Van cranks but wont start , has spark at top of distributer ,but not to plugs

Replace distributor cap AND rotor! .. Mark the plug wires so they don't get crossed up on new cap .. Happy motoring!

If you have spark out of the top of the distributor, it could still be too weak to reach the plugs. Replace cap and rotor. For good measure, if it's been over 75000 miles since you last changed the plugs and wires, it would be a good time to do that, too.