My gold tip 55/75 seem underpinned out of new bow while shooting bare shaft with no weight up front they fly nock left. Will a full length piece of weed wacker just add weight or would that also inhibit the flex of the shaft so as to increase spine. Thanks

Any full length shaft weight increases spine,no matter how flexible it is.

It is because adding weight to the rear stiffens spine.The balance point is already forward of center so adding full length weight,puts a disproportionate amount behind the balance point,hence,a stiffer,dynamic spine.It can stiffen things a lot.

I've experimented with all manner of full length cords,powders and granular substances and all stiffened the spine.You will just have to try them to see.I found them all to be a pain.

Mark your balance point before adding a full length shaft weight.If that balance point shifts rearward after adding the full length shaft weight,that dynamic spine is now stiffer.It will not stay the same,regardless of how flexible a material you use.

Thanks JimB

It will increase the spine.  Haven't used weed wackier line, but I've heard it can rattle a little bit when shot.  You can do the same thing with paracord and it'll be quieter.  I did that for one of my setups.  It added about 2-2.5 gr/inch and increased the spine so that I could use the broadhead I wanted.

Have to agree with Matt - weed line rattles in the shaft. Had deer 'jump the string' in a big way but didn't give it much thought until I stood down range (safely behind cover) and a friend shot some arrows with line. It was LOUD!

Matt's suggestion about using something soft sounds sound!  

Thanks guys

When I shot compounds, I used aquarium filter hose.  It comes in different diameters and can be found at any larger pet store or aquarium supply shop.  It is soft and pliable.  Good Luck!!