I've got a frustrating problem and I've tried just about everything. My nc31 has a fuel flow issue. Around the neighborhood it's fine because when I'm stopped at the traffic lights the fuel in the float chambers refill. If I get it up to speed and open the throttle within a few km the float chambers empty and the bike stalls. I have stop, wait a few minutes whilst sucking on the vacuum tube and the float chambers refill and we're good to go. For a few km again.

So far here is what I've checked:
fuel petcock - I've disassembled, cleaned the mesh screen, checked diaphragms and tested. it works 100%
Fuel lines, cleaned and checked and replaced the main fuel line and checked and replaced the inline filter.
With the fuel line and filter on I've checked the petcock and the fuel runs well.
Carbs - I've taken them off the bike, cleaned the float chambers (not much residue/dirt) checked the needles and seats and checked flow into the float chambers with water whilst the float chambers covers where off...all seems to work OK.

I'm not sure if anything else in the carbs could be the problem, I'm a bit new to this...

help...?

What height are your floats?

OK so this is where I learn.

I did not even know they had a height and I assume if they do there is an adjustment on them?
I'll need to open to check, but I think they're plastic and hence not adjustable?

<update>I just checked and they're plastic an have no ability adjust

I'm fairly sure that they do and there is...

What model do you have?
The f2 float height is 21mm

I opened the float chambers yesterday, the floats are fully plastic with no means of adjustment at all

this is what they look like (mine are made of a different plastic as these are not OEM ones)

Sounds like you could have a blocked vent pipe
If you still have the same problem go for a run until the bike stops running then open the fuel refill cap if you hear a sudden inrush of air your problem lies in the tank vent system.
Remove the tank filler cap there are 6 allen bolts but only 3 actually hold the cap in place
There are two openings under the petrol locking cap the largest is for draining water from the filler well the smallest about 2-3mm may possibly be blocked, there are two rubber pipes under the tank the large one for draining the filler well the smallest is the breathe pipe make sure this is not trapped or kinked.
I had a problem like yours I cured mine by removing the small rubber pipe and inserting a length of Bowden cable into the tank vent pipe from underneath I pushed the cable in as far as I could until I felt something blocking the way I then locked the cable into a small drill and spun the cable until the cable cleared the obstruction.
I made the cable end a bit rough by untwisting the cable slightly but not by much otherwise you may not get the cable to enter the small hole just enough so as to allow the cable to bite into the obstruction.

I've checked the tank is draining properly and that it is not pulling a vacuum...

Did you make sure the small rubber pipe (not the larger pipe) is clear try lifting the rear of the tank and blowing up the small rubber pipe you should be able to blow freely .
When you refit the tank make sure the small rubber pipe is not kinked or trapped anywhere.
Make sure the petrol cap is fitted and closed
Then when you have the rear of the tank lifted try sucking hard on the small rubber pipe if there is any restriction then open the petrol filler cap and listen for an inrush of air if you hear it your petrol breather is blocked somewhere.