I have a 1981 ford 250 pick up,with a 351 cid engine.Last time i started it it was hard starting and stalling.Now i dont have any spark.i cleaned the distributor contacts and rotor,changed the plug wires and coil.i have current on the positive and negitve side of the coil when i turn the key on,but no current going to the distributor.This is an after market distributor rotor and coil,made by MSD.any help would be appreciated!....thanks