I have a 1978 Covette Silver anniversary with a newly rebuilt 350. The engine only has 6k miles on it. I am running stock heads w/roller rockers, dual springs hydraulic lifters and a little bit bigger than stock cam. The engine began running very rough at higher rpms and would idle like it had some huge cam in it. After deciding that the lifters might be bad I pulled them and the cam out! What I found were some of the lobes on the cam were almost shaved off! 2 or three of the lifters had been cleanly gouged on the bottoms also. Needless to say I replaced them with a new crane cam and hydraulic lifters. I initially started the motor up and it sounded beautiful! It idled perfectly and ran great. I followed the break in process to the letter and then changed the oil and filter. I only drove it for maybe 20 miles and l parked it. A few days later I started it again and it seemed to run a bit rough. Now after only maybe 40 -50 miles it seems to be running like it did before. I can hear a lifter and it runs very rough. I attempted to set the timing but to no avail! I'm wondering two things! ONE- do the lifters typically need to be re-adjusted after the initial settings? TWO-could the dual spring set up be too tight and causing the lifters to grind the cam too much? Again I only ran the motor with a new cam and lifters set up for a very brief period of time! The cam and lifters couldn't be fried after only 50 miles could they?
Yes the spring rate could be too high for the cam and lifters. It sounds like you might not be adjusting the lash properly. If the lash isnt set correctly, it will cause premature wear to the valvetrain.I would recommend a roller cam and roller lifters. Also be sure to inspect the cam bearings.