Lessons of the Lawnmower

Ideally, we’d like to engineer ourselves out of existence, by spreading skills and confidence and motivation to as many people as we can. If you go home from a Repair Public event feeling emboldened to handle a screwdriver like never before, then our work was good work.

We want you to be able to take care of your own stuff to whatever degree you’re able. This includes the very important task of maintaining your own items. As much as we rail against planned obsolescence, there are certain powered items that need occasional care and feeding. This is especially true of internal combustion engines, both large and small. Take a look at this lawnmower’s air cleaner, for example.

Clogged air filter picture

Don’t let it get this bad. An air cleaner in this state can kill your mower pretty dead.

I’m not exaggerating, either. Here’s what happens: The engine needs air. The piston and its valves try to draw air in through the air cleaner assembly. If the engine can’t get enough air through the air filter and intake, the vacuum in the combustion chamber will actually be strong enough to start drawing air up from wherever else it can find it, namely the engine’s crankcase. And when that happens, the engine will also draw oil from the crankcase up into the combustion chamber, where it gets burned. This has the effects of both burning oil (worse for the environment), and eventually consuming all of the oil it needs to stay lubricated and run efficiently. Also, because the oil winds up in the combustion chamber, it can also back-feed up into the filter which causes it to get gooey and clog up even more quickly.

That’s what happened here. This engine was completely out of oil when we got it, and pull-starting it was a nightmare because all the parts were pretty much metal-on-metal. We fed this mower a steady diet of 10W-30 (almost half a quart) into the oil sump. The owner ran out and got a new air cleaner. We also dropped the carburetor float bowl, cleaned the bowl and the carburetor jet, cleaned the spark plug, and put everything back together. After squirting some fuel into the spark plug hole, the mower started and ran pretty well.

Even after this tender treatment, the mower remained hard to start. We suspected the spark plug as the remaining culprit, as the electrode was slightly bent when we looked at it. The owner remained in contact with us, and told us that she also wound up replacing the spark plug herself (nice work). After that, the mower started flawlessly.


Here’s a pretty good video on how to clean the carburetor float bowl and jet assembly. The jet in our mower was clogged pretty good, so I used some fine, stranded copper wire to push the crap out of the jet holes. If you do this, make sure to use a soft wire like copper and NOT harder metals like steel, because the jet is usually made of brass and you don’t accidentally want to make the jet holes bigger or otherwise damage the jet.

The moral: It’s often well worth your time to do maintenance on the stuff you use. Change the filter, lube that chain, sharpen that blade.

We’re Repair Public. And so can you!