backround, this was a 2 barrel Imca engine, it was around the 20th engine I have dynoed for him, most of them having 1 or 2 things "differnt" then the last one. IE thinner rings, differnt cam, differnt compression ratio ect. On this engine at around 5200 it went flat, then what sounded like valve float. We usally run these upto 6200-6400.
After playing 20 questions with him, this engine was running a tighter quench and a differnt intake valve. It had a larger head and a smaller stem (yes he was not following the rules) the overall weight was down a few grams.
anyway, with the differnt valve spring retainer, the installed height was around .120 plus the lash, so we installed a -.050 keeper. Then the engine went to 6400 with no issues.
FYI it didn't make any more peak HP, but did make a little more tq down low.