This XT600 is a reliable farm bike which provides it's owner with zero trouble.
Except for the fact that it will not run on reserve (i.e. will only run on mains).
A blocked or faulty petcock was suspected and so was removed for inspection.
When looking carefully at the tank fuel-intakes you can see many years of accumulated scale clogging the port on the left (reserve). Without fuel flowing through this orifice the residue has continued to accumulate over time.
Reinstalled and working perfectly. The XT600 is a brilliant, big, air-cooled thumper, and now with a working reserve provides some warning before running out of fuel.