Project library · Small engines · Adult mode
Fix a lawn mower that bogs down and dies
It starts, runs for a minute, then hunts up and down or drops dead the moment it meets long grass.
Nine times in 10 this is fuel, not the engine. Old petrol turns to varnish and the smallest hole in the carburettor is the first to block.
Before you start
Petrol is a fire risk and its vapour travels. Work outdoors, no flame, no smoking. Pull the spark plug lead off before your hands go anywhere near the blade — a turned blade can fire the engine.
Zero Cost
Fresh fuel, a washed filter and a cleaned plug. Nothing bought.
$0 · 1 hour
You have it
- Socket or spanner set
- Screwdriver
- Wire brush
- Old jug
- Rag
You need it
- Nothing, if you have fuel under a month old in the car can
- 1
Pull the plug lead off. Do this first, every time.
- 2
Drain the tank into a jug. If the fuel smells sour or sharp like varnish, it is the fault. Bin it properly, do not pour it down a drain.
- 3
Take the air filter out. Wash a foam filter in warm soapy water, squeeze dry, refit. Tap a paper one clean and hold it to the light — if no light passes, it is done.
- 4
Pull the spark plug. Wire-brush the tip, clean the gap, refit finger-tight then a quarter turn.
- 5
Tip the mower with the carburettor side up, never the exhaust side down, and scrape the deck. A packed deck alone will bog an engine.
- 6
Refill with fresh fuel and run it. If it now surges rather than dies, the fault is in the jet — go to the next route.
Minimal Effort
New plug, fresh fuel, and cleaner sprayed through the jet without pulling the carburettor.
About $25 · 1 hour
You have it
- Plug spanner
- Socket set
- Screwdriver
You need it
- Matching spark plug
- Can of carburettor cleaner
- 2 litres of fresh fuel
- Bowl gasket if yours weeps
- 1
Lead off. Undo the single bolt in the bottom of the float bowl and catch the fuel in a rag.
- 2
That bolt is usually the main jet. Hold it to the light. The cross hole is the size of a hair and it will be blocked with varnish.
- 3
Spray cleaner through the jet until you can see light through the hole. A strand of strimmer line clears it. Never a drill or a pin — widening it ruins the mixture.
- 4
Spray the bowl clean, wipe the sediment out, check the rubber gasket sits in its groove.
- 5
Fit the new plug, refit the bowl, fill with fresh fuel, run it for 5 minutes under load in thick grass.
Polished Finish
Carburettor off and stripped, full gasket kit, new fuel line and filter.
About $45 · 3 hours
You have it
- Socket set
- Screwdrivers
- Pliers
- Small parts tray
You need it
- Carburettor rebuild kit for your engine number
- Air filter
- Fuel line and inline fuel filter
- Spark plug
- Can of carburettor cleaner
- 1
Find the engine model and code stamped on the shroud. Order the kit against that number, not against the mower brand.
- 2
Photograph the linkage and the spring positions before anything comes off. This is the step people regret skipping.
- 3
Take the carburettor off, strip it, and lay the parts out in order. Soak the body in cleaner.
- 4
Clear the main jet, the emulsion tube holes and the tiny idle passages until light passes through every one.
- 5
Fit the new needle, seat and diaphragm or float gasket. A hardened diaphragm alone causes surging.
- 6
Replace the fuel line and add an inline filter. Refit, set the governor linkage as photographed, and test under load.
- 7
At the end of the season, run the tank dry. Empty carburettors do not varnish up.
The step that bites
Clearing the main jet
Most people spray cleaner at the outside of the carburettor, put the bowl back on and find the mower still surging. The blockage is inside the bolt that holds the bowl, in a hole thinner than a pin. Until you can hold that jet up and see daylight through the cross hole, nothing else you do will fix the bogging.
Further reading
Other people have filmed and written this job well. Their work, their names, their links.
- Carburetor Repair, Maintenance & Adjustment How-To Guide
Briggs & Stratton
- How to Clean the Carburetor on Briggs & Stratton Quantum Lawnmower Engines
donyboy73
- Why Your Engine Keeps Surging (and How to Fix It)
Steve's Small Engine Saloon
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