Seems to me, and I may be wrong, but raising the float chamber needle with the spacer (washer) raises the fuel level in the float chamber above the standard level marked by the dot moulded into the float chamber cover. Presumably this was the result of some experimentation by the factory and was intended to compensate for the use of a smaller main jet than that used on the twin carb bikes.My single carb Atlas had a split sleeve reducing the inlet dia in each inlet port, for slower running, could get 12-15 mph in top gear!
( pilot jet was 15)
Actually, to revise my original speculation about the float needle spacer, by memory, the twin amal set up with the single carb with the float chamber and the other chopped, also spec'd the spacer.Seems to me, and I may be wrong, but raising the float chamber needle with the spacer (washer) raises the fuel level in the float chamber above the standard level marked by the dot moulded into the float chamber cover. Presumably this was the result of some experimentation by the factory and was intended to compensate for the use of a smaller main jet than that used on the twin carb bikes.