Valve springs can lose a lot of their strength over the years, as seen with this R25/3 valve spring:
a new spring measures 16.6 kg, the old spring 10.1 kg, almost 40% lower.
Lower spring tension usually causes no or minor problems on low revving engines, but it can lead to loss of power / misfiring on higher revving engines as a result of valve float
The test device:
