I just installed 2 500EFI's in a '42 Lightning (Fountain Boats)
these are 502 blocks with hp rods,pistons,crane roller cam,roller lifters,rocker arms,and a tunnel ram intake.
one engine sounded great...the other had a loose lifter. Mercruiser Raceing autherized me to readjust(had to cut a security tab) and I found # 8 intake lifter would not compress.
I ordered 2 new lifters,and 1 wouldnt compress,and th other barely would. that one I installed,and still rocker arm noise. Mercruiser Racing then sent me 2 more lifters,and nieher of them would compress.the adjustment on a new hydrolic roller cam is zero clearance and 1 full turn.hard to do with a non-compressable lifter.So the guys in Fondulac were going to send me another engine.I decided to dis assemble the lifter...dumped the cleaning solvent out of it...volla...it compressed!!...reinstalled and redjusted the lifters...VROOM!! problem solved.