Geza,
I was able to un-stick the valve several times. Once you have it out it is
easy to evaluate. The inlet and outlet ports are in-line, so if you can see
through it, the valve is stuck open. I flushed it with carb cleaner. You
could also test the heater inside it for resistance using an ohm meter. If
it open then you need to toss it. If memory serves me, there is a bolt that
provides some amount of adjustment.
Ken Lewis
http://neksiwel.20m.com/