Onan MJA Series Service Manual page 32

FUEL SYSTEM
FIG :3.2. C - MEASURING FUEL PUMP PRESSURE
should be between 2 and 3 psi with the gauge held 16 in. above the fuel
pump.
A low pressure reading indicates extreme wear in one part or some
wear in all parts, and the pump should be overhauled or replaced.
If
the reading is above maximum, the diaphragm is is probably too tight
or the diaphragm spring too strong. This can also be caused by fuel
seeping under the diaphragm retainer nut and between the diaphragm
layers, causing a bulge in the diaphragm. Overhaul the pump and
replace the defective parts.
Low pressure with little or no pressure leak after pumping stops indi­
cates a weak or broken spring or worn linkage and in most cases the
pump should be replaced.
3.2.3.2 Removal and Disassembly. ­
1.
Remove the pump inlet and outlet. Remove the 2 capscrews hold­
ing the pump to the engine and lift it off.
2. Notch the pump cover and body with a file so they can be assem­
bled in the same relative position, and remove the 6 screws hold­
ing them together.
3. Ta,p the body with a screwdriver to separate the 2 parts. Don't
pry them apart - this would damage the diaphragm.
4. Remove tJ;te screws holding the valve plate to the cover and
lift
out
the valve and cage assemblies.
5. Drive out the rocker arm hinge pin.