I have twin 5.7L efi's and I'm having an issue with my starboard engine tachometer. It reads 1200 rpms at idle and the engine is clearly running around 600 rpms just like the port engine. The engine seems to run fine and after 15 minutes or so of running it will come back to 600 rpms at idle. If I stop for an hour or so it will do the same thing. I've replaced the plugs, distributor cap and rotor, TPS and the IAC all with no luck. The engines have Delco distributors and I'm now focusing on the module in the distributor. Any thoughts?

You may be overlooking the obvious. Switch the tachs, and see if it does the same thing on that motor...if not, then you have a bad tach.

on the back of the tach, there should be a little dial, that lets you select 4,6,8. Try turning it back and forth from 4 to 8 several times, stopping at 8 again. Sometime the contacts can get dirty/corroded.

Yea they act wierd if not on set on 8

Good advice, try what they said. It could also be a bad ground.

Tried this today and it worked! Thanks!