Check the Filter on the manifold that comes off the tach wire...If fact, if it's stock just replace it...It's a cap that wave shapes the pulses to the tach and the computer.
Looks like a little round dude about 3 inches long with a tan wire out each end should be mounted on the right rear of the manifold.
ALSO double check that your tach is on the Filter side, not the Distributor side...The other way around will damage or destroy the tach.
If that dosen't Fix it, you need a Tach R&R calibration board, You can get one on EBAY (ongoing add) For 50 bucks.Listed under 1980 Corvette...or maybe look for a whole Tach Cheaper (good luck on that one...)Than 50 bucks...
The Tach's on C3-C4 Corvettes are always a matter of contention for calibration .. If you are handy and careful, you might try a Corrosion Control program on the circuit card, and plug, and the mechanical movement (may have a sticky movement) and bear in mind these aren't always instanious reporting tach's either..they take a second to get up to speed, I have a Monster Comp tach in mine, and it bunches quicker than stock, and about 250 rpm higher...
Thanks for the info. I looked last night when I got home and found round plastic devise mounted on a short metal bracket at the right-rear of the intake manifold. It is only about an inch (or less wide) and about the diameter of a 50 cent piece. I did not find tan wires in and out of it. Could this be the filter to which you refer or should I be looking for something that looks different? Do you know if this is a dealer-item or available at parts stores and about what I might expect to pay?
I found a picture of the filter at the following link, so now I know what to look for and the cost.
Spendy little dog, isn't it? 50 bucks for nothing more than an electrolytic capacitor, that you could probably get at rat shack for $2.49 .....
It's Sole purpose in life is to Clean the spiky Wave coming off the Tach output of the HEI and Shape it to an "Nice Clean" Square wave (or facsimile) for the Computer reference pulse and the Tach.
If I knew the Value, in MFD's and WVDC, I'd try an Electrolytic Cap on mine, and let you know the results, however It's another General Mystery Secret...Nobody seems to know the value...and, Caps can be nasty little bullets when you explode them from over WVDC...or reversing the polarity...
I don't think your local NAPA auto parts is gonna carry one either, although I didn't try..did a check on CSK, Auto zone, and Kragens, and they don't carry them..so It's probably a Corvette specialty item, Corvette Central carries them at $50 plus change.
If I had a Decade Box, I think I'd Find out the value of the darned thing, buy a box of $2.49 Caps, Vulcanize them to a metal manifold mount, and sell them on EBAY for $25..and Re~retire..Fat , dumb , and Real, Real happy..
Well, guess I don't have to tell why my user name is Corvettepoor. Even though I know virtually nothing about electronics and only understand a little of your technical explanations, I understand basically what it does, thanks to you.
I do appreciate the info. and the extra effort you went to in looking for the item at parts house websites.
Is'nt it amazing that the minute you add the word "Corvette" to a part the price doubles. Called the local parts house for a heater blower motor for a 74 Corvette and then the next day for a 74 Chevy and the price was double for the exact same motor.:thumbup: