If they are (very old) replace them,new one's are not that expensive. And while you got them off check for valve guide slop. Also may have lobes going on camshaft.
Originally Posted by fred56
I was out today and on the way back my SBC started clacking(rocker arms?) and as soon as I went to an idle it stopped clacking. What it seems to be doing is back firing/missing out the exhaust. I checked the timing/wiring/vacuum leaks and what I have come to the conclusion is that one or more of the exhaust valve springs are getting weak. At idle it purrs like a kitten but at 2000 rpm and up it pops/misses thru the exhaust not the carburetor. If the springs are weak can you compress(move them at all) them a little by hand? It seems to me that the springs are weak. They are very I mean very old. How can you measure spring pressure without taking them to a machine shop?
Thanks - Jerry