It may be “normal” for fuel to drip but it should not be so and I refuse to believe that they were manufactured to be that way!
I accept that some do, (It’s not just bikes that age!), and by and large we get away with it but it’s not only wasteful it’s also a fire risk. Think not? I have had it happen to me and it has happened to others too, see the bottom of https://www.scotttechnicalities.com.au/technicalities/Chapter%202/2.5%20Carbs+Controls.pdf
I have before me an unused and unmolested 206/151R Amal which, because of a previous post, I had a hard look at. As a result I concluded that the fuel height should be at least ⅛” (3.2mm) below the transverse air holes near the bottom of the main body.
A further point to bear in mind is that due to the steep angle, if the fuel height is permitted to be above this level and with the engine stationary, then there is a likelihood of fuel percolating its way through the pilot jet and then down into the engine which at the very least could upset the mixture ratio and make starting problematic particularly with a hot engine. But then we always to remember to turn the fuel off don’t we? 😉
The fact is that these carburettors are designed to operate with the main jet submerged, so as long as there is a sufficient head above the main jet to prevent air entrainment the fuel height could be made lower. There is a limit of course, fuel height will tend to drop as the power demand rises, (check the line from tank to carb. for flow), and as revolutions rise there will be significant agitation in the float bowel which could make the effective height lower too.
As a reversible experiment it would be a simple matter to make a second float clip groove in the float needle thus lowering the fuel height in the float bowel by that amount.