Basically it was running on 3 cylinders. If plugs and leads are ok, could still be loads of things mate. Dizzy, coil.... Have you got a group a coil on there?? always worth having.

Make sure the crank position sensor is in good working order and is not covered in crap/ and or old and shabby.

Take dizzy cap off and make sure there is no water or crap in there, check overall condition of the dizzy and the points in the cap.

Worst comes to worst could be a blocked injector....

The list goes on..