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5.5 HP Briggs & Stratton mower turns, starts, won't stay running

This is a discussion on 5.5 HP Briggs & Stratton mower turns, starts, won't stay running within the Small Engines forums, part of the Tech Support Forum category. I'm having some serious issues with a YardMan mower. It runs a model 120K02-0418-E1 B&S engine. It was bought new

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Old 12-16-2009, 11:20 AM   #1
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I'm having some serious issues with a YardMan mower. It runs a model 120K02-0418-E1 B&S engine. It was bought new in 2007; unfortunately it belongs to my sister and she has allowed her 11 year old to use it, and now we have a problem.

The engine, when primed, will turn over. It will run for 2-3 seconds before sputtering back out of life again. If I attempt to restart from there, I get nothing; if I prime it and go again, I get identical results.

I've pulled the tank and lines and made sure they were all clean and that everything flows well. I've cleaned the carb. Replaced the air filter. Hell I even made sure nothing was clogging the muffler. But still I'm not able to figure this one out. I'm unfamiliar with adjusting carbs but upon close inspection I'm not even sure how I could; and this mower does not seem to have an adjustable idle.

If anyone has any insight I would much appreciate it. I want to get this thing off my back porch.
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Old 12-17-2009, 05:27 AM   #2
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I suspect you "still" have a gummed up carb. Just how did you clean it, and with what?

I've had great luck with spray carb cleaner, but remember there are many tiny passages that can get plugged by bad gas. Gas today has ethanol and other additives that do not store well. Using a stabilizer does a good job of extending storage life beyond a month or so. This is very important for seasonal engines that sit for months.

First, try a double dose of Sta-Bil, or Seafoam in fresh gas. Go through your starting procedure several (many?) times and see if things clear up. You can even try keeping it running for a while by continued priming.

My second step would be to pull the idle/speed mixture screw and spray cleaner liberally well into the hole. Be sure to count the turns gently into the seat before removal. Use that count to replace the needle for a good starting place.

Here's the Briggs site Type in your model number for info.

From the site:

Adjusting The Idle Speed And Mixture

On some float-type carburetors, you can adjust the air-fuel mixture and engine speed at idle. Check for an idle speed screw designed to keep the throttle plate from closing completely, and an idle mixture screw that limits the flow of fuel at idle. If your carburetor contains these screws, proceed below.


With the engine off, remove the air filter and air cartridge.

Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns.

If your carburetor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube. Then, turn the screw counterclockwise 1 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburetor adjustments.

Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Turn the screw in the opposite direction until the engine again begins to slow (image A). Finally, turn the screw back to the midpoint.

Using a tachometer to gauge engine speed (image B), set the idle speed screw to bring the engine to 1750 RPM for aluminum-cylinder engine or 1200 RPM for engine with a cast-iron cylinder sleeve.

With the engine running at idle (image D), hold the throttle lever against the idle speed screw to bring the engine speed to "true idle." Then, repeat the idle mixture screw adjustments from Step 4 to fine-tune the mixture (image C).

Adjusting The High Speed Mixture

Some older carburetors contain a high speed mixture screw, near the throttle plate and opposite the idle speed screw. Under load, the high speed circuit increases air flow through the throat. Setting the high speed mixture involves running the engine until it is warm, stopping it to adjust the high speed mixture and then restarting for final adjustments.
The screw may have a plastic cap that needs a proprietary tool to adjust, but you could use a Dremel to cut a slot for a screwdriver.
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Old 12-18-2009, 07:05 PM   #3
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I cleaned it with carb cleaner, very thoroughly; and then blew out the cleaner with compressed air.

I also completely removed and cleaned what is apparently the jet-main and float-bowl, to make sure that that was clean and not sitting in old gas.

I looked at the diagram for my engine model - http://www.briggsandstratton.com/mai...S3398_C_LO.pdf -- and I can't find the idle/speed mixture screw that you're referring too, unless it is part number 134 or 843 on that list; in which case the diagram isn't very clear at all as to where that part/those parts is/are located.
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Old 12-18-2009, 08:02 PM   #4
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And actually, just to be on the safe side, I just pulled off the carb entirely and gave it another once-over with the cleaner and the compressed air. There are no blockages to speak of anyplace, that air flows through everyplace the gas would flow as clean as a whistle; and I still don't see any kind of idle adjustment even when I have the carb in my hand. Something I don't know that I would actually find, because according to the diagram, it's a fixed speed choke. (Part 222D)
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Old 12-18-2009, 08:35 PM   #5
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Yours is non adjustable type, like many are any more. There is one passage way you can't get to without removing the welch plug. The welch plug has to be removed and can not be reused. It is part No 127. I would go a head and order a rebuild kit (no 121) for it.

Part no 134 is needle and seat

This site may be of help:


They show how to rebuild several different carbs.


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Old 01-01-2010, 08:53 PM   #6
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yes i agree its only running on choke prime not been feed from fuel idle port and progression ports spray crc through all carb ports and passages to check if clear could be gummed up - check tank cap vent
sounds like a good carb service overhaul repair kit should do the trick
make sure the inlet valve is closing properly and does not have a tight valve clearance

cheers mstpops
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