Opened up valve, everything internally looked clean and ok. No difference after cleanup.
One thing I noticed is that orifice of my valve is pretty large compared to shako valves. Orifice size is around 4.5 – 5 mm whereas shako orifice is approx 1.5mm (if I am correct).
Can big orifice be the reason of my problem (double drops + inconsistent drop size, after each drop operation some water is left at nozzle end)? Shall i try reducing orifice size by inserting some plastic refill piece inside it? (without affecting spring plunger functionality)