tacho issue

I have a kent motor with a bestek electronic ignition and a pricol tacho until recently I was picking up the coil feed from the standard resistance wire in the car wiring ( 9v output ) and the tacho worked properly
I have now changed the coil feed to a direct 12v straight from the ignition switch
now when I start the car the tacho sits on 2000 rpm even though the car is idling at about 900 /1000 rpm by ear.
when I rev the car the tacho drops to about 1500 rpm and then rises
with the revs and is correct all the way up until redline
the tacho is definitely correct at redline 6500 rpm
does anyone know why it is showing 2000 rpm at idle? although I guess
it is the extra volts or does anyone know how to fix it ?
thanks
mal

Just had the same issue with the fitment of my MSD 6AL2 programable ignition, there are to types of taco triggers, voltage or current. The factory taco trigger in my Capri is a current trigger. The original factory install has a 12 volt feed to the taco then off to the resistance wire and coil, then switched via the points to ground.(the coil and resistance wire limmits the current draw of the system)So as ya points trigger the taco see's a current draw in pulses and then indicates the number of pulses on the needle. If you take the coil out fo the circuit and replace it with a relay to start your new ignition, your taco will show as it has done in your case a funny value.(because the overall circiut resistance has changed)How ever in my case, the 12 volt feed wire from the ignition to the taco got very hot and almost melted, because i am useing an optical triger and had hooked the taco wire to the output of that trigger, so I had no resistance in the circuit ie almost a dead short. Not cool batman.......!! To fix that little fault I then wired the triggerto the MSD only, used the MSD's tach output wire to feed the taco, same problem........!!!! Fook I say. The fix in my case way an MSD taco adaptor p/n 8920 I think, the taco output goes to it and then the original taco wire connects on the other side of it. Taco reads well and no melted wires [:x)] I am a very happy camper now.