I believe the 2 sizes of needle valve is to compensate for the inclination of the engine to give even float heights between carbs. However I found that I had to use both the long valves in mine as I just couldn’t get the valve to close without either the float fouling the bowl and hence flooding or having the float shut off the fuel too low down in the bowl. This is if u set it up by inverting the float lid and measuring as per Haynes manual. The better way is to measure Insitu with a clear plastic pipe strapped to the side of the float bowl attached to the float bowl outlet. This will clearly show u where the fuel sits in relation to the jet housing and from that you can set them up evenly approx 23 mm down from the float lid as a guide. What service kit is it, what manufacturer? Mine was a Royston service kit but didn’t include the damper pistons.