Just put new points and condensor on now the bike now it won`t start. I am getting a good spark but all i get is an occasional backfire from the bike. Anybody got any ideas why it won`t start ?

A backfire on a two stroke is usually down to timing, to advanced or retarded, position ofvthe stator plate, or broken woodruff key, causing the fly wheel to move slightly out of position, this can happen and the flywheel still seems tight on the crank shaft and has just spun out of place a little. 

Could be worth trying a bump start if you have access to enough space at home.

 

Just pop the rotor off then you can rule out woodruff key snapping, iv had two break one on an old yz125, and one on a freshly rebuilt gas gas txt 270,it took me ages to find out it was the woodruff key after taking a multi meter to ever electrical component....