depends on your kite mate. Normally kites prefer all lines to be even. So increase/decrease the length of your new fly lines (the rope part of the lines) so that they're even. That's a good starting point.

If it's racing too far foward and stalling, and slow to turn, you probably have too much slack in your back lines, so shorten them.

Just fly the kite and with it overhead and gently turning it left and right you should see that all the lines are contributing and neither the front or back lines are very slack.

As for it turning one way or the other, make sure the left line and right line are equal length. Maybe the new way they are attaching to the bar means they are slightly offset, and therefore pulling unevenly?

I'd help you fix it at the beach, but who know where I'll be kiting tomorrow

edit: a fool proof method to get your lines even is to attach them to a pole/toeball/whatever and really hang off them. Is the bar even or is one line shorter then the other?