Onan Backfires through Carb??

IrnBear · 03-31-2022, 02:44 PM

Hello All,

I have a 2018 Hurricaine 35M
Genrator is a Onan 5500 HGJAB Gas

It likes to backfire through the Carb at startup. Once started it purrs like a kitten.
Last trip I had to pull the cover, hold the choke open (the fly flap) inorder for the Geni to start. It was cold outside 30-40s. Nice little explosion in the carb a couple times each start.
Last trip it also decided to just turn off a couple of times. We were able to restart and no problems.

Research says fuel pump, I can hear the fuel pump working. Do they go bad and yet still run?

I have done a full maintenance on it. Backfire issue was present before maintenance, just didn't go away after.

Again, once running, it purrs like a kitten.

Thoughts,
Thank you,
Bradley (IrnBear)

Chateau_Nomad · 03-31-2022, 03:15 PM

One of the longer term maintenance items is valve adjustment. Maybe check that? Sounds like a timing issue, possibly compounded by carb settings... which all work together. Changing one affects the other. Backfire is usually timing.

The Gritz Carlton · 03-31-2022, 03:59 PM

How many hours on it?

IrnBear · 03-31-2022, 04:04 PM

Quote:

Originally Posted by The Gritz Carlton

How many hours on it?

That I will have to check when I get home.
We boon-dock a lot

IrnBear · 03-31-2022, 04:09 PM

On another site someone mentioned that
Backfire through carb = lean or rich
Backfire through pipe = opposite of

I just can't remember which one is which

Due to the need for the flap to be wide open, I would think it needs more air and less fuel.
Where is the mixture screw on this Generator

I'm a pretty good DIY'er, so not afraid to do the work. Just really don't want to drop the Geni if I don't have to.

Valves sound like a drop geni job.
How do you adjust timing on this Geni?

Thank you for the responses.

**16ACE27** · 03-31-2022, 04:28 PM

Backfire through carb = too lean
No carb mixture adjustment screw
No timing adjustment

Likely clogged passages in carb due to dirt/corrosion/ethanol.

MJC62 · 03-31-2022, 05:00 PM

I did this with a Briggs & Stratton lawn tractor, and it took care of the surging at idle. The crappy ethanol gas around here gums the carb up on my tractor on a regular basis and I didn't feel like tearing the carb apart to clean it out again! Saw this procedure on You Tube.
1. get one gallon of fresh gas in my case I used 90 octane.
2. add 4-6 ounces of Sea Foam to the gallon of gas
3. Pull the fuel line from the carb
4. Drop a line into the tank of mixed gas.
5. Start the engine and run the Seafoam mixed gas through the carb for at least 1/2 hour to 45 minutes. Shut down and let the carb hot soak with the Sea Foam for another 1/2 hour. Start it back up and run another 1/2 hour.
6. Hook up the original fuel tank and run with fresh gas to clean the seafoam out of the float bowl.
On the lawn tractor this worked to cure the surging.
If you are mechanically handy it is fairly easy to do. If it doesn't work all your out is under $10 for a can of the Seafoam. I intend to run the clean on my Onan this spring as it starting to surge a little then smooths out after it is put under a load.

Chateau_Nomad · 03-31-2022, 05:31 PM

After reading (and learning

) I 100% agree with Ace.

Lean mixture at start, then as engine warms it runs fine... because it needs less fuel - lean mix.

It seems odd that when you OPEN the butterfly, which allows more air, it fires up normally?

Everything I read points to the lean fuel at startup... which could be a whole slew of things causing restricted fuel flow.

Ethanol "poisoning" is a huge culprit which can really mess with a fuel system. If our rig sits for over a week, I add the blue marine grade STA-BIL which is supposed to counteract ethanol. Has worked for several years in my lawn equipment AND the motorhome.

The Gritz Carlton · 03-31-2022, 05:38 PM

Maybe a dumb question but have you changed the air filter lately? Is there an inline or carburetor mounted fuel filter on the gas generators? Do they have automatic chokes that can stick like the old engines we grew up with? Any vacuum lines on these that could crimp?

Sappy2 · 03-31-2022, 05:55 PM

The plugs are fouled and your choke break may be bad. Replace the plugs and watch as the engine starts. It should start and the choke break should auto open the choke plate a small amount. If not, find out why since a bad choke break will foul plugs. But backfire is a lean condition, or poor combustion. Also make sure that the carb is tight to manifold. Good luck.

**16ACE27** · 03-31-2022, 07:42 PM

Quote:

Originally Posted by Chateau_Nomad

After reading (and learning

) I 100% agree with Ace.

Lean mixture at start, then as engine warms it runs fine... because it needs less fuel - lean mix.

It seems odd that when you OPEN the butterfly, which allows more air, it fires up normally?

Everything I read points to the lean fuel at startup... which could be a whole slew of things causing restricted fuel flow.

Ethanol "poisoning" is a huge culprit which can really mess with a fuel system. If our rig sits for over a week, I add the blue marine grade STA-BIL which is supposed to counteract ethanol. Has worked for several years in my lawn equipment AND the motorhome.

Generators, when starting - due to the speed governor, will always have the throttle wide open. But the choke is trying to richen the mixture through the idle circuit by cutting off air which basically applies a vacuum though the idle circuit.

But if the idle circuit is clogged, there is no fuel enrichment. Opening the choke allows full air flow and fuel entrainment through the run circuit which allows the generator to start.

Chateau_Nomad · 04-01-2022, 12:43 AM

Quote:

Originally Posted by 16ACE27

Generators, when starting - due to the speed governor, will always have the throttle wide open. But the choke is trying to richen the mixture through the idle circuit by cutting off air which basically applies a vacuum though the idle circuit.

But if the idle circuit is clogged, there is no fuel enrichment. Opening the choke allows full air flow and fuel entrainment through the run circuit which allows the generator to start.

Excellent info! Thanks. I should start a "knowledge base notebook" to keep in the MH and add these tasty tidbits for future reference!

IrnBear · 04-01-2022, 11:23 AM

Thank you all for the response back.

I like the SeaFoam idea, I actually have done the same on an older mh I had.
Might actually put a three way in the gasline so I can do this more often. Our Geni doesn't site long, but I guess a month or two actually is too long.

Air filter can't get any newer then it is
Spark plugs are brand new, and gap'ed to Onan Spec.
Fuel Filter is new.

I'll try the SeaFoam before I go the pull the Carb route.

Thank you all again,
IrnBear

Galobo · 04-06-2022, 08:09 PM

It could just need fresh fuel. If only backfires once and then starts and runs great I would put sea foam in the fuel and go with it.

IrnBear · 06-09-2022, 11:27 AM

So I have a feeling I will need to do this again
So I cut the fuel line and added a 3 way with shutoffs.
I turned off the line to the MH tank
I ran a line to a gas can of fuel and seafoam mix
I ran the Gen a few hours at a time, let it sit, ran again.
Seemed to fix the problem
Gen starts much better now, still has a small backfire through carb from time to time
I will run my Mix through a few more times over the next few months

We run our Gen often. Boondock a lot working checkpoints for off-road racing

Might buy a carb just to have in the cabinet, for that day the Gen just won't start.

Thanks again for the responses.
IrnBear

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