Might be the way I'm cleaning them. I've read material by a ZAMA tech to only use WD-40 to clean out and compressed air from a distance. I'm trying that with next ZAMAs.
Greetings Glen, and it may be possible.
I had no idea, in the early days, that using aggressive sprays (carburetor and choke spray) and high compressed air (over 20 - 30 psi) was so potentially damaging to the inside of these carbs. To be honest, STIHL's position on this is, DON'T use compressed air for internal passage ways at all
(external ok for drying) and use only the recommended NON-FLAMMABLE
brake or contact cleaner to flush the inside. You can, however, use carburetor / choke spray to remove dirt from the outside of the assembled carb.
Both of the basic check valves are made from either Mylar or Teflon, some even use rubber disks. These check valves are extremely sensitive to pressure differentials, as one might expect, because atmospheric pressure (engine off) in the venturi keeps them closed when a vacuum is being applied internally (primer bulb).
I can take a hand pump (Vac. and pressure) and gently exercise the carburetor body check valves with just about 0.5 PSI of pressure, when checking their operation. Old fuel, harsh chemicals, or even compressed air can cause the disks to distort, perhaps just enough, to cause a starting or running issue. Even with an ultrasonic cleaner, and the right chemical, it is too late if the check valve becomes damaged or distorted. As noted before, there is no way to service the idle check valve in any ZAMA carburetors.
In addition, I believe the primer flange can also be a source of internal leaking (fuel pushed out of the progression holes and or main jet) if the flange intake valve is not seating properly, and allowing pressure back into the metering chamber it may force the check valves open.
In a lot of cases, it's not cost effective to replace just the flange. Example: ZAMA CIQ-153S; customer's cost, $ 27.63
Primer flange: $ 17.31
Good luck with future repairs.