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Old 06-29-2022, 01:50 PM   #1
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Join Date: Sep 2015
Posts: 70
Onan 5500 Gas black smoke on start

Recently my Onan generator (500 hours) has started to run very rough and blow black smoke for about the first minute after startup. After that it seems fine.

What I've done:
-Put Lucas gas treatment in RV gas tank and run gen for several hours, rested until cool, then run several hour again. It seems to have helped a little.
- Checked oil level. It's good and was changed 100 hours ago.
- Checked air filter. It's clean and was changed 100 hours ago.
- Checked butterfly valve behind air filter. It feels slightly stiff and if I push it open or closed it stays pretty much where I leave it. (This was done after the engine had warmed up and was running smoothly. I haven't checked this with the engine cold.)

What's my next step? Or does this sound normal and I'm just creating problems for myself where there aren't any!
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Old 06-29-2022, 02:53 PM   #2
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Join Date: Jul 2015
Location: Watertown NY USA
Posts: 5,162
Quote:
Originally Posted by MilesAndSmiles View Post
Recently my Onan generator (500 hours) has started to run very rough and blow black smoke for about the first minute after startup. After that it seems fine.

What I've done:
-Put Lucas gas treatment in RV gas tank and run gen for several hours, rested until cool, then run several hour again. It seems to have helped a little.
- Checked oil level. It's good and was changed 100 hours ago.
- Checked air filter. It's clean and was changed 100 hours ago.
- Checked butterfly valve behind air filter. It feels slightly stiff and if I push it open or closed it stays pretty much where I leave it. (This was done after the engine had warmed up and was running smoothly. I haven't checked this with the engine cold.)

What's my next step? Or does this sound normal and I'm just creating problems for myself where there aren't any!
This is not normal. If it was it would have been doing this since day 1. You have a vacuum diaphram with a link which connects to the rod the choke plate is fastened to. When the engine starts with the choke fully closed vacuum builds up and the diaphram retracts, pulls on the link and opens the choke plate to let more fresh air into the carburetor. If the choke plate doesn't open up right away after starting the engine you will have a rich mixture which will cause black smoke until the thermal spring in the choke housing gets heated up electrically and forces the choke plate to open.
You should be able to find a short rubber hose which connects between the choke pull off and the carb body near the intake manifold. Be sure this hose hasn't blown off possibly due to a backfire in the engine at sometime or worse yet blown off entirely. If the hose is there and connected look for a split in the hose which would leak vacuum and make the pull off inoperable. The diaphram in the pull off housing could have a hole in it which would also make the pull off inoperable. You can test for this by using a longer length of vacuum line, connecting one end of the line to the pull off fitting and sucking on the other hose end. You should see the link move. Hold your tongue on the end of the hose to block it off. If the choke plate doesn't move back to it's resting place both the hose and the diaphram are ok.
There is also an altitude adjustment on the side of the carb. A small black knob with some numbers representing altitude in feet. If you are above 1000' be sure the knob hasn't vibrated back to 0 or sea level. If you are above 1000' adjust the knob for the approximate altitude you are trying to run the generator.
I had a problem with my carb about 5 years ago and found a small split in the vacuum hose. I replaced the hose and it was fine until last year when the generator acted exactly as you described. I did everything I could possibly think of to correct this and I finally did. I ordered a replacement carb from Amazon, installed it and the generator runs better than it ever did before replacing the carb. This was a china made carburetor with no numbers or writing on the box which I still have with the original carb it it and I'll be darned if I can find the receipt where I bought it. I may have had my son or someone else order it for me with their PRIME account.
The whole carburetor cost me just over $40.
Why did I save the old carb? I don't know but I have been known to never throw anything away.
All this information is either going to help you fix your problem or it won't.
Good luck.
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2002 Fleetwood Storm 30H on Workhorse P32 chassis 8.1 gas.
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