The kjet metering rod is ground to zero tolerance, it's just a metal rod in a bore that holds 70psi. Sitting for some time can be enough to gum it up. There's no flow around it to clean it up and if it doesn't move it won't free itself.
They are robust enough to take apart to clean the piston and bore with a little care. But label everything and take lots of pictures to make life easier putting it back together. Having spare o rings is nice for that one which roll of to start it's new life.
Might still be something else like vacuum leaks, or fuel pressure, but it doesn't sound like there is fuel metering going on.