diagram of a fuel servo

My fuel servo is coming back from the shop. I wish I better understood
how they work. One day my plane just stated running so rich that I
could almost not keep the engine runnnig. I was pulling about 7GPH at
1500 (normal is about 3). However, if I shutdown and restarted it would
run correctly about 10% of the time.
So the fuel shop received it and noticed the FF was too high. They
replaced a spring and got the flows in line. However, now I'm worried
that they fixed the 90% case and not the 10% case (which was normal).
If they replaced a spring with a lighter spring, how would that solve
the issue? Why did this happen all of the sudden? Why isn't it
consistant? My plane has been down for 3 weeks now and I fear I'll get
the unit back and it won't work and I'll be down even longer (with a
full FF overhaul). The fuel servo has 900 SFNEW.

Is there an on-line diagram of a fuel servo? I'd like to better
understand how the springs, diaphrams, etc work. I'm told there are
only a few moving parts, but not sure what they are.

-Robert, M20F