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Does sound Carb-y. You mention pressure testing, but did you do a vacuum test?
The nozzle check valve malfunctioning will do just what you are saying is happening. It will allow air from them venturi back into the carb at low speed. So saw will eventually gain rpm at WOT because the low pressure signal from the Venturi is pulling fuel from the metering side. Once you let off, the low speed circuit will pull fuel and instead of the metering diaphragm opening the needle, the area above the metering diaphragm will fill with air. Lather/rinse/repeat.
The carb kits often have the small flapper valve for the nozzle in them.
The nozzle check valve malfunctioning will do just what you are saying is happening. It will allow air from them venturi back into the carb at low speed. So saw will eventually gain rpm at WOT because the low pressure signal from the Venturi is pulling fuel from the metering side. Once you let off, the low speed circuit will pull fuel and instead of the metering diaphragm opening the needle, the area above the metering diaphragm will fill with air. Lather/rinse/repeat.
The carb kits often have the small flapper valve for the nozzle in them.
