I run the datsun GX motor with the 38MM SU carbs. The needle/seat for the float has two kits for the roadster 16010-14625 washer/gaskets and 16010-14628 hardware. The fuel needle in the
roadster is a M-39 or M-43which maybe to rich. The GX carb uses a M-76 or M-83 needle. The difference is the diameter. Measuring from shoudler to the tip of needle the following
distances 0.100, 0.400,0.0700,1.00,1.400. The first two are .090& .091,.088&.0875,.082&.081,.076&.075,.071&.0695,.062&.0665. The next two .088&.088,.087&.0875, .081&.0815,.074&
.0755,.069&.070,.0635&.0625. Also the needle/seat for the 38mm GX carbs will so fit, but I do not have that number here with me. I set the carbs up using s tire thick depth gage. Set
both the nozzles to 4MM as a starting point and use a small hose to hear the difference in both carbs as it is running. If you have any more question just ask.