They are the wrong lifters for the application.

Quote: Speed Pro on SPHT969R
"Speed Pro Hi-Rev (commonly called anti-pump up) hydraulic lifters feature a special high strength steel retainer used in place of the normal spring clip to precisely limit the travel of the plunger during operation. With plunger travel limited, adjustable rocker arms must be used to effect a lash adjustment of .000/.002''. This then allows the valve train to perform more like a mechanical system. Hi-Rev hydraulic lifters can be identified by the prefix "SPHT" and the suffix "R".

Quote: Speed Pro on SPHT969
Conventional Hydraulic Lifters
The lifters are intended for use with both OEM and aftermarket "stock" camshafts. Also recommended with hydraulic performance camshafts with RPM limitations of between 5500 and 6000RPM. Conventional hydraulic lifters can be identified by the prefix "SPHT".