and 3600 RPM would be good reference speeds. Your helper can tell you when you're approximately at 900 or 1800, and then you juggle the throttle to "stop the dot" -- and then helper can read the tach.
The dot will appear to stop (and also appear in multiple spots simultaneously) at various speeds that are harmonics of 120 Hz (remember that the fluorescent light flashes twice per electrical cycle).
--
Don Foster (near Cape Cod, MA)
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