Yes, fuel flows freely from tank. I've also ridden with the fuel tank cap loose so as to eliminate the cap vent as a problem.
No signs of air leaks. Sprayed carb cleaner liberally around the manifolds with no rpm change.
Yes, I use the choke to start but once warm it does not need it. Bike starts easily.
The previous owner did have this issue.
The bike does has the standard airbox and filter.
The bike has an aftermarket exhaust. A Produro.
The jetting is standard now.
Usually leave the fuel tap in the "on" position when I stop.

How can I check accurately that the second slide is working properly. I took off the rubber boot from the air box to watch it with the engine running, but thought that the airflow change might not make it work correctly. What should I observe when watching this? What I saw was the slide jerking up and down when I revved it, but it did so very erratically. It didn't open right up either. Should it have been a smooth up and down action. Again, I thought this may have been because the air boot was off and changing the air flow. Thoughts?

My needles don't have the 5 slot positions. You raise or lower them I assume by shimming with washers. I tried this with the plastic washer but it is a very thin washer. How far apart are the notchs on regular needles. I can try with thicker washers too raise the needle, just not sure by what increments. (Ie. 0.5mm or 1mm at time)

Should I be raising the needles on both sides of the carb? Not sure how to do that on the RH carb as again there are no notchs.

My gut feeling is it is either the secondary slide not opening properly or the needles need to be raised. WHy after 10 mins of riding would these change though?

Thanks for all your help. I feel like I'm getting closer if only because I'm ruling out more stuff.

Maybe a would help me think clearer.
Cheers, Julian