No oil pressure...

I just changed oil in a '75 KZ 900 and now the
light comes on and won't go out until about 4000
rpm. It has 4 qts. of oil and calls for 3.9 qts.
I don't know why all the sudden the oil light
comes on and won't go out. Any help would be great.

spring on oil filter?

Wireman, yes the spring is on the oil filter.
I may have found out what is wrong, I believe
that the by-pass was stuck open, I'll let you
know if that is it.