Tach jumping/inaccurate

[sext7366]

I have an aftermarket tach...it's a cheap tach maybe $30 from O'reilly's  I think its an autometer brand (I think), anyway It always jumps a little between 500 and 1000 rpms while running even at idle.   The other thing is it is correct at idle, but as you increase rpm it becomes more and more wrong.  It is always at a higher rpm than actual, on the highway it will read 5500 going 60-65.  I have 3.23 gears so I know this is wrong.  I do have a mp electronic ignition, and for some reason whoever had the car before me put a gold race ecu in it.  I haven't had any problems with it so I haven't changed it, and as far as I know the main difference is just that it allows good spark up to a higher rpm.  I have no other electrical problems besides my temp gauge not working, and I wired it up to the factory specs, and then to the tach company's specs which are pretty much the same and it happens both ways.   Any ideas.   Also my car was not factory equipped with a tach if that makes a difference.

[Just 6T9 CHGR]

Is the tach set up correctly???
  IIRC there are small dipswitches in the tach to set it for 4 cyl, 6cyl or 8 cyl.
make sure its set correctly.

[sext7366]

Yep set for a v-8 but I tried changing it to six to see what would happen and it doesn't seem to change at all

[hemihead]

You need something that you know to be correct to use as a standard.That jump between 500 and 1000 could be a miss or other things in the engine.Without something proven to be correct how can you tell the rpm's are wrong when increasing engiine speed?

[71_deputy]

seen this before- try putting on a cap from a old points dist. on the neg. side of the coil.  sometimes this helps!!

John Mac

[sext7366]

So if I cant test the rpm while going down the road I am basically screw'd???  Just Kidding I had a smaller than stock cam in my engine before, and now Its being rebuilt, so I wanted to see if anyone of this being a problem.  Anyway, I could tell it wasn't missing or sputtering...in other words correct because of simply listening to the motor.  Very smooth idle, yet the tach was jumpin away. 

I'll try using an old cap thanks.

[Ricardo]

You can also try this:

http://rt-eng.com/mediawiki/index.php/Main_Page

Anyone had install this?