I have a XJ6 ser. II which won't start. Engine died under normal driving conditions, (no advanced warning of any issues), and eventually restarted but would support throttle opening. Now engine just doesn't want to start. I have check fuel delivery, fitted new condenser, rotor arm, coil. I have checked the various vacuum hoses and blanked off the breather system on the carbs. Good spark at the plugs but the plugs appear dry after cranking to try and get it started. Where else to look?

 

1976 so assuming carbs. Try spraying some easy start into the carbs to check if it's a fuel problem.
What carbs are on it? SUs?
Check the pistons move up and down freely. check for fuel in the carb reservoirs. Check carb fuel valves.