The 'E' Type Forum

Greetings All:
I did check archives. I am trying to determine what the proper needles should be. The car is a very late 3.8 bored .040 over, 9-1 pistons, stock cams, head ported and rebuilt with oversize intake valves and running only trumpets on the otherwise stock SU's, no air filter, for now anyway. Tubular headers if that makes a difference. I will guess richer rather than leaner because of increased flow.
Thank You-

Start with UE, but you may need to go to UO.

I would start with UM which were standard on ALL the 3 carburetter engines. If the engine flows rather more intake port air, it will suck harder on the jet and therefore preserve mixture strength. People labour under a perennial delusion that richer = more power. In most cases, it doesn't - the XK engines certainly need a mixture strength rather richer than the stoichiometric ideal of 14.5 : 1 ( usually about 12.5 which equates to about 5% CO at idle ) but the UM will give that in most cases. The case for richer such as UE or richer still UO is really only made where sustained full throttle running is likely to be used, or in some cases where the improvement in airflow is upstream of the carburetters ( e.g. free flow air filter ) rather than, in your case, downstream