Yet another Onan running like **** thread. HELP!

[funk-o-meter]

Hi all!

Short version first:

1999 Onan MicroLite 4k
Been sitting for years
new fuel pump
new fuel filter
new air filter
oil change
pulled and cleaned carb twice
two bottles of Sea Foam

Runs like **** with no load.
Runs slightly crappy with a load. Engine "Hunts."


Long version:

Got an old 1999 Four Winds class C out of Mom's yard and have been working on it. Lots of work. Lots and lots of work. It's been sitting for at least 5 years seeing little to no action. All the fuel was varnish. Plumbing cracked. Brakes sticking, coils where shot. Same old RV Blues.

My current issue, however, is with the genset. Onan Microlite 4k. I replaced the fuel pump, fuel filter, air filter, and cleaned the **** out of the carb a couple of times and got it back up and running but it runs like ****. It "hunts" without a good load on it but runs reasonably well with the air conditioning on with only a few hiccups every now and then. Course when the AC kicks on it dips down momentarily to like 80V which also sucks.

From what I've read this points to the carb being dirty, or possibly the carb needing adjusting. So I worked on the carb... a lot. I exercised the generator several times for an hour each time. Ran like ****. So I pulled the carb off completely, sprayed and wiped the bejesus out of it. I unstuck the fuel shut off solenoid and tested it with a 12v source. Cleaned the float and needle and made sure they move well. Linkage and butterfly valves all move easily. Did all this and ran a can of Sea Foam through it in a 50/50 mix of it and high octane gas. It still ran like ****. So I pulled just the carb bulb, float and needle and cleaned everything again, did another round of sea foam and gas. Same deal. Visual and from what I can test the carb should be good.

I can't really do a carb adjustment because it's only got a throttle stop screw and a main/altitude adjustment screw on this carb. No idle adjustment screw. Also, I don't have a meter that reads frequency. I have no idea the history on this genset or if it's ever been adjusted by anyone. I do know the main/altitude adjust screw was very loose and only a few turns from popping out when I had the carb removed. And the main adjust screw "limiter cap" is missing. I followed the service manual and set the main adjust by turning it until I felt resistance, then backing it out two full turns.

Is there anything else I can do before I just take it to a Cummins repair shop? I'm about at my end with this thing. Got them Old Onan Blues. Lawd lawd....

Any advice is greatly appreciated.

[maillemaker]

Here is a great forum with detailed repair manuals for rebuilding carbs and more:

https://www.smokstak.com/forum/forumdisplay.php?f=1

Mechanic in a bottle solutions seldom work with gummed up carbs. As others noted there are tiny passages in the carb that must be manually cleaned with tiny brass wires or other means (being careful no to damage the orifices!).

Also I recommend you clean the slip rings if the genny has sat a long time.

This is why it is critical to run these gensets every month, and let them run for a half an hour under load so that the heat and vibration can help agitate the fuel inside the carb and help keep it clean inside.

These things do not tolerate sitting well at all.

[dougrainer]

Chris Bryant wrote:
The only really successful cleaning I have heard of used an ultrasonic cleaner.

I have done about 10 over the years using my method. Doug

[Chris_Bryant]

The only really successful cleaning I have heard of used an ultrasonic cleaner.

[dougrainer]

You can pull the carb and CLEAN it. Not rebuild. Remove the fuel bowl and unscrew the jet assbly. You have 16 micro holes in it and cleaners will not remove the debris. Just use carb cleaner and compressed air. You have nothing to lose. If that does not work then replace the carb. Doug

[Chris_Bryant]

funk-o-meter wrote:

Chris Bryant wrote:
Those carbs are near impossible to clean- just replace it.

Are any of the aftermarket carbs worth buying or do I have to spend way too much on the Onan replacement part?

Wow, I just priced those- exactly double what I paid a few years ago. I’d sure be tempted to try aftermarket, but I have no experience with them- just put a $20 carb on my old riding mower rather than the Briggs oem one, at a hundred dollars more. Haven’t fired it up yet, though.

[funk-o-meter]

Chris Bryant wrote:
Those carbs are near impossible to clean- just replace it.

Are any of the aftermarket carbs worth buying or do I have to spend way too much on the Onan replacement part?

[Sam_Spade]

Chris Bryant wrote:
Those carbs are near impossible to clean- just replace it.

Yes BUT.........

SeaFoam is really NOT a good cleaner.

And if you put those two cans into a gas supply that runs ONLY the generator, then that is WAY too much and might actually be causing the problem.

If you can get a fresh supply of gas to the gen, I suggest adding two ounces per gallon of Berryman's B12 Chemtool to the gas and then run it a few times.

Even that didn't fix mine and the Cummins/Onan dealer recommended the above.....and it wasn't as expensive as I had guessed.

[Chris_Bryant]

Those carbs are near impossible to clean- just replace it.

[Scottiemom]

Any of our Onans we have trouble with go to the closest Onan Coach Care. May cost a little money, but never been disappointed and they always get fixed up first time. When we bought our Alaska rig, it had sat for several years without being used. The gas had not been treated, and we had trouble with the generator. Onan fixed it up. Money well spent because it works every time.

Dale

[DFord]

You can try searching for a carburetor repair shop but I think I'm dating myself and they're probably all out of business these days. The varnish from that old gas has plugged the jet passageways (evidently what was known as the low speed jets - on a generator like the Onan, they would be light load jets as the generator must run at a constant speed to provide 60Hz power). Buy some carburetor cleaner, disassemble the carburetor and let is soak for several hours in the solution. Lacquer thinner may be able to soften those deposits but the carburetor is formulated to do the job. After removing and rinsing the parts, blow out every passageway with as high a pressure as possible before reassembling it. Then give it another try. Make sure the hose between the fuel pump and the gas tank isn't cracked from old age and allowing air to be pumped into the carburetor instead of gas from the tank - this is a common problem on RVs over 10 years old.

Good luck with your efforts.