Ignition: Erratic Tach.

[Chuck Larmay]

I replaced my engine on my 1974 Avanti. It came with an HEI distributor. The tach is all over the place. When it gets to a higher RPM it starts to fall. When I accelerate it goes up momentarily and then falls. What the hay. Any suggestions?

[Jeffry Cassel]

Not sure what type of ignition the car came with but if it had points (liklely) and a HEI system was added the tach won't work. The tach can be converted but once again this is $$$$ and it probably won't work any better than it does now. i have found a tach that looks like the original and put it in. Good luck!

[Chuck Larmay]

It was a small block 400 with points. I assume it was the original engine. It is currently a 350 with HEI distributor.

[Michidan]

Chuck I had a similar problem with a brand new aftermarket tach in an HEI application. Called the tach company and they had me add in a resistor to the tach wire off the distributor.
Of course I can't remember the ohms, and it may not be relevant with your tach anyway. But it did solve the problem.

[Chuck Larmay]

Chuck I had a similar problem with a brand new aftermarket tach in an HEI application. Called the tach company and they had me add in a resistor to the tach wire off the distributor.
Of course I can't remember the ohms, and it may not be relevant with your tach anyway. But it did solve the problem.

Thanks, I don't have an aftermarket tach. It is the original tack from the factory. Sense I converted over to the HEI distributor that is when the problem started,

[Hawkowner]

Jeffery what did you use for a Tac I am waiting for the High Voltage Coil to instal my ready to run billet distributor.The Distributor has a TaC wire on it but with my luck I am sure I will have the same Problem.
Hawkowner

[Chuck Larmay]

So far I have done nothing about getting a new tach. I am trying to figure out how to get the in dash stock tach working properly after converting from a points distributor to a HEI distributor. I have a 1974 Avanti with a in dash tach.

[1inxs]

Chuck, This is an old thread from 2018, but I was wondering if you ever took Michidan’s advise and installed a filter to get you factory tach working or did you go another direction? Thanks

[JoeHall]

Pretty sure this is what I used when installing fuel injection with electronic distributor in a GT Hawk, which created the same problem with the tach the OP reports: TURRL4SVXJJMSD Tach Adapter Magnetic Trigger #8920

[jrlemke]

Probably the same one I used. It is an electronic boat tach, and it fits in the dash and looks like the original except the bezel is grey, not shiny chrome. I'll post the brand if I can find the instructions. I bought from someone on this forum, but can't remember who (maybe Jeff?). I haven't hooked it to an electronic ign. yet, but it works great on the point setup. If I don't hear on the forum that it works on electronic, I'll post when I get it installed. My electronic also has a wire marked tach. so I'm hopeful it'll work.About the bezel, you may be able to use the one off the original tach.? I think I have the same electronic ign. as the one posted here- Jim

[jrlemke]

I think the electronic ign. is the same on posted here, about two down from this post. -Jim

[Jeffry Cassel]

A VDO tach looks a lot like original and SW has one too. The old one can be sent away and updated. Avoid AutoIntstument--they used to be good but my luck lately has been bad (avoid Bob's too)