Stale fuel issues

[EFSM]

261 is more lock prone than many other saws. I’m not sure where in the fuel system the problem lies, but my suspicion is the metering side or the main jet of the carb. It’s interesting you are getting zero start. Usually a vapor locked saw will struggle a bit. .

A stiff metering diaphragm is the most common reason for super finicky carburetors, in my experience. However, Zama’s black plasticky pump diaphragms can also swell and not flex as a result.

 

[tree monkey]

changing the gas in the tank doesn't change the gas in the filter hose and carb. 2 pulls won't do it either. you most likely have a tank vent problem

 

[Sloughfoot]

Does it start with a prime?

 

[JT Sutton/candycane]

I thought that you folks on the other side of the pond used the alkylate fuel like Aspen which ought to stay fresh a long while. If the "problem fuel" will work in another saw then I really doubt the fuel has anything to do with it. There has to be another issue. Have you checked or replaced all the easy stuff like spark plug, vent, filter?
I've never noticed one saw performing differently with a given fuel as opposed to another saw. I have been surprised a few times by how long fuel has sat in my old 028 Super and the thing fires right up.



Side note: Last night I watched the first half of the 1984 BBC production of "Threads" and it holds up very well considering it's 42 years old. Still a relevant story line unfortunately.

Vent ?