Doc here, imp:

Check your Coil, It sounds like it may be going out.

PRIMARY RESISTANCE = (BATT to TACH) Less than an ohm, but more than "000" on the readout.

SECONDARY RESISTANCE = (BATT to rotor pickup) 6,000 to 31,000 Ohms .

Meter set to RX1 (for primary)RX20K or higher(for Secondary) on OHMS SCALE.

If out of those ranges toss the coil, and get a new one.

The coil may be breaking down under load, (internal Arc over) and an ohms test won't find it you need an oscilloscope for this..Of you suspect the coil is doing this, swap it with a known good one.

Doc imp:

 

Thanks Doc. Coil tested good .
Swapped out the cap and rotor from my truck (that the Tach works fine in) and tried it in the Chevelle and Tach is still bouncing around ...

Plugs or wires don't effect the tach even with a miss right ??? (I don't hear a miss anyway and wires look excellent )

Engine runs smooth on the stand.(No insurance for a test drive !)
This Car has been sitting for a few years..I want to work out all the bugs Before I put it on the road .
All my eng. grounds are cleaned up and test good ...
HHmmmm ???What to check next next ??

 

Maybe the ignition 12V supply wire is not sending enough voltage to run the tach properly, but still enough to run ignition ? I have my electric choke hooked to that circuit as well....I might be on to something here ??? Maybe I have some rusty fuses ??

 

Doc here, imp:

Is this a Stock GM tach?

IF so, the Filter/Buffer may be bad..OR if it was discarded, the Tach may be going south..Common problem on stock GM Older Tachs.

The Filter/Buffer Is supposed to Shape and Buffer the Voltage/Waveform the Tach Calibration Card see's into something it can "Live" with.

A lot of times the wires will break and guys will toss them thinking they are just a "Noise Suppressor" (looks just like the one on a Coil or alternator, but it isn't..)

Without it on the older systems, the tach will soon die.

Try (if you have one) a second tach, see if it reports accurately, If so that probably is the problem.

BTW: You SHOULD move that Choke heater to it's proper system (Choke RELAY operated from the Alternator field wire) It can consume tons of current.

Doc imp:

 

Ran 12V power from my fuse box to dist. and the everything works great !
When I put the HEI unit on ,I hooked up both 12V power wires that used to go to the coil to my HEI.
Testing both those coil 12V supply wires,neither one of them had 12V ???
So I just wired direct to my fuse box ....
The old coil system didn't have some kind of resistor in line did they ? (ballast resistor)
It was a very long time ago,,,,
Why was there two power wires going to my old coil points system ?
Where should that coil power be coming from ?

 

Doc here, imp:

IF the Old ignition system was points type, then a ballast was used and will not work with an HEI..If it had an HEI it will not have ballast.

YOu SHOULD have mentioned you had a points system before..

YES, two wires are used for a points type coil, one of which comes from the Ignition switch to a ballast resistor, then to the + side of the coil. The other wire is SECONDARY Ignition, used to supply 12 volts Directly to the coil + in CRANK mode only From the starter solenoid "I" or "R" terminal.

If you had a ballast wire or resistor in line The Car SHOULD have performed POORLY..(If at all, because of reduced coil power on the HEI) Not just effect the tach, however maybe , after firing the plugs not power was left over for tach support..dono..

YOU need to find the wire coming from the ignition switch, determine if it has a ballast resistor, OR resistor wire, and REMOVE the ballast, Or Ballast wire, graft in a new wire and run that to the HEI BATT terminal. A Ballast wire is usually white , cotton clad, and be marked "Resistance wire Do Not Cut.."

Running a coil off the fuse buss is problematic, Coils at full load can pull up to 15 to 20 amps for stock, and a performance coil can pull up to 48 amps..It's hard on the fuse buss..in addition to that, the coil MAY run in ACC mode, causing heat and premature failure..The Ignition wire is dedicated to the switch "Hot In Run Only"..AND is the proper gauge wire for the task.

Doc imp:

 

Had a major volt drop somewhere between fuse box and up to the ignition switch and back down to the fuse box. 13v. on one side of the fuse box and 11.6v. on the other side of the box.
I took the fuse box and the ignition switch connection apart and cleaned them with electro-wash ,,,.Volts are up ,,. Hooked my ignition back up to the right wire,and tach runs smooth and true !!!
BTW it is a Sun Super Tach II .
Thanks for the help Doc !!

 

There Is not enough Volts @ Idle in the Alternator Field wire to activate relay properly .
I just used Ign. power from coil wire to activate a relay.
Choke Relay power supplied from Horn relay terminal. (fused line)

 

Doc here, imp:

If you have a GM , SI series Alternator, and your not reading 12 volts at idle on the brown wire..you need a new alternator..or soon will..the diodes in either the regulator or the rectifier are breaking down and reverse Biasing, causing the lower voltages..and it will get worse.

GM has hooked the choke heater up here , or at the oil pressure sender for years..Via relay, and it never had a problem..

BTW , IF the relay was "Buzzing" that is a SURE sign the alternator is shot.

Doc imp:

 

SI Series alt. ????
71 chevelle ....Field wire is BLUE..external reg.
Alternator is FINE.!@!

 

NO ....They do NOT use the FIELD wire to supply choke in any of my Tech. books 1967-1987 !!!!
Electric choke used from 81-87.
They did use the oil press. switch Dark Blue wire.
Except that it is a diff. kind of oil press. swich ( TWO WIRE )
I'm talkin old school here Doc.
Remember when ..... ?

 

NO ....They do NOT use the FIELD wire to supply choke in any of my Tech. books 1967-1987 !!!!
Electric choke used from 81-87.
They did use the oil press. switch Dark Blue wire.
Except that it is a diff. kind of oil press. swich ( TWO WIRE )
I'm talkin old school here Doc.
Remember when ..... ?

yes they did. good manuals ( not Chiltons or Haynes ) will have good correct information.