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I have a serious misfire and the car conks out when driving.
I've changed the coil, condenser, fuel pump, fuel filter and checked the points gap.
What else could it possible be - help!

Have you tried another Rotor arm, They often break down when they get warm.

Thanks I'll try that.

Check for a crack in the distributor cap.

OK bought a new distributor cap, rotor arm, points, condenser, coil and plug leads. Problem persists.
Conclusion it must be fuel. New SU fuel pump fitted, will now buy a fuel pressure meter so what should the pressure be?

7psi max

Pressure tested OK, dismantled carburettor and it's clean as a whistle.
So what next to check?
Could it be a coincidence that the serious misfire (so bad it's undriveable) only started after fitting a Kenlowe fan? If so, why does the misfire occur regardless of the engine temperature?
Could there be a problem with the ignition switch?

How about running it with a strobe timing light fitted to the distributor king lead and then to each of the plug leads in turn - watching the flashes will at least tell you if it is a spark failure causing the misfire.

Problem is that when the car won't start there is still a spark from the king lead. If it was a single spark plug problem the car would start but be very rough.

I broke the number one rule when trying to cure a problem - do not replace several components at once.
It turned out that the new condenser was faulty!
Thanks for everyone's help tackling this, at least I now have new points, distributor, ballast resistor, coil fuel pump and rotor arm!