I'm impressed that you put in the methodical time and attention to get this right, and that you compared the 2 ways of measuring to calibrate the easy manometer method against the fiddly but convincing direct method, rather than just assume the easy method is accurate to avoid doing the hard work!
I like your float spindle retention method, I will adopt that assuming you haven't patented it yet! With the tweak of using a bent staple to hook over the spindle, so it is easy to swap floats. This is so much simpler than my "bent bit of metal clamped by nut/bolt"!
It is obvious how you levelled the bowl laterally, though small errors in this won't affect your measurement much at all at the centre of the bowl, the bowl and float have symmetry laterally.
How did you level the bowl front-to-back? This is much more important as float and bowl are totally asymmetrical about the centre in this direction. Say the bowl leans toward the engine side, more fuel will move to that side, where the vast majority of the float is, raising the float (and hence closing the valve) earlier than it would if the bowl were level. Vice versa of course also applies. A small inaccuracy in levelling in this direction could invalidate fuel level measurements.
To level the bowl in all directions, I made 2 rubber cups that held the float bowl in the vice below the flange, with careful tightening I was able to hold the bowl from moving while being able to make fine adjustments to it in all directions.
I hear folk saying "the bowls are at an angle on the bike anyway". True, but to reliably set things you must have a reliable, repeatable baseline to work from, and level is an easy baseline to work from, and is what AMAL
would have used in their technical advice, though their carbs are fitted to bikes at all sorts of angles.
Also, how did you actually measure the fuel level in the centre of the bowl accurately? It is easier said than done, as I'm sure you know.
I farted about with all sorts of ideas that required too many hands, none of them producing reliable REPEATABLE results, until settling on a bridge of straight machined 12mm square bar across the bowl. From this I could drop the depth leg of a vernier down until it just made contact with the fuel, several times to check for the smallest reading, and again several times after refilling the bowl. Only then was I sure of my results.
On your using 6" head of fuel. In my calcs I used 0.74g/cm3 so same as the stuff you're using. I found that the alloy needle lifted from the seat at ~12-13cm head. So 6" should be ok, just. However, if that is just in a tube, rather than a header tank, it maybe borderline for accurate testing. I used a funnel in a tube that could be kept at different heights between nails in a board to see the effect of different heads - no smoking! Then have your manometer plug as a drain, so you can drain and refill the bowl with ease, by lowering the tube.
There is much confusion over the phrases "fuel level" and "float height". The important thing is fuel level, that is what the engine sees, float height is only of secondary help, and only if certain conditions apply, namely that you are talking about the float and needle used in the recommendation.
Because of changes to float needle weight, and more recently the advent of the stayup float, the time-honoured relationship between FUEL level and FLOAT height no longer applies.
The .080 Float setting came (I believe) from the US Triumph agents in the early days of Concs, when they had plastic float needles and floats. I'd call it a "rule of thumb", because it is a crude, indirect method of setting Fuel level, but it was good enough for most.
Introduction of the brass needle changed the relationship between float and fuel height bigtime, and I think that is where a lot of this tapping the seat up and down has come from. Anyway, all praise to the alloy needle! It behaves close to the original plastic one, so as I found, putting it in with the old plastic float restored Fuel level to about where it was intended by AMAL
Now you've put in a stayup float, made of different materials, not just air inside, and a different balance about the pivot. I think it is also deeper from top to bottom.
There is no reason to think that it will be bouyant to the same level as the plastic float. Forget float level, remember fuel level.
The bowl flange is just a reference point, the gasket and recess in the body allow the float to rise considerably further, so as long as it doesn't contact the roof before closing the valve (which will be obvious with flooding) it will be fine. Do use your setup to compare all your bowl components to make sure you haven't got a "freak".
Because an engine with taps closed can idle till it dies at tickover doesn't help much, would you do that at full throttle? An engine will run on a wide range of air/fuel ratios, but I'm sure we all know the difference in disaster between weak at tickover to weak at full throttle?
Brett, do you get any pinking when you open the throttle from say 2500 rpm, not wide open but say to half?
Enuf for now, Dave