Mechanical Tach Bounce-C2

[Vette66AirCoupe]

I have a '66 427/390 4 speed. My tach was understating RPM's by about 1/2 as well as bouncing a lot and I took it to a shop that was supposed to diagnose the problem and fix it. They said the tach needed a rebuild. I agreed to the repair and the tach reads accurately now according to my tach/dwell meter. However, I still have a lot of bounce. Since I was installing a Pretonix electronic ignition I decided to replace the driven gear and the tach cable while I was at it. The needle still bounces. Also, the parts I replaced didn't look badly worn. Now I'm wondering if some bounce is normal and maybe I'm trying to fix something unfixable. I know it won't behave like an electronic tach but can somebody clue me in here. Do they all bounce? :confused

 

[maxphunn]

Mine started to bounce a year or so ago. Lubed the cable and it's been fine ever since. By any chance is there a sharp bend in your cable?

 

[67HEAVEN]

Max is pointing you in the right direction..........the cable.

Check for lube and tight bends.

 

[Vette66AirCoupe]

I'd say the cable bends a little less than 90 degrees over it's entire length with nothing approaching a kink. I lubed it with graphite before installing it.

 

[studiog]

You might want to try disconnecting the cable at the tach and hook a variable speed drill (a small cordless would do fine) to it and see how it acts. Get someone to operate the drill while you watch or vise versa. This will also allow you to straighten out the cable a bit and see if it solves the problem.

 

[Vette66AirCoupe]

Do you mean disconnect the cable at the distributor???

 

[studiog]

Do you mean disconnect the cable at the distributor???

Yes... at the distributor

 

[JohnZ]

And run the drill in reverse, which is normal tach cable rotation; the tach will read double the drill rpm (distributor runs at 1/2 crank rpm).