kasai

The effect of the rave valve would come into effect from 4-5k rpm so in that sense it could be related. But your problem might sound a little heavier.
Have you pulled out the valve to see if it is intact. (they broke both at stem and at the flat end on my 787 engine)

PS! where did you buy your bellow, I live in Norway, where it is impossible to get parts. So I need a good cheap dealer to order from.

Sounds like you have condensation in your gas, was the gastank full when you stored it. Have you tried "condensation remover" (dont know hat its called in english)