With a carb set at the correct mixture, if you were to enrich it manually, initially the engine speed would increase, but enrich it too much, the engine speed would decrease, and the engine would begin to run unevenly.
When the choke is operated to start the engine from cold, the mixture is temporarily enriched to facilitate the cold start. To help with the smoother running of the car on choke, the fast idle cams increase the tick over by rotating the throttle linkage, therefore resulting in a faster tickover whilst the choke is in operation. Without the fast idle cams, the richer mixture would just cause the engine to stall almost immediately.
The basic principle is the same for both SU's and Stombergs.
Adjustment of the fast idle cams is basically turning the stop until there is a slight clearance between the stop and the cam, so that the cam doesn't operate on tick over, but with a slight operation of the choke, the cam moves to the stop and then begins to move the throttle linkage , and increasing the tickover to compensate for the richer mixture.
When re assembling your carbs, make sure the piston falls freely and come to a stop on the bridge with a metallic clunk. Check the jets aren't worn into an oval, because SU needles are biased to one side and this can cause the jet to become oval instead of round, and this can result in an over rich mixture.
I find that when setting up from basics, release all the throttle and choke cables and linkages first, and fully unscrew the idle screws. Make sure the butterflies in each carb are completely closed before retightening the linkage. Then do the same for the choke linkage, (ensuring the choke knob is fully home) so that both carbs operate simultaneously. For a basic setting, I turn the idle screws one full turn from the point they begin to contact and operate the throttles. You can then either turn each idle screw equally, or use a air flow meter to measure the air, flow, or use the old way of a length of tubing to your ear to ensure the 'hiss' is the same.
Then go onto your mixture adjustment!
Hope that makes sense!
Register Member no. 1596
1967 Mk1 2000 in Gunmetal Grey
1969 Mk1 2000 in Royal Blue
1970 Mk2 2000 in Valencia Blue
1972 Mk2 2.5 PI in Triumph White
1973 Mk2 2.5 PI in Sienna Brown
1976 Mk2 2500S in Carmine Red