In a spring/mass system, at some typically high frequency, the inertia of the spring system. Tends to overcome the springs and they don't return the valves to the seat. Stiffer springs raise the frequency of the occurrence and reducing the mass of the system does also. Add pushrods and lifters adds mass, making control of the valves harder yet. OHC's certainly help in this issue.
George
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"...out to my ol'55, I pulled away slowly, feeling so holy, god knows i was feeling alive"....written by Tom Wait from the Eagles' Live From The Forum
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