Generator Blowing Black Smoke

Here's another issue I'm having besides my furnace... :)

My generator... an Onan Gold Marquis 5500, is blowing black smoke intermittently. It didn't run for the better part of six months because I'd fire it up, it'd run until it was time to produce a load, and then it'd die. I had a carb guy come out and look at it. He cleaned the carburetor and then decided I had a problem with the fuel line at the gas tank because it wasn't getting fuel. I paid him his $50 and he went his way.

I decided to test that theory by buying a gallon of gas and running my own line. So I pulled the line off the back of the Onan fuel pump and found it POURING gas out of it. Interesting, it shouldnt be doing that. So I reconnected it and fired up the generator and it ran fine. Turns out the gas cap was off.. I had removed it just recently to put some stabilizer in the tank (though I never did because I didn't find the bottle of it), but never screwed the cap all the way back on. So I'm assuming my gas camp has a bad vent and dumbass didn't figure that out.

So anyway... it ran, and ran fine for maybe a 90 sec to two minutes, then would blow black smoke for thirty seconds, then go back to normal again. This would happen non-stop. Figuring the gas was probably old, I pumped it all out (12 gallons worth) and then dumped in five gallons of fresh plus some water remover/stabilizer/etc stuff.

Fired it back up... runs exactly as before. 90 seconds of just fine followed by 30 seconds of lower RPM and black smoke. Then back to normal.

I'm guessing at this point maybe it's still just getting what's left of the bad gas in the bottom of the tank and the fuel line? It's had a total of maybe two hours of run time since the gas change.

BTW, when the generator is running, I put on the A/C and a space heater to provide a decent electrical load. It continues to produce power during the slow-down, but every now and then my UPS will bleep, indicating it's not happy with the power quality.

Thoughts?

Thanks!

RobG
Explorer
Feb 29, 2016
It's a 5th wheel, not a motorhome... so yeah, not an option. :)

I'll dump a couple cans of sea foam in it and ride it out.
Sam_Spade
Explorer
Feb 29, 2016
RobG wrote:

Now with it running off of the regular gas tank, it's doing the load-down thing every 90 seconds, but instead of black smoke, it's white like before. WTF?

If this means that you are pretty much right back where you started, then it's pretty obvious that you have some water or other contamination in your main tank.

SeaFoam will actually help get rid of that but if you can't drain the main tank, you should burn the treated fuel off with the main engine and NOT the generator. If you have fuel filters, NOW would be the time to change them.....AFTER you get rid of the old fuel.
Sam_Spade
Explorer
Feb 29, 2016
RobG wrote:
I'm doing the Sea Foam treatment today.

I'll mix the rest of this can of sea foam in with the remaining half(ish) gallon of gas and run it til it's out.

Despite the "does everything, snake oil" marketing claims, SeaFoam is NOT a good cleaner. It's original purpose was a marine fuel stabilizer.

Before you go tearing anything apart, get yourself a can of Berrymans B12 Chemtool. It is designed and formulated to BE a fuel system cleaner and it does an excellent job.

Regardless of the additive used, you should not mix a concentration higher than 2X the standard recommendation (that would be 2 OZ per gallon for most products) because the excess additive can cause it's own problems with not running good.
Your intent to "mix the rest of the can" of SeaFoam with a gallon of gas and then use the mixture is NOT a good idea.
rcmiller32
Explorer
Feb 28, 2016
How old is the gas in your main gas tank? If it is over 30-90 days old it maybe stale or contaminated. You might also be picking up dirt and water from the bottom of the tank. Could also have a fuel pump that is on the way out, or a fuel filter that is partially plugged up.
RobG
Explorer
Feb 28, 2016
Another Update...

Almost finished off that gallon, so shut it off, dumped the rest of it into the main gas tank and reconnected everything. The generator hadn't loaded down at all in the last 10-15 minutes so I figured we were good to go.

Now with it running off of the regular gas tank, it's doing the load-down thing every 90 seconds, but instead of black smoke, it's white like before. WTF?
RobG
Explorer
Feb 28, 2016
Update...

This Sea Foam stuff seems to be pretty good. Once I got it started again, it blew a ton of white smoke, but then settled into a good RPM. It stayed there for close to ten minutes before its first load-down, in which it blew more white smoke. Now the load-downs are only happening about every 5-6 minutes instead of every 90 seconds, and there's no black smoke... instead it's white, but not as bad. And the load-down isn't as severe either.

The gallon of gas is almost done. I think when it is, I will reconnect the regular fuel hose and plan on dumping at least one more full can of Sea Foam into the gas tank (which has 10 gal of fresh gas in it). This may just keep me from having to pull the carb!

I'll keep everybody posted. Thanks for the tips!
TEXAS
Explorer
Feb 28, 2016
I agree Sam. But after 55 years of working things start to run together.You tend to forget things. Maybe some day I will win the lottery and I can quit working. Thanks for catching my mistake.
RobG
Explorer
Feb 28, 2016
I'm doing the Sea Foam treatment today. I let it run for a 15-20 min with a mixture of one gal gas/4 oz Sea Foam. No change in performance, fwiw. Now will let it sit for an hour, and unless somebody chimes in differently, I'll mix the rest of this can of sea foam in with the remaining half(ish) gallon of gas and run it til it's out.

That failing, I will pull the carb, disassemble, and soak it in Pine Sol overnight.

BTW... I felt and smelt the exhaust when it was blowing black smoke... it was very dry... no gas, no oil that I could smell.

Also, I removed the air filter while it was running... RPMs dropped a lot and it was struggling. I replaced the filter and it returned to normal.

This carb doesn't have a choke that I'm aware of (unless it's electronic). It does, however, have a dial-a-jet.. it's currently set to sea level, as my elevation is 200' above.
Sam_Spade
Explorer
Feb 28, 2016
TEXAS wrote:
After 55 years of working on engines. I have found.
Black smoke is caused by clogged air filter or to much fuel.
Blue smoke is from burning oil.
White smoke is from water vapor or a diesel starting in cold temps.

While this is true............
A tiny bit of oil burning can produce smoke that looks white.
A LOT of oil burning can produce black oily smoke for a short time until the plugs foul.

When it is producing black smoke, you have to feel and smell the resulting tailpipe residue to distinguish between too much fuel and too much oil.
TEXAS
Explorer
Feb 28, 2016
After 55 years of working on engines. I have found.
Black smoke is caused by clogged air filter or to much fuel.
Blue smoke is from burning oil.
White smoke is from water vapor or a diesel starting in cold temps.

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