This vehicle is was an original 1970 Mk2 1275 Austin Mini Cooper 'S', which had been running ok until the owner started to introduce E10 petrol after which it wasn't running the same.

This old 1275 Cooper 'S' was running at a slight overbore of 1310cc and had a period fast road cam fitted thought to be the AEG 510, and a maniflow stage 2,  1.75" freeflow twin box exhaust system.   The car retained the original twin HS2 carbs, fitted with relatively new AH2 needles and red springs.  Hence a quite a normal Cooper S set up.  However, in recent weeks since switching from E5 to E10 petrol the car was more hesitant in mid range. 

It is a little know fact that E10 petrol has a lower calorific value than E5 or prior unleaded petrol, so the first step was a new needle proposal to recover this energy loss, via a change from AH2 to Needle #5.   This was shown to the client and explained fully.  The carbs fuelling mixture and airflow were then balanced at idle, and also choke idle balanced.  The client was using a 123 multi curve distributor, so the correct ignition advance curve was chosen in comparison to the original Lucas Cooper S curve and also CSI Classic 'S' curve with slightly enhanced low rpm torque.

After a quick test run the mid range throttle response had returned and this little classic was back to running how it should.