Response to Mark:
Did they slide freely before the rebuild?<- Yes,
Use a Sharpie pen on the piston. <-I'm assuming you mean to look for scuffs when I move the piston up/down?
You might also try leaving the 4 screws slightly loose and gradually tighten them as the piston is being slid up and down. <- I need to hold the dome down or it will lift when I move the piston
Did you try switching the pistons too? <-I'll try that maybe tonight.
Response to 240260280
Remove needle and plunger, Install dome but with all 4 screws very loose and not touching dome, Rotate Dome CCW and CW while lifting and dropping piston to explore where it binds and does not bind. Put dome in middle of its free rotation limits. Turn down 4 screws so they just kiss the metal dome <- this is where I don't understand. The dome screw holes will only permit two orientations. Neither resolves the problem, even with the needle removed.
Response to the Great CO:
If the piston is sticking even without the needle installed, then it should stick even without the dome installed on the carb body. <- No, it freely moves without the dome but sticks with the dome.
With that in mind... Did you try mixing and matching all the domes, pistons, and carb bodies? Maybe something got mixed up during the rebuild? <-That is a strong possibility. I've only tried switching the domes but not the pistons. I'll try that tonight.
Did you accidentally drop a piston or a dome? <-not that I recall but if I can figure out how to mount the piston and dome on a rotating jig, I can use a precision micrometer to look for deformation.
My next steps:
1) Switch the pistons and domes around in all combinations to try to isolate the problem.
2) Mark the piston with something that easily rubs off and figure out what is rubbing against what.