ATF fluid is an excellent starting point for SU's and similar type carbies. The weight of the spring above the piston in the carby also needs to be considered when selecting the correct weight of oil,as they both work together to determine the mixture delivered to the engine. Generally speaking, the heavier the oil & the spring, the greater enrichment under acceleration. This can be helpfull, but you aslo need to check that the piston open fully at the point of max HP. At one time I was using 2 by 1.75 inch SU's on a 1400, and needed to use the lightest springs available to ensure full openeing. Hope this helps.