I have a ZZ4 engine with a HEI distributor. It seems like my engine runs fine, but at idle my tachometer bounces around 700 RPM's to around 2500 RPM's. The engine is still idling when the tach is jumping around. I checked my electrical connections, but they seem fine. I am not to familiar with an HEI distributor. If the cap on the HEI is cracked would this cause the tach to jump around, but wouldn't this also cause the car to idle rough? We have an Autometer street tach.
Thank you

Make sure you have your engine grounded to the frame and to battery. also make sure tach ground wire(black) is grounded.

Look at the things mentioned above. If everything is good, measure the voltage that is feeding the tach while the engine is running and while the needle is bouncing. It would be preferrable to test the voltage with an analog multimeter. Make sure that the voltage that the tach is getting is constant 13-14v or so. The mulitmeter needle should not be bouncing while doing this test.

There is also the possiblity that the tach has malfunctioned. If you have a buddy with an extra one, hook it up on your car and see if it does the same thing.

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69 Camaro
400 Small Block
Trick Flow Heads
Comp 282S solid cam
Performer RPM intake
750 CFM Holley
and a Muncie four speed
Pics of My Car (http://cards.webshots.com/cp-25368180-AyNI-album/38182394akfhbj)

Good suggestions from jhow66 & Cameron.

My suggestion is the HEI unit has a TACH output on the connector. It is the unused terminal.

I guess if you had a stock wiring harness, it would be the unused terminal, but, with a custom set-up, I believe the terminal to use would be the outside upper left terminal looking at the dist cap with it removed and looking directly at terminals, place the four-pin connector at the 12 o'clock position.

Hopefully others here can verify this location.

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Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"