Can't figure this out. It's a fully modded 305 and modded 700 r4. I put the car in gear and the rpm's fluctuate between 650-850 and doesn't settle. I checked the usual possible vaccum leaks, just did a tune up, tapped on the MAF(nothing happened) no codes, can't figure it out. Not a daily driver, just use in spring through fall. It does this when cold or when warmed up. Any ideas please feel free. Thanks.

 

this isnt goin to help any but I had a similar problem when idleing. Does it bounce all over the place or does it slowly go up then down back and forth... Cause my 305 TPI had a tach where it would bonce all over the place between 550 and 900 and all I did is replace plug wires and it went away but if you have already done a tune up then I dont think it could be the wires.