Just thought I'd update this RE the 'underfueling/lean top end issue' incase anyone is interested or it helps someone else.
As already mentioned, I did indeed just have a million different issues that were making it very hard to find the causes of problems. After getting it to run much better with the over-run valve blocked, I finally got into investigating the rest of it.
I added more fuel to the tank, pulled all the plugs and cleaned them, and took it for a good thrash round the garden. Pulled the plugs again, black? Ok so it can't be lean. It has to be an ignition miss..made sense after I thought it through properly, my timing light wasnt rubbish after all. Thinking back, I had on a whim much earlier swapped in the high voltage coil I had kicking around from my old hilux, and I'd had made a slight but noticable improvement, of course, something else in the system was breaking down under load and the extra spark voltage must've been enough to get an OK spark at the plug higher in the rev range. This tells me that the coil was probably not the cause, but to be safe I went all out and brought new Bosch coil, leads, cap, rotor, and plugs. Now it runs brilliant, revs its guts out every time not an ounce of a missfire. Feeling much better now, brought 50mm wheel spacers to celebrate that fixes another problem
Getting closer to being usable.
I havn't yet tried changing back to the BFM needle, and don't really see the point, the taper is almost identical to a BDL anyway just the overall size is thicker compared on mintylamb so as long as the idle mixture is right it shouldn't really make much difference.
Have since thrown the fuel pump away too as that seemed to be about as healthy as the ignition system and swapped to a locally made diaphragm type electric SU style.
Thanks to all those that did offer input, it all helps, but as they say, 90% of carb problems are electrical
