This has probably been answered in another thread somewhere but I have a 92 Z28 and replaced the stock 350 with a 383. I used the MDS Pro Billet distributer that is a direct stock replacement distributer. The problem is my tach will come on when the ignition is turned on but won't read the rpm's, it just sits there. I had a heated O2 sensor installed at the time of the engine swap and they wired it to my fuel pump relays....not a good idea. But when the O2 was hooked up to the relay the tach would work, it just bouned around everywhere. After I disconnected the O2 sensor from the relay the tach stopped reading. I thought it could be a ground wire issue but that isn't it. Could there be something in the wireing of the O2 sensor that could cause the tach to mess up? It worked fine before the swap. I have also had trouble with my fuel pump not shutting off when the ignition is turned off and flooding my engine resulting in a new oil and filter change as well as new spark plugs. This has happened twice already. Don't know if these are related problems.

 

All is fixed...tach problem turned out to be a loose wire connected to the coil. The fuel pump that kept running after the engine was turned off turned out to be a setup in the computer that didn't make the proper adjustment during engine warm up. Once the engine warmed up, the pump would shut off fine after the engine was turned off. So far so good.