Firstgens.com

A lot of times, when using an electronic ignition or HEI with an original or a reporduction factory tach, you get a "bouncy needle" that just won't sit still, it lags behind the engine, then overshoots, or just bounces around when you're cruising steady.

With my repro tictoc tach, even a point ignition I tried had the needle bouncing everywhere. My MSD probillet distributor was bouncy too.

So, a long time ago, I designed this simple "tach filter." It uses a resister and capacitor that you can get at radio shack to filter out the high frequency ignition noise and dampen the nedle movements.

Instructions are EDIT here http://firstgens.com/tech-articles/46-tach-filter.html

Nice tip Jim! Never had this problem but know I have a fix if I ever do.

Interesting. Someone asked me about this problem just last week. I have never had this problem. I wonder why not?

Dunno? It just depends I guess.

My first time was back in the 80's, a 67 vert with a "I don't remember" cheap aftermarket tach. Fixed it by putting an autometer tach in the dash.
With my current car, first ignition was accel, and my new oer repro tic toc tach was fine.
The Accel died, I put the points dizzy back in, and the tach was horrible. Not having a resister wire could have been part of this, along with the hi-po coil. I tried a mopar style ballast, but it burned up dead.
Then I went to the MSD 8360 RTR dizzy, with their blaster coil, and the tach was bad with that too.

That was when I designed that tach filter. It fixed it right up.

Today I'm using a cheap chinese copy GM small body HEI with electronic advance. Still using the blaster2 coil, and still using the tach filter. Haven't tried it without.

Jim -If using an MSD ready-to-run (8360) should the filter be used with the MSD tach feed wire or the neg side of the coil, or does it matter?

Thanks,

Al

I don't recall the 8360 having a separate tech feed wire, mine did not for sure.

If you have a separate tach feed wire (most CD ingnition boxes will have one) absolutely use it.

Interesting. One of the guys over at Brand-X is having trouble with his tach. Pretty sure he has an 8360. MSDs instructions show a separate tach feed for that dist. My E-curve has it. (No box) http://www.msdignition.com/uploadedFile ... ctions.pdf

I'm planning on a repro tach install this summer, and will use the tach feed wire. If I DO get a goofy needle, would your filter work in the tach feed? I wonder if the tach feed wire somehow has the signal cleaned up.

Thanks

The tach feed wire should be a cleaned up signal, they usually work fine.

I Have a std GM HEI no issues with the tach at all.

When I had the orginal points dizzy, I stored the orginal factory coil away, and dropped a 12v 'hi' energy coil in ...by passing the resistor wire, again never had any issues...
did have to change the capacitor in the dizzy thu, burnt out pionts too quick..

link to tach filter does not work.

Can someone explain method for building a tach filter.

I have a pertronix dizzy, msd 6al with tach adapter and a new OER tach for my 69 z/28

thanks in advance

Erick