the onan 2800 manual is available for download as a .pdf-
here's an excerpt:
An electric fuel pump is used to supply fuel to the carburetor.
If the pump malfunctions or insufficient fuel
delivery is suspected, use the following procedures to
test the fuel pump.
Warning Do not substitute automotive electric fuel pumps for
Onan supplied electric fuel pumps. The output pressure
is much higher andcan cause carburetor flooding
or fuel leakage, creating a fire hazard.
Fuel Pump Test: Test the fuel pump by checking the
pump outlet pressure as follows:
1. Remove fuel line from carburetor inlet and install a
2. Press start switch and hold it for several seconds,
until pressure reading is constant.
3. The pressure reading for a good pump should fall
within 3-1 12 to 5 psi (1 7.2 to 34.4 kPa). The pressure
should stay constant or drop off very slowly.
If pressure reading is below 3 112 psi (17.2 kPa),
replace fuel pump.
If pressure reading is at zero, stop engine cranking
and check electrical connections. Press the Start
switch and recheck pressure reading.
There are no serviceable components in the fuel
pump. Replace complete fuel pump assembly if
**** many autoparts stores will have fuel pumps for carbs ( not efi) which will give correct pressure of 3 to5 psi about $30- I recently put one on my '72 Eldo w/ carb ( so i can turn off for theft prevention)
if working on the fuel line you may want to add a petcock so that you can run the carb float bowl dry for storage and prevent gum up.