I have had two four stroke lawnmowers that behaved in exactly that way, this year.

I am told it is the way high-ethanol fuel affects the carbs. I do not really believe this, but have no better explanation.

I put a "Fuel system" cleaner chemical in the fuel tanks, and then sprayed fuel directly into the carb to keep the engine running until the system cleared itself.

A two person job, really. Dangerous if you get a backfire. Worked, though.


Simon

Location: On a hill in Wiltshire

If you think it's fuel, it will probably be electrical! Have you tried another coil? Or a leak in inlet manifold gasket?

Location: Wales

I'm with Ian. Make sure the simple things are OK first. Yes it can, of course, be some obscure problem to do with fuel or whatever that has never affected anyone before but there's a much better chance that it's something simple.

Malcolm Parker will tell you a story about a friend of ours who'd convinced himself 'it' was a problem with his new timing gears and, when we found him, was in the process of stripping the front of his engine on a wet and muddy trials hill with his wife/passenger berating the quality of repro parts to anyone who'd listen. Re-setting a distributor that had come loose and retarded itself solved the issue in under a minute........!!

Steve

Location: North Yorkshire

Simple check with an SU take the piston out and ensure the float chamber is filled with petrol to the point where the fuel supply is cut by the float chamber valve. You can now look down into the main jet and see where the petrol sits, of it is overflowing the top of the main jet in the bridge the fuel level is too high, if it is significantly below the top the fuel level is too low, either way the float will need adjusting, and in the case of the latter may explain your difficulties.

Location: NZ

Rings very true that story.
It's confession time. I've got it started and it's in rude health. The carb was set too weak. In my defence I set the mixture nut 12 flats down as per SU spec, and assumed that it would start even if that setting was approximate - but it didn't. My carb needed a quite few more flats down before it started to cough a bit then start. A bit of fiddling with the Colortune and it seems fine.
I suppose it's logical if you think about it: if it's obviously fuel starvation and all else is mechanically fine, the only thing that can possibly cause it is the carb setting.
Moral: as ever don't overlook the obvious.
Thanks all for the help.

There's something not quite right here. The nominal position for an SU jet is 12 flats down, and you shouldn't need to move it more than 2 or 3 flats from there. It suggests that the fuel level is too low. Ok it will run but I think it would run better if you checked the fuel level first.