I second Julian’s comment, the carb settings would be closer to that of the Spitfire Scrambler than that of a golden flash. Depending on what compression pistons and cam have been fitted might need tweaking further slightly. But the spitfire settings will get you much closer than what you have now.
Open pipes prefer a leaner mixture, you have proved this further by turning the fuel off and it running better.
Bigger cutaway slide, smaller needle jet, lower the needle and smaller main jet are all things which will lean the mixture off.
To fine tune. Mark the throttle off on the housing with tipex or similar, then with the throttle closed, mark it off with a 0 next to it
Then open it fully and mark it off with a 1 next to the line you marked on the throttle housing.
Then find half way, mark it “1/2”
Then find the 1/4 and 3/4 and mark it respectively.
Settings at “0” are idle mixture screw ONLY (if your adjusting this and blipping the throttle then you don’t know how to tune carbs) this is the tick over and you want to set this on a hot engine, do about 15 miles and then adjust it. Turn the screw (horizontal) out until the engines runs its fastest, then screw it in until it just starts to slow and find that sweet spot where it is JUST starting to run its fastest. This sets the air to fuel mixture. If the engine is now too fast, then use the diagonal screw to slow it down. Ie back it out.
Positions between 0 and 1/4, these are slide settings, you can blip the throttle and it should pick up cleanly, if the engine whisps and flutters and possibly cuts out, then the slide is too lean. If it doesn’t pickup but keeps running then the motor is too rich and you want a bigger cutaway.
1/4-1/2 is the needle jet size. Jets and needles do wear - it’s a rubbing action which wears the needle oversize. Again if it’s too rich it won’t pick up cleanly, if it’s too lean it will loose power, closing the throttle might make it run better, if it does.. it’s too lean. A new jet and needle is always advised if any doubt.
1/2-3/4 is needle position. (There is a slight overlap here, so if you’re closer to the 1/2 mark when checking the 1/4-1/2 then checking needle position is always worth while too). Again if it’s too rich it won’t accelerate cleanly, lower the needle which means put the clip on a higher ring position. If the clip is already at the highest spot (meaning the needle sits lower in the jet) then try a smaller needle jet and fit the needle in the middle slot. Again, if it runs better with the throttle
Closed down a bit then it’s too lean. And lift the needle by using a lower clip position.
3/4-1 is main jet… same rules apply, this works best if you can find a long hill and get yourself a full throttle. Any 8 stroking should be evident and means it’s too rich. A loss of power means not rich enough. 1 jet either side will usually run ok. So pick the one in the middle if you have 3 that run reasonable. If you do a flat out speed on a motorway. Then the right jet will give a couple mph improvement.
Start at top end and work backwards.
