low speed steering shake

[68C15]

my 65 Johnson Tri-hull / 100 HP V4 with cable and pulley steering has a shake at slow speeds. it moves the wheel about 1 or 2 inches back and forth. it is a 3 blade prop (unsure of pitch yet). after I hit 1/4 throttle it smooths out. is this normal? I have the recommended 75% compression on the spring.
BTW, it has one spring and it is on the starboard side.

 

[QC]

Re: low speed steering shake

Sounds like a bad prop to me. Condition? A good spare you could try? Borrow? A slight deviation on one blade could do this and may not be visible.