Metering valves can get varnished quickly with bad fuel and air. It doesn't take much as the fit is very precise. Clean metal from a new rebuild is the problem as most transitional metals are oxidation catalysts for fatty acids. See Japan Drier additive for linseed oil based paints, it is a cobalt salt. Anyhow, biodiesel is made from methyl Ester of fatty acids.
The metering valve is in the back center, roll the throttle shaft to see it move. Note how the ETR solenoid in the cover pushes the metering valve closed through the governor arm.
When replacing the cover, drop it on the pump about a half inch forward, then slide it back so the solenoid fingers catch the governor arm and push it back.