Tach jumps at low RPM

[slant6billy]

As mentioned on another thread, my tach is suffering a needle jump at low RPMs. It is not the actual idle. My vacuum advance is on the pass side front of the carb (Edelbrock performer). It is the timed vac port recommended in the Edelbrock manual. My tach is a Sport comp Monster 5 inch. I have it wired like Autometer suggested it- on the signal to the Distributor. Note: the tach needle only bounces at low rpm. I think the tach is hosed up. I had a cheap tach from JC Whitney that wired to the negative off the coil. The cheap tach did not jump at low rpm, but it was too small to see while driving. Anyone see this bouncy needle on the Sport Comps?

 

[slant6billy]

My idle is 1100 on a cold day (choked) and about 950 on a warm day. I like it that low. My Ram runs 750 on a warm day, but that's fuel injected.

 

[NoCar340]

What are you using for ignition, and what model tach? AutoMeter tachs are notoriously fussy about where they're connected depending on what style ignition you have. If you have factory-style Mopar electronic ignition, it should be wired to coil (-).