my tach's been reading high in my 78 Z. it used to read 30 @ 60mph, after the car sat for about a year it started reading 33 @ 60mhp, after sitting for another 3 months it now reads 35 @ 60mph. how can i fix this? also, it's always read 15 when the engine is off, is that ok?

thanks,
-mike

Mike,
The first thing to do is to verify that the tach is wrong. See if you can borrow a diagnostic meter or some other type of "portable" tach, hook it up and compare the readings. The reason I am suggesting this is that it is possible, although kind of rare that if you have an automatic trans, maybe something has gone wrong and is causing some slipping ? I think 78 may have been one of the earlier years with a lock up type converter, if so it could not be working correctly ? I guess the whole point is, make sure the tach is actually wrong. I think a lot of the factory tachs tend to stop wherever they feel like it when you shut the car off so I would not be too concerned about that. I may be wrong on your particular year, maybe someone else will pitch in more info.
Hope this helps.

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

thanks for the advice, but i've got a 4 speed. i'm pretty sure it's reading wrong because it used to read 30 at 60mph, and not it reads 35 at 60mph. i haven't changed any gearing (tranny of diff) nor have i changed out the cam or anything like that.

Mike,
I guess the 4 speed sort of rules out any slippage. The only thing else I would check would be all the connections to the distributor, especially the one that goes to the "tach" connection on the HEI. Also any kind of missfire can possibly make the tach read wrong.

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

okey dokey, i'll check, but one last question, what is the HEI?
(i feel so stupid)
-mike

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1978 Z-28
4 speed
Carmine Red

High Energy Ignition. ie the distributor. Sounds like a pretty nice car, by the way. tom