It seems like my tachometer isn't accurate and is sometimes bouncing...
When I turn the engine off it spins for a couple of seconds like an heli rotor...
Any idea why? Could be a filter (which i don't have on my HEI) issue?
If you were recently installing it, or playing around with the ignition box or such, perhaps a loose wire, loose ground, or bad connection? Some cars had a factory inline filter that some people remove. Some put them back in place, some don't. Daniel did my speedometer years ago, but it did take him a few months to get it back to me.