Diagnosis of Briggs & Stratton Lawnmower Problem, Model # 11A-414R229

I was trying to start the lawnmower to mow the lawn. It started for a few seconds originally, after not having been used for many months. However, it was sputtering and on the verge of stalling, and once it made contact with the grass, it stopped completely.

I took some of the mower's engine apart today, to try and diagnose what the issue is, and why it stopped working. I watched one Youtube video, by 1beastlydude, where he suggested that the carburetor being clogged from stale gasoline was the most common problem in lawnmower malfunction. That video is linked here.

MY EXPERIENCE:

I haven't worked with lawnmowers at all, this was my first project. I've done some amateur-intermediate car repairs, but I don't know totally how an engine functions, though I have a pretty good idea. This diagnosis was partially for me to find the problem and partially for fun and for me to learn more about engines and mechanics, to help me with future car repairs.

CONCLUSION:

There was oil on the spark plug, which suggests that there is an oil leak inside the engine, perhaps caused by the failure of a piston ring seal. From what I have heard this requires either new rings or maybe a larger piston, and taking the engine to a machine shop to have the cylinder bored to make it fit the new larger piston.

I will consult with those wiser than me in these matters and see what the next step should be. For now, I am recording my process of diagnosis.

Supplies

Husky Ratchet set from Home Depot, ~$22

Carburetor cleaner ~$5

Phillips Screwdriver

Flathead Screwdriver

Spark plug socket

Step 1: Taking Apart the Engine Cover, and Seeing What's Under

I started by just taking off the cover of the mower. I didn't really know where I was going to go, but I figured I would take a look at the different parts and check on the carburetor. I started by removing the exterior plastic engine cover, which was a matter of one 8mm nut and two plastic clips by the air inlet. The plastic clips are hidden, and adjacent to the flywheel air fins, as shown in the first picture above.

Step 2: Removing the Air Filter

This step could also have been done first, since access to the air filter is not restricted by any other parts. There is simply one long screw that can be unscrewed to remove the air filter. The filter comes apart, and the filter itself is a sort of sponge. Mine felt oily and smelled like gasoline (I don't know what those things mean), so I took it inside and washed it off with lots of dish soap and let it dry in the sun for a while.

Step 3: Removing the Gas Tank, and Inspecting the Carburetor

The gas tank is held on by one 13mm bolt, and is connected to an air tube and a gas tube. The tank will just slip right off the tubes when you try and pull the gas tank off. The 13mm bolt is also holding some kind of bracket that is also attached to one of the 10mm bolts on the metallic secondary engine cover. There are two seals on the air tube (the larger of the two tubes). Be mindful of these, as they were sitting on the intake tube for me and I didn't even notice them until the third time I removed the gas tank. They're supposed to be pressed into the carburetor opening, as you can see in one of the pictures above, with the white ring on the inside of the carb opening, and the black rubber ring behind it.

I removed the 5 or 6 screws holding the carburetor onto the gas tank. There was a 2-layer gasket underneath (according to a video I watched it should be 3-layer: one paper, one rubber, and another paper gasket).

The carburetor didn't look dirty or anything to me, but I don't have much experience in these things. I sprayed with some carb cleaner just to make sure.

I didn't have a new gasket, so I installed the old one which was probably bad. I don't know how much this would affect functionality of the engine, but if I plan to complete repair of the mower, I will definitely purchase new gaskets.

Step 4: Removing the Spark Plug and Inspecting, Spraying in Carb Cleaner

This should probably have been the first step in hindsight, because it's much easier to do than all the other steps (virtually all you do is unscrew the spark plug), and because it told me the information about the internal state of the engine that led to my diagnosis.

The spark plug wire was removed, and the spark plug was unscrewed with a spark plug socket. A deep socket would do here too, since the spark plug is external and it's not sitting 10" down a tube like in my '97 Toyota Corolla.

It was pretty obvious that the spark plug was grimey with oil, which suggested to me that the combustion chamber was being flooded with oil. I was guessing that this meant a leak between the cylinder and deeper parts of the engine, but maybe there are multiple possible causes. I'm not sure. I don't know engines well enough. But I'm fairly certain it must be due to some type of internal engine failure, as oil can't leak into the combustion chamber from anywhere but the engine.

I believe the pictures I took above were after I had initially wiped the oil off the plug, before I knew I was going to make an Instructable out of this. However upon reinsertion, engine turning, and removal again, it seemed that the plug was wet with oil.

I viewed a diagram of an engine from the net, and it seems like a leak coming from around the piston would be the most likely, as it seems much harder for oil to leak in through the valves.

In this Hunker article the author suggests that the spark plug being oily could also be caused by oil overfilled in the engine, but this was not the case for me. The oil in my mower was very dark, and I'm sure it's never been changed, so perhaps this caused the engine to overheat, and the piston seals to fail.

I sprayed some carb cleaner into the combustion chamber through the spark plug hole, thinking that this would clean some of the gummed up oil and maybe make the engine run better for a few moments until the chamber was flooded with oil again. This would be further evidence that the flooding of oil was the cause of the mower failing to operate.

Step 5: Removing the Alternator to Access the Intake Tube

The alternator had to be removed to access the intake tube. The intake tube was removed. There was a gasket, but it was also not replaced. Carb cleaner was sprayed into the intake chamber.

Step 6: Testing the Spark

CAUTION!!! I REMOVED GAS TANK FIRST AND MOVED IT FAR AWAY SO IT DOESN'T IGNITE FROM THE SPARK! I should probably have known from the fact that the lawnmower would start at all, but I tested the spark plug wire by inserting a screwdriver and seeing if it would create a spark that would jump to a metal piece of the engine when the motor string was pulled.

Step 7: Reassembling, Pouring Out Carb Cleaner, Starting Up.

I reassembled all components except for the spark plug, and tilted lawn mower to pour out extra carb cleaner. Then reinserted spark plug and tried to start motor, to see if it would run better for a few moments. Sure enough, it ran at full power for a couple seconds then died and wouldn't start again. I believe there is an internal engine failure. I will look into if this is even worth fixing.