How do I troubleshoot my lawn mower?

August 7, 2005 6:09 PM

My walk behind lawnmower stopped cold while I was cutting the grass. It appeared to be out of gas, so I filled it. Now it won't start. How do I troubleshoot the issue?

The lawnmower is a Sears Craftsman and has a Briggs and Stratton 6.75 HP engine. I removed the spark plug and it looked kind of dirty, so I replaced it with a new one. I cleaned the air filter, and while it was off I confirmed that the primer was in fact squirting gas into the the carb. It has sufficient oil in it. I thought maybe I had flooded it so I left it overnight and attempted to start it as normal the following morning with no joy. What should I do next?

posted by tcskeptic to Home & Garden (17 answers total)

What happens when you try to start it?
posted by winston at 6:39 PM on August 7, 2005

If you pull the plug out, and hook it back up to the plug wire; you can crank it over and see if the gap in the plug has sparks dancing across it. Or you can have somebody hold the end of the plug wire and pull start the motor. If they jump and cuss you it is sending spark to the plug.

Having a fire extinguisher close by is a plan I hope you already have in effect.
posted by buzzman at 6:59 PM on August 7, 2005

Have you cleaned underneath, where the blades are? Sometimes things get fastened in there.
posted by alteredcarbon at 7:06 PM on August 7, 2005

starting fluid is your friend.

Take off the air filter and spray some in the ol carb there. then pull the cord and go (it may require a few tries, so don't put the airfilter back on til your'e under way)
posted by freq at 7:10 PM on August 7, 2005

I initially had the starting fluid listed, but I did not want to be responsible for blowing out a piston or causing valves to drop. The 6.75 Briggs has overhead valves and a greater compression than a typical flathead lawnmower engine. Use very sparingly if you do use any, perhaps a 1-2 second burst.

If you chose to clean underneath, and the mower on/off switch is on... please remember airplanes used to be started by turning the prop; which in this case is mechanically the same thing as a lawnmower blade.
posted by buzzman at 7:49 PM on August 7, 2005

Try a reel mower that needs no gas. You'll also burn more calories, pollute the air less, and send less money to our Saudi overlords.
posted by davy at 8:17 PM on August 7, 2005

For such a sudden failure I would suspect either an electrical problem as suggested by buzzman or a fuel clog, most likely in the carburetor as the primer seems to be working.
posted by caddis at 8:21 PM on August 7, 2005

If you take davy's advice I strongly reccomend good medical insurance. Hard manual labour from newfound jobs like that is a prime catalyst for heart attacks. (Ask any doctor in Canada what season causes the most heart attacks -- driveway shoveling season -- because so many people here still do it manually).
posted by shepd at 8:25 PM on August 7, 2005

Had the gas been used recently? Gas can go bad after a few weeks.
posted by mhaw at 8:49 PM on August 7, 2005

Shoveling snow is a lot more strenuous than mowing the lawn with a push-reel mower, even with hills (I have both).

Shoveling snow is done a lot less frequently than mowing the lawn -- that is, it's a sudden amount of hard work, as opposed to a regular, weekly amount of work of medium difficulty (hmmm, exercise?).

Hence, shepd's concern is bogus. Good luck starting the mower. My push-reel always starts. Can't let the weeds get high though ...
posted by intermod at 9:22 PM on August 7, 2005

shepd's concerns are not completely "bogus" depending upon one's level of fitness, etc. However, shoveling snow is a particular risk due to the factors cited by intermod (especially the intensity of the effort) and also due to the cold weather.

Reel mowers are basically for people with small lots or zealots. Pushing one of these over a typical one third to half acre lot is quite an undertaking. Electric mowers are pretty environmentally friendly, and less taxing on the ticker. One clear advantage of a good reel mower is the quality of the cut. The propeller blade gas mowers, even when freshly sharpened, produce a pretty ragged cut by comparison, especially if you keep the grass very short (but that itself is decidely environmentally unfriendly).
posted by caddis at 9:45 PM on August 7, 2005

It may be dirt in the carb. I don't buy the stale gas story--I've often used year-old gas with never a problem, but if you empty the last bit of your storage gas into the mower, the accumulated grit plugs the tiny carb. It doesn't take much. Try cleaning the bowl under the carb. Always fill your gas container before it's completely empty, and never use the last bit.

A broken shear pin will also stop the mower.
posted by weapons-grade pandemonium at 10:28 PM on August 7, 2005

I'd also see if the air cleaner is clogged. I'm not familiar with that engine, so I don't know if you can clean the filter on it.

Required items for a running engine:
Spark
Fuel
Air

Failure to start is almost always due to the lack of one of those. If it were something else, like lack of oil, you'd probably have noticed some extreme symptoms of distress.


If you follow Count Ziggurat's advice, I hope you have a good health-insurance plan.
posted by Kirth Gerson at 4:38 AM on August 8, 2005

Wash the air filter, clean the plug.
posted by Pollomacho at 5:49 AM on August 8, 2005

Old gas in small quantities will quickly turn to varnish and foul up everything it touches in your engine, but there's no mention if the lawnmower had been used before this season, nor if fuel stabilizer had been used prior to taking it out for its first usage. Get some carb cleaner and go to work on the carbs, then check the air intake as pollomacho recommends.
posted by Civil_Disobedient at 6:47 AM on August 8, 2005

Response by poster: Thanks for the suggestions (with the exception of the reel mower thing, 3/4 of an acre + reel mower + Central Texas in Aug = Death.)

I think I have it narrowed down to an electrical problem. There does not appear to be any spark jumping across the gap of the newly installed plug (or the old one before replacement for that matter) So my guess is that whatever thing it is that makes the current that makes the spark isn't making anything.

When I try to start it, it spins and chuffs, but not the kind of chuff that happens when something is exploding in the cylinder.

I don't think it is a fuel line clog because i can see fuel squirting into the carb when I take off the air filter and push the primer button.
posted by tcskeptic at 12:27 PM on August 8, 2005

Response by poster: Also, wrt old fuel, I have been using the mower all season, and have refilled it several times with fresh fuel. The fuel in the mower was stabilized before storage, and the fuel in the gas can that I fill out of is fresh.
posted by tcskeptic at 12:29 PM on August 8, 2005