6500 MCCK generator loses fuel pump prime after sitting for a few days

[sdowney717]

Then I can only start it by dribbling some gas into carb until it can reprime the line.

I have 2 tanks, 3 engines, each tank has one pickup.

This year it is having some troubles, so perhaps the check valve in the fuel pump is leaking back fuel?

I rebuilt that pump at least 5 years ago.

 

[gonzo]

That problem has nothing to do with priming. The carburetor bowl holds enough gas to start the engine, or it should. Most likely the gas in it evaporates and you start with a dry bowl, which requires a lot of cranking before it fills. Also, if the fuel pump in old, it may only start building pressure at higher RPMs after the engine starts. That means that at cranking speed it is not able to fill the carburetor. You can test the pump with a pressure gauge. Also the flow by putting the output hose into a can while you crank the engine.

 

[sdowney717]

The fuel line is 1/4 inch.
How far in inches do you think the pump can squirt fuel when it cranks?

I also should check the filter.
These pumps you can buy new rubber diaphragm and check valves. Been a while since I ordered any parts for it.

The other symptom is my main engines after sitting over winter were not able to pump fuel unless I turn off the gen line fuel valve. So maybe the gen fuel pump is letting air back into the system when the engine pumps try to suck fuel.
Engine pumps are Carter electric.

All 3 engines share a single source of fuel or 2 tanks sucking fuel through dip tube of each tank. Fuel then is distributed at a center valve type manifold where you can turn off tanks or shut off fuel feeds to motors.

 

[gonzo]

The problem may be in one of the main engine fuel pumps letting air go backwards too. Electric pumps should have no problem self-priming. Is the pump running when you turn the key on?

 

[sdowney717]

Could be I have no leaks.

I put a brass antisiphon one way valve on the output of the carters to prevent any backflow.

I am thinking of using an electric pulse pump on the mcck gen.

The pumps run fine. I could only prime the main engine carbs when I turned off the valve feeding the mcck gen.

 

[FAST FRED]

Many small noisemakers have a solenoid valve in the fuel line that solves many problems.

 

[sdowney717]

I found a pump in my garage came out of a GM 350 diesel from 1985. Looks just like the onan electrics on ebay.

I tested it PSI is 2 and suction is 5 to 7. It pump 1.5 cups in 15 seconds.

So going to use it. Dont want to buy another kit for the mech pump. This way I can also prime the system without the motor having to crank.

 

[gonzo]

If you install an electric pump it needs a shut-off connected to an oil pressure switch. It also needs to have a starter or solenoid with a bypass to make it prime as you crank. I thought all your pumps were already electric, but re-reading seems that only the main engines' have them. It will be much easier and cheaper to rebuild or replace your existing pump. Probably, it will be safer too.

 

[sdowney717]

I installed the pump.
Onan wiring schematic had an electric pump as an option, so I installed where Onan told me to install.
Works fine.
Runs when cranking
Pump is in the ignition circuit before the dropping resistor to coil, so when the safety system shuts down the gen, by killing the ignition, the pump turns off also.

 

[sdowney717]

schematic in case anyone else does this. Pump is dashed line circuit designated E4
RT1 is the volt dropping coil resistor.
Electric pump gets its power from relay K1, which is the start-stop relay.
K1 is powered from emergency relay k7 which kills all power when overheats or oil pressure drops.

The system is sophisticated and from 1970.
It has onan control-o-matic which auto starts and stops the gen when load demand is detected or dropped.

 

[gonzo]

That option made it easy and safe. Thanks for posting the circuit.