I've got a problem with my 1966 model that misfires and partly backfires (some light pops through the carburetors) when idling. When you drive the car at normal speed you don't notice it, and the engine seems to pull ok.

The misfiring occurs at both cylinder banks, and as far as I know at all cylinders. Therefore my first idea was ignition problems. I have renewed the distributor cap and rotor, the breaker points and the spark plugs without any luck. Breaker point gap was adjusted to 0.4 mm. Resistance in the plug wires was equal and good.

My car was fitted with wrong spark plugs, Bosch WR7DC, which I assume is for a 3.2 Carrera engine. The original W250P21 was gone out of production, and according to my supplier some WR3CS plugs were found in an old dealers list.

The same plug was specified for the 'S and RS Carrera. This is a much colder plug than I had (and I think a bit colder than it should have been), so I hope they are worth the money. My engine is fitted with the later type heat exchangers and is found to produce 145 bhp compared to the original 130.

The new plugs had a standard gap of 0.6 mm compared to the factory specification of 0.35 mm for the original plugs. I didn't change this before I had them fitted.

So, now I'm hoping to get some ideas from you out there. Maybe check of the ignition timing? I believe one should have a perfect idling, no interruption of the pure music.