[yo2001]

I don't get this one. My 78 280z the tachometer started to jump around between 2000-3000rpm? Looks like when you have the speedometer cable loose, it just goes up and down about 500rpm. It seems like it drives fine and there isn't hestation. The car drives smooth. It started when I change the muffler to 2 chamber Flowmaster to 2 chamber Flowmaster with Delta Flow (by the way, it really sux. it's not quieter at all. It vibrates my feet.)but could that be the prroblem? I have the MSD 6A with tach adapter and the tach has been working fine before. Or could it be my distributor vacumn advance going out? I checked for a vacumn leak and I didn't find it. The timing is set right and the idle is pertty good (not great with 6 in 1 header and 2 1/2 in exhaust) Well if anybody have a clue, help me

 

[lifegrddude]

an idea

I dunno if this will help but on my old 510, I had the ignition wires too close to the tach wire and the radio interference caused the tach to bounce. You might want to check that out. Hope it helps.
John