Mower Troubleshooter

Quick answer: when a mower won’t start, work from free to expensive. Is the fuel valve on, is the choke right for a cold or warm engine, is there fuel in the tank, is the spark plug wire on? Then a flooded engine, old fuel, the air filter and the spark plug. The carburettor comes after those, and the ignition and compression are a shop’s job. That is the order Honda’s own troubleshooting table runs in, and the tool gives you the same kind of list for ten problems on push mowers and riders, each check quoting the maker that prints it.

How to use it

Pick the machine — a gas push mower, or a riding mower or zero-turn — and what it is doing. The list that comes up is every check the makers print for that problem, grouped by what it costs you to try: free to look at, free with a basic tool, a cheap part or fresh fuel, and a shop job. Tick a check once you have ruled it out and the next one moves to the top, so you are never guessing where you were. The ticks stay on this device. When you run out of things to try, print the list and take it to the dealer: it saves them charging you to repeat it.

Why cheapest first, and not “most likely first”

Because nobody publishes how likely each cause is for your mower, and a list that pretended to know would be inventing it. What the makers do publish is what to check, and their own tables already run roughly from the free checks to the dealer. Honda’s walk-behind manual starts with the control positions, then the fuel, then the spark plug, and only then says to take it to the servicing dealer. Toro’s zero-turn manual does the same for a starter that won’t crank: blades disengaged, levers in the brake position, you in the seat, then the battery, the connections, a fuse, and only then the dealer. Checking the fuel valve costs nothing. Taking a carburettor apart costs an afternoon, so it comes later.

Where the makers disagree: old fuel

Old gasoline turns up under almost every symptom, and here the makers part ways. Honda’s table says to drain the fuel tank and the carburettor yourself and refill with fresh gasoline. Briggs & Stratton says untreated gas starts to break down after about a month and gives the same drain-and-refill steps, float bowl included. Toro’s zero-turn manual, for “dirt, water, or stale fuel” in the fuel system, says to contact an authorized service dealer. The tool shows Honda and Briggs & Stratton’s steps and Toro’s line under them. On a push mower, draining it yourself is ordinary. On a rider with a fuel pump and filters, Toro’s caution is worth weighing. Before you drain anything, the gas shelf-life calculator tells you whether the gas is past what the makers and the fuel companies say, and we wrote up what old gas does to a mower.

Smoke: read the colour

Briggs & Stratton splits smoke by colour. White or blue means the engine is burning oil: the crankcase is overfilled, the mower was tipped or run at more than a 15-degree angle, or the breather tube behind the air filter is blocked. Worn rings are the last and dearest answer. Black means too much fuel for the air: a dirty air filter, bad fuel, an overfilled crankcase or a flooded carburettor. Smoke after tipping the mower should clear after a few minutes on the flat. If it does not, work down the list. Our post on what a smoking mower means goes through each colour.

Vibration and a bad cut are usually the blade

Both Honda and Toro put the blade near the top of the vibration and cut-quality lists: packed grass under the deck, a loose blade bolt, a bent blade, or a blade thrown out of balance by uneven sharpening. The spark plug wire comes off before your hands go near it, every time. We have written about fixing a vibrating mower, telling whether a blade is unbalanced and why a Honda leaves a strip of grass. On a rider, Toro adds the deck level, the anti-scalp wheels and tyre pressure. A soft tyre on one side tilts the whole deck.

What this does not cover

Battery and robot mowers have their own electronics and their own manuals. Two-stroke trimmers, blowers and chainsaws have their own fuel and their own faults. The 2-cycle mix calculator covers the mix. And anything that means lifting a riding mower is blocked with stands, never trusted to a jack.