I have made a fairly successful 6mm bore SU type carb. At this size an overflow tank was easier than a float chamber. The fuel level is quite critical, it should be a bit below the top of the jet. The orientation of the piston is also important so that the air connections to the top side of the piston are on the engine side, not the air intake side. There is usually a guide slot to ensure this. Finally the piston must be free to rise and fall with the jet in every position. My needle has a fairly shallow taper, difficult to measure accurately.
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Peter, SU carbs are very simple I used to tune 3 on a 1959 Healey 3000 Mk11 and 2 on a Healey sprite,I remember reading about SU carbs a while ago (because originally on early RR Merlin engines IIRC), anyway that was a long time ago, all I remember now is that SU are complicated and possibly finicky, you might want to start by reading up on them. I'm sure they've been modeled before, but I don't remember ever seeing any in for example BAEMClub.com, and searching this website for "SU carb" or "SU carburetor" doesn't turn up anything useful. You might want to start a new thread with the title "SU carburetor help needed" to see if anyone can assist.
start with the easiest possible fixes and work your way through "all the usual suspects" list, compression, ignition and timing, carburetion.
firstly if your battery voltage is low then at more open carb you get more air which means higher compression which means the spark might not be strong enough, try a slightly higher voltage battery and see what happens.
you probably have already done this, but just in case, make sure the SU piston moves freely, make sure the needle it controls is straight and clean, etc, etc, etc. And was it really designed to be in a non level installation. Also give this a try, remove the float valve and reservoir and just position your fuel tank so its fuel level is at the carb level, if it runs then the problem is the float valve is too slow and you're starving for fuel.
if all else fails, put a plain old RC model airplane engine carb on it (between 7.5 and 9.5 mm bore, I'm guessing, for engines with near 1" cylinders, that's what I've been using) and give that a whirl. I've never had a problem with them. if it does run then you've eliminated a lot of other potential problems.
Found this on youtube for you. It gives a good description of how the SU works. Well worth watching. Start at 2.30 to see the How it works.
0.5/0.6mm should be fine. Would love to know what the coil is too if anyone recognizes it.