Remove ht/coil lead from dizzy cap and place about 4mm from engine block/earth with ING on open and shut points by a small screwdriver, should see a spark at points and from coil lead to earth(be careful ht lead high voltage)
if that's ok check brush in top of dizzy cap ,some rotor arms had a resistor fitted in them(across from middle of rotor to sweep point) they fail ,new rotor or you could try to solder a piece of wire across the resistor to prove a point some thing like the earth wire from 2.5 twin and earth.