Fuel starved?

Hey Mark, you nailed it with those symptoms. You didn't mention however how it was resolved. What did you do to fix your bike? Was it the tank vent?

I was trying to remember how I resolved, I seem to remember doing a few things at the time but not just because of the fuel starvation am going back 15 years. Tank Vent sort of rings a bell. You can test that quite simply though, take it for a run with the cap unlocked and see if the problem goes away. Not trying to be unhelpful but at the time did a top overhaul on it as well as other things. Lol not saying that is your answer. Others may have ideas but check the obvious first and discount them. Just remember the fuel is gravity fed so any type of blockage will impact on your flow dramatically.

I had those exact same symptoms & it turned out to be the original coils breaking down with heat.

I checked the 2 vent hoses...they're clear and actually the gas cap is definitely clear because it leaks gas (bad rubber seal). The gasline hose is a replacement which is just regular rubber hose and I'm thinking it should be braided or heavier fuel injection type hose due to the heat from the engine. I'm going to replace the hose with the proper gasline hose and try her. Phil: I never thought about the coils because it feels like a fuel starvation issue, but I guess I'll have to check those next. Would one bad pickup coil ruin my day? It would still run with 2 out of 3 working wouldn't it? My issue is that it slowly, slowly dies, but I can prolong the misery for an extra mile by choking it, so that suggests fuel. This really bugs me.

If all the obvious is good and clear and fuel is flowing freely. My only other thoughts are.
1. Carb floats set to low (I doubt it)
2. Diaphragm Suction (is your timing correct and valve shimming correct) I would imagine if valves are not shutting correctly would create a problem on the vacuum required to suck the fuel from the bowls. Bear in mind I carried out a top overhaul on mine and problem went away.
None of the above explained to me at the time why the problem was there, would fire up fine, run fine and just gradually lose power, there was no erratic running (which might indicate coils), no difficult starting problems etc. All I had to do was wait a couple of minutes and away again for next few miles till it all repeated itself. It was a strange one like you I checked everything obvious.
I carried out a top end re-build because I was also at the time when this started happening getting water pressure back into my coolant expansion bottle.
Perhaps the Likes of Kawboy, Strate Scotch and others need to input here

If you're going to replace the fuel hose, make sure that it's hose for fuel injection and I say this ONLY because the fuel injection hose is a viton rubber product compatible with the newer ethanol blended gasoline. The older fuel hose was a nitrile Buna N hose and it swells in the presence of ethanol. If you chose to go with clear tubing try to get clear fuel line that's specific for Karting. Those guys like to run methanol blended fuels and the clear fuel line that's marketed for karting is usually rated for alcohol and methanol. Some guys have been known to use Tygon hose for fuel line and it tends to harden up within a year. If you ever pulled a line off of a carburetor and then tried to put it back on and it leaked like a sieve then you know what I'm talking about.

Phil's comment about the ignition coils rings a bell with what I had mentioned earlier. I knew someone had that problem, just couldn't find it.