Yardman walk-behind - runs rough, low power!

[Mr. Howard]

Hi! I have a Yardman walk behind power mower, model # 11AA18M055 with a Briggs and Stratton engine. Bought it on Craigs List for 50 dollars last week. When I first got it home, it ran very rough and had no power at all - just stalled out whenever I hit grass. I don't know much about small engines, but I started taking things apart and quickly found that the air filter was missing and the inside of the carburetor was badly gunked up. I poured about a half a can of Seafoam through the carburetor as it ran, then made a temporary air filter out of a paper towel and poured the rest of the Seafoam into the gas tank and finished the lawn. It definitely ran better, but was still badly underpowered.

When I was finished, I replaced the spark plug and bought a new blade. The old one was sharper on the back side than it was on the front, so I figured that was probably part of the power problem.

Trouble is, the mower still doesn't really run all that great. It still sputters a bit, and tries to stall as soon as I hit moderately heavy grass. My next step was going to be to replace the fuel filter and adjust the carburetor, but I can't find either the filter or the adjustment screws on this mower, and the owner's manual that I downloaded doesn't show where the fuel filter is located. Nor does it offer any information on how to adjust the mixture - it just says that the carburetor was preset at the factory and should never need adjustment (yeah, right, thanks a lot), and that if it ever does need tweaking, take it to a repair shop.

Can anyone give me any ideas on what I ought to try next here? If I could just find a schematic showing where the adjustment screws are on the carb, that might help, but they're not very easy to get at on this engine and I can't tell by looking at it.

Thank you!

 

[natenkiki2004]

If it was ran before without an air filter there might be engine damage from dust particles. Being such a new mower, it would have taken a LOT of abuse to get to that point. What kind of Briggs engine is on it? There should be numbers by the exhaust on a plate or somewhere around there. It might have an adjustable carb and if that's the case, you'll have to adjust it. Another issue that is likely, it's probably running bad due to no air filter and no resistance to sucking air in so it's probably running lean. Engines are designed to breathe in such a way and the carb is built with the resistance to flow in mind. Grab an air filter off eBay or something, you should find one pretty cheap. Also, change the oil ASAP and fill it up, run 10 minutes (once you get an air filter) and drain it again to flush out any crud.

 

[Mr. Howard]

Oh, I never considered that the oil may be contributing to the performance issues. You're right, it's filthy, and was definitely on my list of things to do. Now I'll do it first.

And I did replace the air filter.... sorry, forgot to mention that. Did it when I replaced the spark plug. I'll check the engine for an identifying number....

Thanks for your help so far!

 

[natenkiki2004]

The oil (unless it's gear oil or something waaayyy different than what's specified) won't really affect performance as long as it offers some lubrication and cooling. It is probably heavily contaminated and will contribute to premature wear and that's why it needs to be changed. Maybe put some seafoam in the oil when you run it for 10 minutes on the flush. Whenever I get a used mower, I flush at least once. If it's one I'm keeping and not selling, I'll likely flush twice. I try to be green in life but oil is cheap and when you recycle it, it has another purpose. It's all good when it means your engine will live longer.

Other things I could think of might be valve adjustment, dirty carb (cleaning and adjusting is always good to do), flywheel key partial shear (probably not but it's worth mentioning), and spark plug gap.

Once you get the Briggs numbers, we can get manuals and instructions on further repairs. Very easy and, in my opinion, fun to work on Briggs engines. They always document everything and parts are cheap and plentiful on eBay.

 

[Mr. Howard]

Thanks, I really appreciate all of this. I always feel funny popping into forums like this and picking peoples' brains when I don't have anything to offer myself, but I really appreciate that there are people willing to take the time and make the effort to help out.

I'm not sure what numbers I'm looking for; there was no plate near the exhaust, but there were some characters stamped into a housing above the spark plug. It says, "Model," and below that "10T802." Next to that, it says "Type," and below that "778 B1." And next to that it says, "Code," and below that it says "09113034." I don't know which of those (if any) are relevant. Near the carb, there's a plate with emissions information, and among other things it says "Engine family - 9BSXS.158VG," and also says that it's a 158cc engine. While looking at that sticker, I also found what may possibly be an adjustment screw at the base of the air intake on the carb, a couple of inches below where the fuel line enters the carb, but I'm not certain.

Is any of this helpful? This is going to be fun. I know next to nothing about small engines - and not much about engines at all - but I love fixing stuff. I'm hoping to learn a lot of basic stuff about small engine repair with this mower. This should be a lot more fun than just buying a new one.

 

[natenkiki2004]

Thanks, I really appreciate all of this. I always feel funny popping into forums like this and picking peoples' brains when I don't have anything to offer myself, but I really appreciate that there are people willing to take the time and make the effort to help out.

Not long ago, I was the exact same way. There will be a day when you can help someone and it makes asking questions all worthwhile


Go here:
Engine
Put in the info you just listed and it will give you an illustrated parts diagram as well as an engine manual. One issue though, look again at the "Type", it should be 4 digits.

With the manual and diagram, it will tell you how to adjust the carb and you can see how the carb comes apart. From the info you provided, it looks to be a flathead/L-head engine so adjusting valves isn't as easy/necessary. Chances are, they're still fine and within spec.

 

[Mr. Howard]

Thanks, that parts diagram may help a lot. It can't hurt to pull the carb and give it a good cleaning. Pouring Seafoam through it probably got a lot of crap out of there, but that only goes so far.

As far as the manual is concerned, though, it looks as though the only one available is another version of the one I already found - the one that says not to bother adjusting the carburetor, because it's preset and never needs adjusting. I'm digging around right now for an actual repair manual, and we'll see if that gives me a few more options.

 

[natenkiki2004]

Thanks, that parts diagram may help a lot. It can't hurt to pull the carb and give it a good cleaning. Pouring Seafoam through it probably got a lot of crap out of there, but that only goes so far.

As far as the manual is concerned, though, it looks as though the only one available is another version of the one I already found - the one that says not to bother adjusting the carburetor, because it's preset and never needs adjusting. I'm digging around right now for an actual repair manual, and we'll see if that gives me a few more options.

Personally, I like those non-adjustable carbs. No needles to fuss with so once you clean it, you know it's 100% ready to go. Being a newer mower, you might be lucky and not have to buy any carb gaskets if you tear it down. Maybe e-mail Briggs for a repair manual, if they have one.

If all else fails, you might just pull the head off and see about carbon build-up, make sure the valves are opening and closing fully and that the cylinder isn't scored.

Any chance you could get a video of your mower's behavior?