18hp Onan B43g backfiring

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  #1  
Old 03-05-10, 08:21 PM
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18hp Onan B43g backfiring

1983 318 B43G. Was removing snow and the tractor was running fine. Started backfiring a little ( not loud but pop,pop,pop,pop softly) and not running smooth. It only happens after it warms up. When you first start it, it runs smooth.

New parts in the last year. Carb, intake, plugs, plug wires, points.

Just replaced the plugs, condenser and coil. Checked points gap. Still have the same problem. What do you think?

Here is a link of how it is running.
YouTube - 318 running rough
 
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  #2  
Old 03-05-10, 08:58 PM
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Sounds like the main jet in the carb has a small clog in it. Try running some SeaFoam thru it.
 
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Old 03-05-10, 09:08 PM
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My first guess would be "lean mixture" This can be determined, to some degree, by closing the choke while the engine is missing.
You might also try spraying a small amount of WD40 or just plain water on the entire intake system. If a difference is noticed, there is an intake leak.
The first thing I noticed in your video was the the center porcelon of the plug is extremely white. If this plug has been run in the engine, it is apparently too lean. After about 30 minutes of running, the plug should be a light tan or grey.
Is the machine down on power as well?
 
  #4  
Old 03-06-10, 12:26 AM
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It is down on power once it starts running rough and the more load you put on it the rougher it runs. Pulling the choke out makes no difference.
 
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Old 03-06-10, 12:47 AM
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Pulling the choke out fully doesn't change anything at all? If not, I'd say try loosening the gas cap...it may have a clogged vent and be developing a vacuum, starving the engine of fuel once it has run a while. If not, check fuel flow into the carb bowl. If fuel is flowing slowly, it would run fine for the first little while but as the engine uses fuel faster than it flows, it would eventually begin to run lean.
 
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Old 03-06-10, 01:13 PM
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If you pull the choke out all the way it will die. Pulling it out partially does not make it run better. I put comprised air through the gas line and you could hear the gas bobble and the cap whistling, so I think the vent in the cap is working.
 
  #7  
Old 03-06-10, 01:29 PM
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Your spark plugs are white. They should be tan. White means it is running lean (not getting enough gas). Internal parts of the carb are varnished up. Carb needs to come off, taken a part and soaked in a carb cleaning solution, rinsed with water, then blown out with compressed air.
 
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Old 03-07-10, 11:59 AM
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Indy, I am trying to learn as I do this. I have a question. If it is the carb why does it wait until it is warmed up to do this. Looks like if a passage is gummed up it would run bad all the time?
 
  #9  
Old 03-08-10, 09:36 PM
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Plugs are ngk v-power tr5 2238. If I pull the plug wire from the left cylinder the right runs rough and backfiring, if I pull the right the left runs perfect nice and smooth. Could it still be the carb or are we taking ignition?
 
  #10  
Old 03-08-10, 11:09 PM
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If it has NGK plugs, I'd change them just because. I do not like them and have bad luck with them. Put in some autolites or champions (regular cheap ones, no fancy gimmick stuff) and them you will either solve the problem or eliminate plugs as a potential cause.
 
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Old 03-09-10, 06:01 AM
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I agree with Cheese. I only use NGK on Hondas engines. I use Champion on American made engines. I find the NGK plugs don't seem to run very well on American made engines.
 
  #12  
Old 03-09-10, 11:15 AM
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Hmmm...NGK plugs are highly recommended by a lot Onan owners.
 
  #13  
Old 03-17-10, 05:38 PM
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Changed plugs no difference. Since one cylinder runs good when you pull a plug wire does that rule out a fuel problem?
 
  #14  
Old 03-19-10, 04:22 PM
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I couldn't watch the video before, but the link is working for me now. The very first pics of the plugs say that it is running too lean. Then, the video of the engine running sounds like classic lean condition due to carb problems or insufficient fuel delivery to the carb. Often when the inner lining of a fuel line collapses, the engine will crank up and run fine until the engine uses what fuel is in the bowl, then as the engine uses fuel slightly faster than the fuel line will allow, it begins to run lean. When you shut it off, enough fuel drains to the carb to fill it well enough to run well again for a while. This may or may not be what is happening here, but it is very likely. If this is a pump supplied carb, the pump could be weak or the pump could be sucking in some air through a crack at a fuel line, drastically reducing the volume of fuel it can pump. I suspect since it runs well at first the carb is ok and the fuel delivery to it is suspect.
 
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Old 03-21-10, 06:36 PM
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I was thinking it might be the fuel lines to. They have never been changed and I was planning on doing that. I have also read that ethanol in gas can make old fuel lines collapse. Can this engine be gravity fed? I would like to try and run it with a can just hooked up to the carb to see if that is the problem. You have to practically take of the entire body of this tractor to get to all of the fuel lines.
 
  #16  
Old 03-21-10, 07:20 PM
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Yup, on an engine with the carb and pump separate you can do what you're talking about. Most engines that are pump fed have to do with the location of the tank relative to the carburetor. With the pump you have a consistent delivery of fuel to the carburetor while the mower moves through different positions.

Just take a few precautions as to spilling fuel and make sure the discharge from the pump is dealt with to avoid spraying fuel.
 
  #17  
Old 03-23-10, 04:29 AM
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of all the posts I've read, it doesn't suggest a compression test. I've seen on these older onans, and oil delivery problem do to low oil pressure, I would do a compression check, possible exhaust valve sticking slightly open when warmed up.
Great engine though,
I love mine!
 
  #18  
Old 10-07-10, 06:22 AM
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I wish people would finish the post with what was actually wrong so we don't have to start another post
 
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