Breaking down under load

[Jonesy :-)]

Steptoe, you know I'm a nut spinner, ohms and all that malarkey is not my forte. S'pose I could Do some research and work out how to check that. Would a bad cap(s) affect all the rev range?

[steptoe]

Did you happen to check the caps for ohms? I’m sure they’re relatively new though no?

[lambs]

Dyna Coils and leads are probly only 10,000 km young, waddaya reckon about not advancing or over fuelling?

I'd still possibly try swapping for another set of coils, etc, if at all possible.    At least do some basic checks on primary secondary resistance, etc.    Also try changing ignition unit if you have a spare, and check vac advance unit + mechanical advance if fitted.

There's a lot of mythology surrounding dyna coils.   They can go bad just as easy as any other coil.

Over-fueling has to be a possibility given the sooty plugs.    WOT is all main jet for fuel but float height also has an effect....maybe sticking float, but then all four at once ?

[Jonesy :-)]

Dyna Coils and leads are probly only 10,000 km young, waddaya reckon about not advancing or over fuelling?

[lambs]

Whenever I had that problem (but not XS11) it was nearly always coil/lead/plug cap related....eg ignition leads breaking down or corrosion at connections.     Another possibility is the wiring/connections to/from the ignition control box.

[Jonesy :-)]

Took the bike out on my quick spin circuit recently after a couple of months not being used,  approaching a hill I gave it full throttle and it started missing.pretty bad, half throttle, no worries
Cleaned fairly dirty air filter, k&n cleaner and oil, synchro'd and colourtuned, replaced battery which unexplicably at the same time up and died after 9 years. Took it for another run and plug chop (all pretty sooty) still breaking down, chucked in some new plugs still breaking down. Visibly the pickup coil wires look ok.
Any ideas?