I just put a new B50 string on my long bow, and now it has hand shock that it did not have before. ?? it had B50 before. what could be wrong?  

If your bow is rated for a low stretch string switching back will reduce handshock.

is it at the same brace height ?  Did you put new string silencers on it yet ?  Both can cause some vibration.   What exactly do you mean when you say hand shock.  There is a dull thud (OK, sometimes not so dull)  or there is a vibration that keeps going for a short time after the shot, like plucking a string might do.  They might signify different things and fixes might be different.
ChuckC

Same Brace Height, string silencers are on, Vibration after the shot.

xia...   does the new string serving fit the arrow nocks tighter than the old one ?  Just curious.  Are the arrows holding on tighter and causing their own "plucking" when they release ?  I have a bow I am working on right now that was doing the vibration thing and some silencers on the string stopped it.  The "thud", to me, means structural concerns or brace height issues while the vibration means string issues.  Again, my thoughts only.  also, try moving the silencers on the string to different positions, see if that helps.
ChuckC
ChuckC

Are you certain the previous string was dacron and not a fast flite type material?  Just is no reason for the same material to act differently, all other things being equal.

yeah i'm thinking your orininal string wasn't/isn't dacron.


did you reduce the number of strands?

I now they were the same type, I will check the # of strand. I made this one at 16 strand. but the old one I don't recall.

Check your nock point as well.  Moving your string hand up or down the string is going to change the way the limbs bend and recover and could cause some shock if there was a large change.

If it is ff capable put a D97 or 450+ string on it, I have found them to be quieter than B-50 anyway. Did ya wax the hell out of the new string? If ya did get it rubbed in real good with a piece of leather or something this may help with the hand shock as the strands may not be real tight together due to too much wax. Shawn

lower or raise brace maybe it has changed.

Let it stretch in for a couple days and see if that helps at all.  I agree with Orion.  If nothing changed...well nothing should change.
ChuckC

What Chuck said and keep it strung while it stretches out.i usally dont like the feel of the bow when i put a new string on it initially.Especially B-50 until it stretches out.might take a day or 2 and the more you shoot it the faster it will set.