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Engine Running Problem


Pendragon's Avatar
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02-03-03, 01:18 PM   #1  
engine lope/surging at idle

Hi all, got a mower with a 21 hp B&S twin.
Out of the last 3 times of used it, the 1st and 3rd time, the engine doesn't want to idle right.

It will idle, then drop, like it's trying to die, catch itself and speed back up, drop like it's trying to die, catch and speed back up.. etc.

Any idea what would cause this? The mower is only a year old. I haven't checked the plugs yet (I was planning on changing them when I changed the oil before mowing season really got going).

 
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02-03-03, 01:24 PM   #2  
Joe_F
Sounds like there is trash in the carburetor. Did you leave gas season to season? If so---trash in the carburetor.

A cleanout should restore it to good running order.

 
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02-03-03, 02:55 PM   #3  
There isn't really a season we don't need a mower here..

Actually, I ran it dry and let it sit empty for a while. When I went to refill it, it still had some in the bottom of the tank. I think now that may have been water. I refilled the tank and that's first time it did it (and it was bad, doing it even at speed, especially on bumps and direction changes). I thought hey, water in the carbs, it'll eventually burn it out. I try to keep it covered, but the wind keeps blowing my cover off (even weighted with bricks).

Next time I used it (~2 weeks) it was fine. Then I used it again today, and it was back to the bad idle.

I haven't looked, but do the carbs have drains that I can just unscrew and see if it's water, or do I have to completely remove the carbs and take the bowls off?

 
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02-03-03, 07:27 PM   #4  
Hello Pendragon!

Those engines are prone to that problem. It is in fact the carburetor. There is a drain plug on the side of the carb...a 5/8" wrench will remove it. I have found that this usually doesn't do it though. You will probably have to remove the top of the carb and clean out the bowl. Also, use an allen wrench to remove the jet in the bottom. You have to insert the allen wrench through the drain plug hole to get that jet out (look in the hole and you'll see it).

Let us know how it goes!


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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02-03-03, 10:08 PM   #5  
Just wondering though, how would there be 'trash' in the carbs?
The gas comes from a filtered pump, to a filtered can and runs through an inline filter on the mower itself. Surely, anything big enough to affect them would have been caught by one of the three filters it's passed through before hand?

 
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02-04-03, 07:23 PM   #6  
Moisture

Condensation builds up in your fuel tank and carb from nothing more than changes in temperature. Moisture in the air condensates inside the tank and carb when the temp falls, just as it builds up on a cold glass. This moisture collects oxidation from metal components. Since water is heavier than gas, it sits at the very bottom of the carb...exactly where the main jet is. And so, just a few drops of water can eventually keep your engine from running.

Another thing could be gas varnish deposits. When gas is left in a container...especially a small area like a carb bowl, it breaks down. It leaves a gummy varnish that clogs up passages in the carb. Usually this happens over a period of months.

I'm sure that a thorough carb cleaning will solve your problem.

BTW: if the mower has the red circular fuel filter...it really only filters out large pieces of debris. It is just a screen.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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02-04-03, 08:35 PM   #7  
mikejmerritt
Shot in the Dark......

Roll your low speed idle screw out until the idle levels out. Sounds like post break in syndrome...LOL...Really, check the things in the above posts but try that low speed idle screw. It's the little used screw on the side of the carb. Try it...I bet it works....Mike

 
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02-11-03, 08:46 PM   #8  
Just an update.. finally got to it today.

I took out the drain bolt (it's a 3/4" bolt) and let the fuel drain out.
I didn't see anything in the gas, but the end of the bolt (inside the carb) was rusted, so it definately had water in it at some point.

I took off the old fuel filter (1 yr old) and replaced with a new clear one (so I can see it's condition!).

Suprisingly started right up, considering there was no fuel in the bowl or lines. The filter instantly got trash in it. What looks like small pieces of paint or plastic, probably from the black plastic gas tank. At least I know the filter is doing it's job.

Anyway, it ran fine, this time. Only a few more uses will tell.

 
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03-04-03, 02:36 PM   #9  
Well, it's doing it again.

This time I took off the 3/4" bolt on the side and what looked like drops of rusty sludge came out. I even blew out the fuel line between the filter and carb, took off the plate on the side of the carb and blew it, took out the jet behind the 3/4" bolt, it was clean.

Still does it.
Am I going to have to completely disassemble the thing to get it to work right?


<img src="http://members.cox.net/pen411/carb.jpg">

 
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03-04-03, 05:12 PM   #10  
I would suggest taking the top off of the carb and cleaning out the entire thing, also drain and clean the fuel tank, and let it dry, flush the fuel lines, replace filter, and you should be rid of this problem. Once this stuff gets in the system, it's hard to get out without doing this. Check your fuel can that you use to fill the mower. Make sure it closes fully and has no water in the bottom. Make sure your mower fuel cap is not letting water in also.

If this water accumulation is due to condensation in the mower's tank, then keeping the fuel tank completely full when not in use will minimize the chances of it happening again.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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03-05-03, 07:12 PM   #11  
MrWrench
Pendragon, Mostlikely the black flakes or "trash' you saw in your fuel filter were actually pieces of fuel line. The gas today will do weird things to fuel line. We have this problem all the time up here in the Northeast. Also, do you have an old metal gas can?

 
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03-24-03, 01:08 PM   #12  
Just wanted to give everyone an update...

Started doing it again, after draining the bowl several times without fixing it, I finally said F it and pulled off the top of the carb.

The bowl was about half full of orange water, but here's the real problem, and I wish I had taken a picture.

The "bowl" is a U shaped deal, just before the top of each leg of the U, is a rise. The drain bolt is just PAST one of these, between the rise and end of the U. When you pull the bolt, you only get the 1/2" x 1/2" or so space created there drained, everything else stays put!

Everytime I pulled the bolt, I was only draining that little space. Once the bowl filled again and got sloshed around, more water would mix in and get over that rise, so that when I did drain it, I'd get a little bit of water every time.

Long story short, at least on this carb, only way to truely drain it is to take off the top and blow it out with compressed air, the drain bolt is useless.

 
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03-24-03, 05:18 PM   #13  
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Pendragon,

Make sure you park that mower with a full tank of fuel to help avoid the condensation what I always told my customers when I was in the biz, never a steel storage can and don't keep gas for more than 30 days, after that, pour it in your car. Not wasting it and will mix with the gas in the tank and burn fine.

Also, Briggs color codes their filters. Different colors, different micron ratings for different fuel delivery systems. I still use to install a bronze stone filter (like the old GM cars) in-line where possible. They are clear to see contamination and even catch cat hair which will go through most lawn mower filters and I have seen probs due to cat hair.

 
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03-25-03, 10:50 PM   #14  
The plug (which I call a drain plug) is more like an access plug for the main jet. It does drain some fuel, but not the lower portion of it. That is why I always reccomend pulling the top off to clean these carbs. There is really no other good way. Glad you got it straight.

I agree about keeping the tank full. You could also buy a screw-vent gaqs cap for the mower, and when you store the mower, tighten the screw vent. This keeps air from entering and escaping with changes in temp. The moisture that causes condensation comes in with that air. You have to loosen the vent when you get ready to use it again (just a small thumbscrew). Also, try to avoid pouring the last tiny bit of gas out of a can into the mower. This is where the water and trash will be.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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