I have an xt 550 1982 which is up and running but:-

when you open it up it feels like its holding back and you get a back fire off it when you ease off

had the carbs cleaned and checked

had the valves checked

some times it will run ok but the next time it is flat as shit

any idea what to do next

Cheers

Beefy

Sounds like fuel starvation , make sure theres no crud in the tank or tap.

TTB

Sure there are no air leaks carb side?

how about the fuel filter in the tank...attached to the fuel tap?

Next time she holds back , pull on the choke ,, if it responds , then shes lean.