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I bought an Accucraft pop valve for my K-27 that did the same thing. I replaced the ball with a precision stainless steel ball and the problem went away. I can't remember whether it was a 3 or a 5 mm ball, but I got it from Quisenberry Station. Another thing I did was once I found the sweet spot on the adjustment for 60 psi, I put a small drop of paint on the edge of the screw in thingy so it stayed in place. I noticed that over time, it moved slightly with each pop, reducing the top pressure. Screwing it in raises the pressure and screwing it out lowers the pressure.
 

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Curmudge,
I accept what you say, but you must realize that most of us are subject to the design we purchase and can only make minor adjustments and improvements since we don't have precise machine tools to do otherwise. I merely suggested ONE solution that worked for me so that Joe could try it and not have to return the device or purchase another one.

I recently talked to a person at a steamup and observed his Accucraft K-28 with an Aster pop valve in it that worked as precisely as yours.
 

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Eric,
I don't know about the Mogul, but my K-27 has a screw-off dome and my K-28 has one little allen set screw to release the top half of the dome, which then lifts off. I had to use a flashlight to see the set screw on the front and not on the side of the dome about half way up.
 
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