Onan float woes

Kelpiesgmc

Kelpiesgmc

Active member
Aug 19, 2023
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Tucson Az
I recently swapped my Onan. It starts and runs reliably— for a while. After a few minutes it starts to run rich and eventually dies. It will then refuse to start for a while.

Assuming it was slowly flooding, I disconnected the fuel pump's 12v supply and started it up. It struggled to start, but then started, and ran better and better and better until running out of gas. This reliably fixes the problem whenever it dies.

I adjusted the float to the specs in the manual. With this setting it flooded so badly that gas flowed out the intake. I set the float ridiculously high, hooked the fuel line back up and powered the pump. With the float in this position the bowl wouldn't fill at all.

I've adjusted the float a dozen times and I can't stop it from eventually flooding, or completely starving.

Is there a better way!?

The process is so painful — disassemble, adjust, reassemble, hook up fuel line, test, disassemble, adjust, reassemble, hook up the fuel line, test, etc

I've already rebuilt the carb, and I have another recently rebuilt carb. I've tried both seats. I'm using the fuel pump from Applied, so I assume its pressure specs are correct.

New carbs are like $30, making it VERY tempting to find a "generic" replacement. If someone knows a viable replacement I'd be very interested in the model that worked.
 
It sounds like you have the wrong pressure fuel pump on it. It needs to be a low pressure pump, like 2.5 - 4 PSI. Most likely the pump is overcoming the needle valve and causing the flooding. I had the same problem with a "reconditioned" Onan that I put in my coach a while back.

It needs to be one like this:

Amazon.com: Facet 477060N, Facet Cylindrical 12v Fuel Pump, 1/8 NPT, 2.75-4 psi : Automotive

Buy Facet 477060N, Facet Cylindrical 12v Fuel Pump, 1/8 NPT, 2.75-4 psi: Electric Fuel Pumps - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
 
Carl Stouffer said:
It sounds like you have the wrong pressure fuel pump on it. It needs to be a low pressure pump, like 2.5 - 4 PSI. Most likely the pump is overcoming the needle valve and causing the flooding. I had the same problem with a "reconditioned" Onan that I put in my coach a while back.

It needs to be one like this:

Amazon.com: Facet 477060N, Facet Cylindrical 12v Fuel Pump, 1/8 NPT, 2.75-4 psi : Automotive

Buy Facet 477060N, Facet Cylindrical 12v Fuel Pump, 1/8 NPT, 2.75-4 psi: Electric Fuel Pumps - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
Click to expand...

I agree that does seem to be what's happening. It's the pump from Applied

www.gmcrvparts.com

ONAN FUEL PUMP EXHACT FIT - GMC MOTORHOME

We are a major supplier of GMC Motor Home, RV Parts. Serving GMC MOTORHOME owners everywhere.
www.gmcrvparts.com
 
That's the one I got as well, and it works just as it should. There s a chance that it is still the wrong one. Otherwise, the float might be sinking. I can't remember, is the float brass, or composite? If it's brass, it could have a pin hole in it and is filling with gas.
 
I found a very cheap 2psi pump on Amazon. I'll report back if it works better.

I also have two floats, they both float in fuel.

The manual says this about running rich:
"Nozzle boss gasket
leaks. Engine runs with power needle seated"

Only I'm not sure what the nozzle boss gasket is. I think the manual also says not to remove the nozzle 🤔
 
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tyler said:

are you a "by the book" kinda guy? page 9 of this manual has a pic of the idle and main fuel adjustment needles. not all small carbs are the same. there is a little o ring i think in the main cavity that is prone to trouble. a fresh kit and settings by the book are a start. some are too far gone
Click to expand...
I did a rebuild kit. I'm not saying I couldn't have messed something up, but I was shooting for "by the book". I've looked at the exploded diagram many times and I think all the gaskets are accounted for and look viable.
 
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Does this have a neoprene tipped needle, or a neoprene seat? Could one be hardened or nicked?

There's a nominal force required to get the needle and seat to seal. If something isn't ideal, you'd need more than the nominal force. The float can develop more than the nominal buoyancy force by becoming more submerged than normal. But if float height is set to specs, this extra force can only develop as the result of an elevated fuel level.

So symptoms sound like either:
  • Needle and seat are fine, but fuel pressure is too high
  • Fuel pressure is fine, but needle/seat have problems
  • Float isn't buoyant enough (either a composite float getting spongy and absorbing fuel, or a brass one with a pinhole)
 
pvfjr said:
Does this have a neoprene tipped needle, or a neoprene seat? Could one be hardened or nicked?

There's a nominal force required to get the needle and seat to seal. If something isn't ideal, you'd need more than the nominal force. The float can develop more than the nominal buoyancy force by becoming more submerged than normal. But if float height is set to specs, this extra force can only develop as the result of an elevated fuel level.

So symptoms sound like either:
  • Needle and seat are fine, but fuel pressure is too high
  • Fuel pressure is fine, but needle/seat have problems
  • Float isn't buoyant enough (either a composite float getting spongy and absorbing fuel, or a brass one with a pinhole)
Click to expand...
For the float, it is the seat that is "rubber". It looks "new". No crack or swelling apparent. Like I said, I can get the float to stop ALL flow by setting it very high. Seems with any float travel, however, the pump pushes past the needle.

I agree with your assessment.

I don't have a way to measure the pump's output pressure unfortunately. I ordered a cheap 2psi pump. I'm interested to see if it plays nicer with the float. The shipping was delayed so I'll have to wait until tomorrow to find out.
 
I may have tricked the carburetor into being happy.

I took an extra bowl retainer gasket and put one on the inside of the bowl as well so that fuel can't leak into the nozzle from the bowl. I adjusted the float just a little tiny bit higher for good measure.

With this setup I just ran the Onan for over an hour with a good load. Seems pretty stable 🤞

Now let's see if it runs next time I actually need it 😅

1767557701862.webp
 
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I put a Walbro LMB-230 carb on mine and works great. It bolts up to the intake manifold and fits perfectly. It is a larger carb so I had to adjust the idle speed and stuff. The throttle linkage on top of the carburetor didn't match, but I corrected that by turning that upside down and bending one of tabs, if you get one you'll see what I'm talking about. My advice is unplug everything from your MH before you power it on, being a larger carb it will run faster and put out higher voltage, just adjust the speed until you bring the voltage down to the 110-120v range.
 
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My go-to for small engines is carb replacement if available at a reasonable price. I believe their small passages get partially clogged and even soaking in carb dip does not seem to get them running right. Go for the replacement!
 
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Luke said:
I put a Walbro LMB-230 carb on mine and works great. It bolts up to the intake manifold and fits perfectly. It is a larger carb so I had to adjust the idle speed and stuff. The throttle linkage on top of the carburetor didn't match, but I corrected that by turning that upside down and bending one of tabs, if you get one you'll see what I'm talking about. My advice is unplug everything from your MH before you power it on, being a larger carb it will run faster and put out higher voltage, just adjust the speed until you bring the voltage down to the 110-120v range.
Click to expand...
Awesome! I'm actually waiting on a LMB-230 replacement carb that I ordered yesterday thinking it looked plausible as an alternative. Great to hear it works.
 
Luke said:
I put a Walbro LMB-230 carb on mine and works great. It bolts up to the intake manifold and fits perfectly. It is a larger carb so I had to adjust the idle speed and stuff. The throttle linkage on top of the carburetor didn't match, but I corrected that by turning that upside down and bending one of tabs, if you get one you'll see what I'm talking about. My advice is unplug everything from your MH before you power it on, being a larger carb it will run faster and put out higher voltage, just adjust the speed until you bring the voltage down to the 110-120v range.
Click to expand...
Got the LMB-230 style carb on with not much effort. Got it stared and it runs well! Few small tweaks to the linkage to make it run at the correct RPMs.

Generally it seems pretty happy with this carb. But it does see weirdly reluctant to start. The choke isn't working, but even manually applying some choke doesn't help. I have to spray starter fluid in the carb, then it starts right up. It hunts a lot for the first minute or so and then smooths out and runs well. Takes the AC + water heater without complaining.

@Luke did you have a similar experience?

Edit: it starts right up when warm. Seems to be specifically cold starts.
 
Last edited:
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Kelpiesgmc said:
Got the LMB-230 style carb on with not much effort. Got it stared and it runs well! Few small tweaks to the linkage to make it run at the correct RPMs.

Generally it seems pretty happy with this carb. But it does see weirdly reluctant to start. The choke isn't working, but even manually applying some choke doesn't help. I have to spray starter fluid in the carb, then it starts right up. It hunts a lot for the first minute or so and then smooths out and runs well. Takes the AC + water heater without complaining.

@Luke did you have a similar experience?

Edit: it starts right up when warm. Seems to be specifically cold starts.
Click to expand...

I have to slide open the drawer and manually choke mine to get it to stay especially when cold.

There is a way to add a priming switch to the control panel inside and it should start right up that way if the fuel bowl is full.
 
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