I have had a problem when trying to tune the carburettors on my TC, I couldn't get the mixture rich enough despite having the jet adjusting nut at the bottom of the thread.

In the appendix on tuning SU carburrettors in Classic Engines,Modern Fuel it became obvious that the carbs had the lighter suction piston but no spring fitted. I have now fitted a red spring as recommended and the mixture is correct very close to the default position of the jet adjusting nut. I have had the car for over 20 years and I am sure I used to be able to adjust the mixture with older petrol.

A lighter piston (or no spring) allows the piston to rise too high, weakening the mixture.

This is not obvious as one would expect the higher the piston, the richer the position on the needle (i.e. the annulus between the needle and jet is greater - so more petrol can flow). However, this is countered by the depression in the choke which is lower (because of the lighter piston). A lower pressure means less petrol will flow out of the jet.

The comment "I am sure I used to be able to adjust the mixture with older petrol" is difficult to understand. The only property of the fluid that affects its flow through an annulus is its density or specific gravity. Interestingly, the specific gravity of petrol in the 1950's was 0.784 and is virtually unchanged in modern fuel. This suggests any differences were not due to the properties of the petrol.

The only possibility I can suggest is that the suction pistons were perhaps sticking which prevented them from rising as high as they should. This would have made the mixture richer.

Kind regards

Paul