It's not that difficult .... just difficult to find the problem ...

An engine needs mixture and needs a spark to run.

A. Fuel / mixture
So, you have been looking at the fuel system extensively, but let's talk about that one first.
- Fuel arrives in the carb? If you open the drain screw at the bottom of the carb float bowl, there should be about 15 liters per hour or 1 liter in 4 minutes running out of the overflow. This is approximate of course.
But, if the amount is much less, there is a problem somewhere: tank aeration, petcock filter, filter on top of the float needle
- Carb settings ok: did you change anything lately?
- Air filter clean and correctly fitted?
- Carb connections to cylinder head ok?
What was / is your fuel consumption; this bike does something like 1 liter : 20 km, if yours is far off that might be an indication of the problem.

B. Ignition
As per a previous post, the symptoms might be in one of the electrical ignition components. Best way is to swap them one by one for the items from a known good bike:
- Ignition coil
- TCI box
- Wiring (difficult to swap, check the wires for chafing and the connectors for cleanliness and tightness), and do not forget the sidestand switch, clutch switch, emergency stop etc.
- Stator: a bit of work but worth it. Another option is to measure it's resistance values when cold; if they are far off from the factory data this might be broken. Further, when your bike is hot the resistance value of one of the pickups might be way off from what it should be. So, ride till the problem occurs, stop, jump off and measure with a multimeter.

Good luck!
Auke

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Auke
http://aukemarieke.blogspot.com