Are you sure there is such a thing as a aluminium/viton tip ?

Yes, I've just bought some.

The viton tipped aluminium needle weighs 0.5 gram.This will give you the same fuel level,as long as you have a fuel head 5.5" above the bottom of the float bowl.That means it will work OK if there is some fuel in the tank.It takes 5.5" of fuel head to overcome the weight of the needle.If there isn't enough fuel head to overcome the needle weight,the fuel level in the bowl will be about 0.200" lower than it should be.The float is being used to open the needle,instead of closing it.

The viton-tipped brass needle weighs 1.7 gram.It takes about 13" or 14" of fuel head (above the bottom of the bowl) to overcome the weight of this needle.With less fuel than that,the only chance you have of getting the correct fuel level in the bowl is if vibration is lifting the needle off its seat.
If the fuel level ever became high enough,the float would then hold the needle closed.

The original nylon needle weighed 0.3 gram.
This should give you a fuel level 0.205" +/- 0.035" below the top of the bowl,when the float toe is set 0.080" below the top of the bowl.
The viton tipped aluminium needle weighs 0.5 gram.This will give you the same fuel level,as long as you have a fuel head 5.5" above the bottom of the float bowl.That means it will work OK if there is some fuel in the tank.It takes 5.5" of fuel head to overcome the weight of the needle.

If you see how much clearance there is in the needle groove,the float has to drop a long way before it can lift the weight of the needle (about 0.200" at the toe).
It shouldn't have to lift the weight of the needle.The fuel pressure should lift the needle,and the float should be on the closing side of the groove.

The figures are from tests using fuel in a fuel line (static,not on a running engine).There is a sudden change of fuel level in the bowl at 5.5" with an aluminium needle,and about 13" for the brass needle.The height of the float changes approx 0.200".

The free play between the float's fork and the needle surfaces above and below it is noticeably huge on a Concentric.

A Monobloc float, which lifts the needle to close the valve, looks like it would work properly the way Magnetoman describes.

I experimented with narrowing the gap on a Concentric, but didn't really get anywhere.

Did you get those direct from AMAL ?

I'm in an airport waiting for a weather-delayed flight, so I haven't taken the time to reverse engineer your calculation, but it appears you've calculated the pressure of a column of gasoline (pounds/sq. inch, or grams/sq. cm) and equated that to the mass difference. If so, that's an incorrect calculation. And, it's not how the float needle works anyway.

The needle is pushed against the seat by the float acting through a ~4:1 lever arm, and flow ceases when the pressure on the seat equals or exceeds the pressure head of the fuel in the tank above. It's true that the float has to lift a bit more weight if the needle is brass instead of plastic or Al, but that's a minor effect. The level of the gasoline just has to rise enough around the large float to displace the additional 1 gram (or whatever) of weight, at which point the pressure on the seat will be the same as if the needle weighed less.

Since there's no Amal handy in the airport to measure, I'll guess that the cross section of a float is ~2.5 cm, so the area immersed in fuel is ~6 cm^2. If the fuel level rises an additional 0.1 cm (i.e. only 0.039", which is within the specs needed to set the level) above where it was with a plastic needle, it will displace 0.6 cm^3 of fuel. The density of gasoline is less than that of water (1 gm/cm^3), so rather than googling it let's guess it's 0.6 gm/cm^3 to make the calculation simple. That's 0.6 grams of additional fuel that has been displaced. With the 4:1 leverage, this means the rise of only ~0.04" of the fuel in the bowl will lift a needle weighing 2.4 grams more than the plastic needle. So, the weight of the needle does affect the height of the fuel in the bowl, but by very little.

As for me, I like the old nylon needles as they seem to cause me less problems.

Only one side of groove matters. Float always pushes on it. The amount of play in groove doesn't matter.

My parts man gets them direct from AMAL.
HTH

When you have time, please start again and make it brief. I'm baffled: a rise in the fuel level in the bowl of a Concentric doesn't lift the needle. It lifts the float, which pushes the needle down.

This is not the case,when needle weight alone is enough to overcome fuel pressure and prevent fuel flowing into the bowl.With a brass needle,this will happen when the tank fuel level is below about 3/4 full.

The needle then needs to be lifted by the float (or some other means) to allow fuel into the bowl.For the float to lift the needle,it must drop a long way and operate on the other side of the needle groove.The float will drop this amount only if the fuel level in the bowl drops too.

This discussion is as old as Amal brass needles themselves...

There continues to be a small vocal group who bemoan the discontinuation of the nylon needle and introduction of the brass needle jet. This some 30 years after the introduction.