This late 60's Jaguar 420S was running quite rough with occasional backfiring and popping through the twin 2" HD8 SU Carburettors.  It simply refused to run off choke, so the owner called AG Classic Car Tuning of North Yorkshire for some help.

As per any tuning session carried out by AG Classic Car tuning the first task is to establish good baseline settings and health for all the key fuelling and ignition components to ensure that they function correctly and are ready to be fine tuned.

There were no major issues found with this car, however quite a few smaller issues which collectively stacked up on top of each other to hinder smooth and correct running conditions.  Issues included excess PCV resins in the Twin HD8 SU carbs, differing mixture settings, idle draught imbalance, ignition 3.5 to 4 deg retarded, air leak on advance and retard pipe, different spark plug gapping to name but a few.  After readjusting and balancing all of the above and more, this Jaguar was ready to be fine tuned to then tick over and perform as a 4.2 Litre straight 6 Jaguar should do.   

The above video is the car coming from a choke fast idle, down onto normal idle at just below regular running temperature.  Turned off with no engine overrun (dieseling as it is known).  Ignition timing stability tuned, and set to 9 deg BTDC at idle (no Vac).