So, progress, almost...
I took the carb off (a Weber 28, which is on a 126 650cc engine) and gave it a good clean out. There wasn't as much grit/ grime in the reservoir as I was hoping for (!)..but I did replace all jets, gaskets etc. (I didn't go as far as replacing the spindle as it looked in good order). Two things I came across were:
1) The end of the choke cable was bent such that I question whether there would be sufficient tension in the cable to engage the choke lever on the carb (i.e. the bent cable would, unbend, and take up the slack). So, first question is, how easy is it to push another choke cable through the existing outer or should I attempt to straighten the existing cable in the first instance? I suspect this is the source of the issue that the choke lever didn't make any audible changes to how the engine ran..
Its simple enough just to push a new cable in as it is a solid cable. Just cut off the bent bits on the old cable and pull it through. You need to take the centre console off to get to the levers. Squirt a bit of WD40 down the outer first and hopefully it should slide through
2) The fuel return outlet was all taped up and underneath the tape, a large pin had been jammed down the eyelet... I didn't tape it back up, leaving it (presumably) blocked. I pieced everything back together and installed the carb back on the block. I unscrewed the choke and idle screws quite a bit and after a few goes it spluttered into a start, with some accelerator encouragement, and died after I pulled up on the pedal. I went back to the engine to start adjusting screws (see below my thoughts on turning) and saw fuel everywhere! My initial thought was that the return feed had not been blocked afterall, but having since given it some thought, the excees outlet points forward yet the fuel was in the tray and pooled in other dips on the front side of the engine (i.e. the other side from the return value). Frustratingly, I ran out of daylight before I could investigate further... but when I get the chance, I'll dry dialing in the mixture screw and starting again. So my other question is, can dialed out mixture/ idle screws cause such significant leakage? I took great care piecing it back together, with all screws done up tightly. If it is the fuel return valve, then I would be tempted to put a t-piece in back to the fuel hose from the pump to the carb leaving whatever is jammed in there...
The adjustment screws shouldn't cause any leakage unless possibly they are way screwed out. It's probably the fuel return, if it was a tight seal then there would be no reason to tape it up further. See if you can get the pin out completely and the put a T piece in before the fuel pump and return it to there. Don't put it between the pump and carb as you may screw up the fuel being pumped up to the carb at pressure and end up forcing it up the return line.
I'm grateful for any pointers ahead of me getting the chance at turning it again later in the week.
Finally, and on tuning... my thoughts were to get the idle screw set such that it ticks over, winding it back 1/2 turn from a position that sees a warmed engine cut out. Then, with that set, I'd do the same with the mix screw, taking it back 1/2 turn from a position that see a warmed idling engine splutter. Again, any points on this would be much appreciated.
Thank you for your time and patience!
