The thread regarding water/vent plugging issue has missed a potential
cause of this problem. Having both Left/Right tank selections responding
exactly alike would lead me to a common cause. One is-- a
slit/crack/porosity in the diaphragm in the fuel manifold valve on top
of the engine.
If one exists --the engine driven pump will overcome it
initially-- but then as leak (crack) enlarges-- it'll require electric
pump lo-- then high to keep the engine running. This might be on one
flight--or ten. But eventually the diaphragm will not contain the pressure to
overcome the spring, closing the valve and the engine will
quit.
To test--Put a bucket under sniffle valve-- Remove the drain line (rubber
hose on bottom of manifold valve) and turn pump
on high mixture and throttle in. ANY fuel coming out drain is
serious and the valve must be overhauled.
Engines stored for long periods are really susceptible to this
problem.
Charlie K.