I have recently upgraded and installed a factory tach in my 1971 chevy truck with a HEI distributor. I'm having a problem with the needle bouncing around when I have the lights on or have a load on the engine, sitting at idle the rpm's are steady. I have heard from somewhere that I will need to install an adapter so the tach can read the hei......When I tried to buy one, no one carries it. One guy even called MSD, the tech said that the tach should read the HEI w/ no problem. do I need an adapter or do i have a wiring/ground problem?

any help will be much appreciated.

thanks,
Jim

No adaptor is needed. The needle bouncing around might be voltage surges.

Is there any cure for this voltage surge?

I am interested in this too. I just installed a factory tach in my 74 w/HEI and the tach is waaaaaaaaaaaay off. I think I am going to yank it out and go back w/ the old one until I figure out how to fix it. It is real irractic, says I am 3000 RPM doing 20MPH, idle at 2000 which I know is not right.:crazy: I did read somewhere that we have to put a resistor or something in line from the dizzy.

HEI to tach, straight wire. No resistor or adapter or anything like that.
Are these 30+ year old tachs you are messing with?

I just bought the tach upgrade from Classic Industries about a month ago. I'm going to check the voltage when its under load to see if there is any fluctuation.

Thank you