Onan generator starts but cuts off again. - Watts Onan Portable Generator - 4000 , Model# 5500

Is it diesel generator? Because i do not know Onan 3600 lp generator much. If it uses diesel fuel to drive, maybe you can reference some reason:
1.The fuel pipeline has air, and the fuel can't keep up with it. Solutions: Remove fuel and air according to instructions.
2.Air intake is insufficient or intake pipeline is blocked.Solution:check the intake pipe of the engine and replace the filter if necessary.
3. There is no oil in the tank or the oil valve has not been opened. Solution:Check fuel level of the fuel and fuel valve.

Try under the bench seats around the table, near the heater.

Check the oil filter.

Again, what generator model? Onan have thousands. Be more specific please.

Could be vapor locked. Prime the line and try again.

Adjust the carberator. You have to do this while the engine is running and it usually takes a straight slot screwdriver. You can turn a spring loaded screw to adjust fast or slow which ever is needed.

your going to need to check the carburetor. sounds like your fuel sys is contaminated. when you unplug the fuel solenoid the plunger pops up and cuts the fuel off. the solenoid is used to shut off fuel to keep the unit from back firing

Standard 30W motor oil.

Your remote start wires going into your transfer switch are closed, which means you transfer switch is the problem.