autometer tach going crazy!!

[wichitacorvette]

Ok, I got my tach up and runnign but its obviously not working right.... at idle it hops around 3-4000 rpm...... give it a little gas and she shoots to max......
yes its set up for the 8cyl (no wires cut) and yes chassis ground and positive from ignition wire....

What is going on????

I do have a tpi motor..... with a msd blaster II coil, but no msd ignition..... using the factory tach wire that usually goes to the factory gauge. should i go to the - on the coil?

HELP!!!!!!!

 

[Robin7TFour]

was there a 4-6-8 switch on the tach? i know it seems like a silly question,but i have seen it overlooked

 

[wichitacorvette]

there are wires on the back (loops) that are to be cut for a 4 or 6 cyl motor...... for a v8 it says to leave them intact...... since i posted it has calmed down in jumping around..... but still off on reading... about 2000rpm higher and still bouncing..... does this thing need to "break in"?

 

[Jack]

Yes, I'd first try connecting autometer's green tach wire to blaster coil's negative terminal ... disconnect & tape up factory tach wire. That should fix it. If it don't ... you may need a "tach adapter" Even though you don't run msd ignition box ... suggest you contact msd for their recommendation of one of their tach adapters. If you DO run an msd ignition box ... DO NOT connect tach to coil!
JACK:gap