I have a 1993 Chevy 4x4 with a 5.7L TBI that I built. The RPMs will jump around 1000-1500 rpms at a time like the gauge is messed up while driving. I don't feel any RPM differences while driving everything seems smooth. It only does it when I apply light to moderate gas on the pedal. If I give it lots of gas then the RPMs are smooth while they increase when I drive. I have been told that it could be bad plugs, loose timing chain, or faulty distributor or gear. Oh yeah I tested it in park and they won’t jump around when I give it gas at different RPMs. Any suggestions????

Lots of truck those years have bad distributors now. the tach takes a signal from the coil, and it receives a firing signal from the module, etc, etfc, etc. so i'd see if you have a known good dist, drop it in and see what you get. db

Lots of truck those years have bad distributors now. the tach takes a signal from the coil, and it receives a firing signal from the module, etc, etfc, etc. so i'd see if you have a known good dist, drop it in and see what you get. db

Or take a known good tach and see if you get the same thing, and you should check the wiring for the tach at the distributor