Onan Gen--hard starting and surging

When we first got our coach, the genset would not run. It would turn over, but would not run. After a little studying, I figured out that it was getting plenty of fuel from the fuel pump, the fuel line was in good shape, and it had plenty of spark.....only thing left was the carb. I figured out how to disassemble the carb..removed the bowl...removed the needle valve and found the float to be slightly stuck in the closed position. Seafoam to the rescue! I put all the removed components in a bowl of Seafoam and let them soak for an hour or so, and then used a wire brush to remove all the varnish that had accumulated from the crappy gasoline. Put it all back together, and WAL-Ahh! She fired right up, ran like a sewing machine, and has been running great for the past 5 years. I put Seafoam in every tank of gasoline in the RV mixed at 1 oz per gallon of fuel. Primary engine runs great, and so does the genset!

My unit is a 5000, not a 5500. The carb faces the rear of the coach.

I never could get the brass connection to the bottom of the filter. So I cut the line leading to the filter, from there I removed the filter, blew out any fuel and gunk that might have been there.

After removing the filter from the carb, I pumped some SeaFoam spray down into the carb. Cranking the motor just briefly I could then determine the fuel pump is working...so the line was plugged with a bolt so I could crank the motor without getting a face full of gas.

I got it to start with the inlet fuel line loaded with Seafoam sorta kinda. I opened up the connection to the carb from the air filter, where I sprayed the innards also with Seafoam.

It ran better----smooth, after connecting back the inlet fuel line. UNTIL I put some load on it. Then the surge came back slightly, then got worse and worse until it quit.

I fired it back up, this time playing with the throttle, I am guessing that there must be a FAILSAFE that if the RPM's get too high the unit shuts off. I tried to get the RPM's up higher to see if it would self-clean with some fresh gas from the main tank, but nope...nada.

Opening the fuel line back up I poured more Seafoam into the line leading to the carb, where I let it sit for an hour. Again, it surged and finally quit.

SO.....I guess I will try one of those carbs off Ebay. I see I can get one for about $50-60 bucks. The swap looks easy enough. I am sure if I take it to the Cummins shop they will sell me a new carb at $400 plus labor.

One thing I did do is blocking the inlet gas line with a bolt, I checked after cranking to see if any of the leading gas lines from the main tank were wet indicating a crack. None.

**Footnote** The adjustment at the bottom of the carb for altitude, I have it set at Sea Level. If I turn it to 5000 or upwards the unit really does run rich.

I guess I could try some more carb cleaner, like BG or another product, but it appears whatever is going on with this Genset, the years of sitting in that storage yard has destroyed the carb just enough that it does not want to operate anymore.

Your Onan NHM 7K generator has an adjustment on the bottom of the carb for different altitudes. Try turning the screw counterclockwise for a slightly richer mixture, and see what happens.
The ethanol in the fuel is actually a solvent that helps keep the carb clean. Remember when we used to add ethanol to our gas to help pass any water that was in our fuel tanks. Now we don't need to do that.

Richard

I had a surging problem on the Onan 5000 in our 96 Southwind.

I used a can of Spray Seafoam. I removed the air filter. With the generator running I sprayed the Seafoam into the intake until it almost stalled.
I did that several times, then sprayed until it stalled and left it sit overnight. The next day it ran smooth.

Try that a few times, it's cheap and easy. Leave it sit several hours after stalling the generator with Seafoam.
I left it sit overnight because I was busy with another project.

Most would agree that it's fuel related - likely gummed up carb and/or fuel filter or fuel pump. I would start with the gummed up carb since most rigs that sit have that in common. I would try the seafoam approach first - ain't fast but it's inexpensive and easy to apply. Small gas container with lots of seafoam can be connected to your fuel filter with small fuel hose - run the generator - let is sit - repeat.

Russ, your problem may be the fuel line from the tank to the gen. Get some fuel line the right size, and connect it to the inlet of the fuel pump. Then insert the other end in a container of gas of know quality, and start the generator to see if it will run. If it does, the fuel line from the tank to the metal line under the MH may have dried out and cracked, allowing air to enter the line causing the problems. It is possible to change the hose without dropping the tank, if you are not too muscled up.
Good luck

Small engines and 10% ethanol gas are incompatible. The carburetor could be all gunked up as a result. It could require a rebuild or replacement to make things right. After taking care of the carb try to find a way to run the generator with ethanol-free gas and maybe your problem could go away.

You might find that filter below the carb has been discontinued. There is another on the fuel line just before the fuel pump on the lower backside.

OK...going to study this genset a bit more. Also get some Seafoam. I think maybe trying to get the cleaner to ungunk it first before I buy a carb. Looks like Ebay does have aftermarket models.

After years of setting the carburetor is probably gummed up. Replacing it may be the easiest fix. I've heard there are 3rd party carbs on eBay at reasonable prices. You'll need the model and serial and "spec" numbers of the generator to find the right part.

The fuel pump may be on the back side of the generator. The fuel line to the gas tank may have a crack allowing the fuel pump to suck air instead of gas after so many years. You can use a short piece of hose stuck in a gas can next to the generator bypassing the fuel line to see if that related to your problems.