Weak spine on heavy bow.

[capt eddie]

I have a 3 piece hybred longbow.  Two sets of limbs shoot great with the proper spined arrows.  But I have one set of limbs that take an unusually weak spine to fly straight.  I have noticed that on the two short 60" and 62" sets of limbs, the arrow is centered on the shelf and centered in the limbs.  the riser is cut past center.  So everything lines up centered.  But on 64" limbs the arrowpointing to the left, RH bow.  These limbs need about 15 # weaker arrows to stop the left kick.  Shooting the same arrow as the 60" fly 18 inch left at 27 yards.  The string on the 64" appears to be centered on the limbs but at 30 in arrow it is 1/8 left.  How can this happen?  Can the limbs be colapsing to the right on the draw and kicking the arrow ?  Neither limb is twisted.  When I input in the Stu Miller chart the results equal out when I input 3/8 inch in the sideplate. I am trying to correct this problem because these are backup limbs and they need to be able to shoot the same arrows as the main bow.  There is no sence in carring two sets of limbs and two sets of arrow on a trip.  How do I correct this problem?  Thanks for any input.

[macbow]

Is the only difference the length of the limbs?
No difference in weight?

[capt eddie]

They are both supposed to be 74#. The bowyer just checked them months ago when i received the new 64#  I can see that when an arrow is on the shelf of the 64", it points to the left a good bit more then the 60"  The riser is cut past center and the arrow align perfect with 60" and 62" limbs.  But not with the 64".  I don't know how this can be.