Hi there,
I have a 1978 xj6 4.2L. We had it firing with old plugs and wires. Now we have changed out plugs, Wires and Distributor. Now it doesn't fire at all. we have ensured all is well with Firing order. Any ideas?

 

1-5-3-6-2-4.
Number 1 cylinder being at the rear of the engine, number six at the front nearest the radiator.
Pic below of series 3 engine(1983), numbers 1 & 4 facing radiator.

 

Normally if you cross leads you at least get a backfire or two for your trouble, not completley dead. Assuming from what you have said you havent touched anything but plugs and leads. If Clarkes diagram brings no joy , maybe start at the HT lead into the distributor and check for spark going in. If present check for spark going out to the cylinders. Thinking maybe a broken HT lead into the cap? If you have changed other bits , I would check for presence of a rotor button and the small brush in the top of the cap that contacts the rotor button.