Brand New Mower Won't Start Next Spring? The One Mistake Everyone Makes
Date Released: 29 Dec 2025 Category: Blog
It's a scene we witness every April and May. Someone has a lawn mower—maybe it's only a year old, perhaps even just six months—and they're genuinely baffled.
"It was running perfectly in October," they say. "I've barely used it. Now it won't start at all, and when it does catch, it runs terribly. Is it the engine? Did I get a dud?"
They take it to be fixed, the technician pops the fuel cap, takes one sniff, and knows exactly what's happened. The same thing that happens to dozens of mowers every spring. The same mistake that's entirely preventable and costs absolutely nothing to avoid.
They left fuel in the tank over winter.
That's it. That's the whole problem. And it's going to cost them anywhere from £60 for a carburettor service to £150+ for a complete rebuild if the fuel system is badly varnished. All because of a five-minute job they didn't do before putting the mower away in October.
Let's talk about why this happens, why modern fuel makes the problem worse than ever, and exactly how to prevent it from ruining your next season.
The chemistry is actually quite straightforward. Modern unleaded petrol is a complex cocktail of hydrocarbons, and when exposed to air, these compounds start oxidizing. Light hydrocarbons evaporate, heavier components remain, and what's left behind gradually polymerizes—basically turning into a glue-like residue.
In your car, this isn't much of an issue. You use a tank of fuel every week or two, long before it has time to degrade. But in your lawn mower, pressure washer, or generator? That last tank full sits there for five or six months, slowly transforming into engine poison.
Here's the timeline of fuel degradation:
Weeks 1-4: Fuel is perfectly fine. Your equipment will start and run normally. Weeks 4-8: Lighter fractions begin evaporating. Fuel starts losing octane. You might notice slightly harder starting or rougher running. Weeks 8-12: Noticeable deterioration. The fuel smells stale and sour. Starting becomes difficult. Running is rough and inconsistent. Weeks 12+: Serious degradation. Fuel has formed gummy deposits. Varnish is coating internal components. Your carburettor is well on its way to being blocked. Month 6 (typical storage period): Complete disaster. The fuel has turned into something resembling thin syrup. Your carburettor jets are blocked solid. Fuel lines may be gummed up. Internal passages are coated in varnish.
And here's the really frustrating part: this happens even if your mower is barely used. We've seen brand-new mowers with under five hours of use with seized carburettors, simply because they were put away with fuel in the tank after that first autumn cut.
Ethanol Blends
Most UK petrol now contains up to 10% ethanol (E10 fuel). Ethanol is hygroscopic—it actively absorbs water from the air. That water then separates from the petrol, settling at the bottom of your tank and causing two problems:
First, water and ethanol mixture is corrosive. It attacks fuel system components, particularly on older equipment not designed for ethanol fuels. We regularly see carburettors with corroded internal parts, all thanks to ethanol-attracted moisture.
Second, water doesn't burn. When your engine tries to pull this water-contaminated fuel through, it won't run properly. The separated ethanol-water mixture can actually freeze in winter, potentially causing physical damage to fuel lines and carburettor passages.
Modern Formulations
Today's petrol is formulated for modern cars with sophisticated fuel injection systems, not for small engines with simple carburettors. The additives and lighter hydrocarbon fractions that make modern fuel work brilliantly in cars make it less stable during storage. It simply doesn't keep as well as the petrol of decades past.
The upshot? What might have been a minor nuisance twenty years ago is now a major problem that catches out thousands of equipment owners every spring.
Your Briggs & Stratton engine has a carburettor with tiny jets and passages—some as small as 0.5mm across. These precisely control fuel flow into the engine. When fresh fuel evaporates from these passages, it leaves behind those heavier compounds we mentioned. This varnish doesn't just coat surfaces; it actively blocks these critical jets.
Come spring, you pull the starter cord. The engine turns over—that's fine, nothing's seized—but it won't fire. Or maybe it catches briefly then dies. What's happening is this:
The carburettor is trying to pull fuel through blocked jets Little or no fuel is reaching the combustion chamber What fuel does get through is stale and has lost its volatility The spark plug may be fouled from incomplete combustion attempts Air-fuel mixture is completely wrong
You keep trying. Pull, pull, pull. Maybe you're flooding it now, washing raw fuel past the piston rings and diluting what oil is in the sump. Eventually, you give up and call a repair shop.
At the workshop, they pull the carburettor apart. Inside, it's coated with a brown or amber varnish. The jets are blocked. The float bowl has a layer of sticky residue. If it's been left really badly, the float needle might be stuck, or the diaphragms (if it's a diaphragm carb) may have degraded.
A standard carburettor clean and service: £60-80. A complete carburettor rebuild if components are damaged: £100-150. A new carburettor if the old one's beyond saving: £80-120 for parts alone.
All completely avoidable.
Option 1: Run It Dry
This is the method we recommend for most people because it's foolproof and costs nothing. Here's what you do, ideally in late October or whenever you're done mowing for the season:
Mow your lawn one last time, using up as much fuel as possible When you're finished, let the engine cool for five minutes While the engine is still warm, close the fuel tap if you have one (many mowers don't—that's fine) Restart the engine and let it run until it dies from fuel starvation Try starting it once more to confirm the carburettor bowl is empty
Job done
That's it. The entire process takes ten minutes at most, and five of those are just waiting for the engine to run out of fuel. Your carburettor and fuel system are now empty—there's no fuel to go stale, nothing to form varnish, and nothing to gum up those tiny jets.
Come spring, you'll add fresh fuel, give the primer bulb a few pushes (if fitted), and your mower will start on the second or third pull like it should.
Option 2: Fill It With Treated Fuel
Some people prefer to store equipment with a full tank, particularly for generators or equipment they might need to use during winter. If that's you, here's the method:
Add fuel stabilizer to your fuel can before filling your equipment (following the product's instructions)
Run the engine for five minutes to ensure treated fuel reaches the carburettor
Fill the tank completely to minimize air space (less air means less moisture and oxidation)
Store in a dry location
Fuel stabilizer works by preventing oxidation and absorbing moisture. A decent bottle costs £8-12 and treats multiple tanks. Popular brands include Briggs & Stratton's own fuel treatment, STA-BIL, or similar products.
The catch? Fuel stabilizer only works if you add it before the fuel degrades. Adding it to already-stale fuel achieves nothing—you can't reverse fuel degradation. And even with stabilizer, you shouldn't store fuel for more than 6-8 months.
Personally, we still prefer running equipment dry. It's free, it's definitive, and there's zero chance of something going wrong.
If it's less than a month old, you can use it in your car—just pour it in before your next fill-up. Your car's fuel tank is so much larger that the old fuel will be diluted and used up long before it causes problems.
If it's older or you're not sure, dispose of it properly. Your local household waste recycling centre will have a facility for old fuel and oil. Never pour it down drains or onto the ground—it's both illegal and environmentally damaging.
For next season, buy fuel as you need it rather than storing large quantities. A 5-litre can is plenty for most homeowners. You'll use it within a few weeks during the cutting season, so it'll always be fresh.
The DIY Approach:
If you're handy, you can often sort this yourself:
Drain the old fuel from the tank completely Remove the carburettor bowl and clean it thoroughly Spray carburettor cleaner through all passages Clean or replace the spark plug Fill with fresh fuel and try starting
This works for mild cases. If the varnish build-up is minimal, a thorough clean might be enough.
When to Call the Professionals:
If your equipment still won't run after cleaning, or if you're not confident working on carburettors, that's when to bring it to us. We have ultrasonic cleaners and proper solvents that can shift varnish from places you can't reach with a spray can. We can also test fuel flow rates and make sure everything's calibrated correctly.
The earlier you bring it in, the better. Come May, every repair shop is drowning in mowers with the exact same problem. Book in March or early April, and you'll get it back quickly and avoid the seasonal rush.
Time to run your mower dry: 10 minutes Cost: £0 Carburettor service after winter fuel damage: Time without your mower: 5-10 days in season Cost: £60-80 Carburettor rebuild if seriously varnished: Time without your mower: 5-10 days in season Cost: £100-150 New carburettor if beyond repair: Time without your mower: 1-2 weeks waiting for parts Cost: £80-120 for parts, plus fitting
We're not trying to frighten you. We're just pointing out that ten minutes in October saves you money, time, and considerable frustration in April. It's possibly the best return on investment you'll ever get for ten minutes of your time.
The carburettors fitted to most Briggs engines are beautifully simple, which is part of their charm—fewer things to go wrong. But that simplicity means there's no high-pressure fuel pump forcing fuel through blocked jets, no electronic compensation for poor fuel quality. When those jets block, the engine simply won't run.
The good news? That same simplicity means prevention is straightforward. Running the carburettor dry takes minutes and guarantees it'll be clear in spring. There's no complex procedure, no special tools, just running the engine until it stops.
The frustrating thing—from our perspective—is how completely avoidable it all is. Ten minutes. That's all it takes. Ten minutes running your equipment dry before storage, and you'll never have this problem.
"It was running perfectly in October," they say. "I've barely used it. Now it won't start at all, and when it does catch, it runs terribly. Is it the engine? Did I get a dud?"
They take it to be fixed, the technician pops the fuel cap, takes one sniff, and knows exactly what's happened. The same thing that happens to dozens of mowers every spring. The same mistake that's entirely preventable and costs absolutely nothing to avoid.
They left fuel in the tank over winter.
That's it. That's the whole problem. And it's going to cost them anywhere from £60 for a carburettor service to £150+ for a complete rebuild if the fuel system is badly varnished. All because of a five-minute job they didn't do before putting the mower away in October.
Let's talk about why this happens, why modern fuel makes the problem worse than ever, and exactly how to prevent it from ruining your next season.
Why Last October's Fuel Becomes This April's Nightmare
Here's what most people don't realize: petrol goes off. Properly off. And when it does, it transforms from a volatile liquid that makes engines run into a sticky, varnish-like substance that makes engines stop.The chemistry is actually quite straightforward. Modern unleaded petrol is a complex cocktail of hydrocarbons, and when exposed to air, these compounds start oxidizing. Light hydrocarbons evaporate, heavier components remain, and what's left behind gradually polymerizes—basically turning into a glue-like residue.
In your car, this isn't much of an issue. You use a tank of fuel every week or two, long before it has time to degrade. But in your lawn mower, pressure washer, or generator? That last tank full sits there for five or six months, slowly transforming into engine poison.
Here's the timeline of fuel degradation:
And here's the really frustrating part: this happens even if your mower is barely used. We've seen brand-new mowers with under five hours of use with seized carburettors, simply because they were put away with fuel in the tank after that first autumn cut.
Modern Fuel: The Problem Has Actually Gotten Worse
If you're thinking "My dad never had this problem with his old mower," you're not imagining things. The issue has genuinely become more severe in recent years, and there are two main culprits.Ethanol Blends
Most UK petrol now contains up to 10% ethanol (E10 fuel). Ethanol is hygroscopic—it actively absorbs water from the air. That water then separates from the petrol, settling at the bottom of your tank and causing two problems:
First, water and ethanol mixture is corrosive. It attacks fuel system components, particularly on older equipment not designed for ethanol fuels. We regularly see carburettors with corroded internal parts, all thanks to ethanol-attracted moisture.
Second, water doesn't burn. When your engine tries to pull this water-contaminated fuel through, it won't run properly. The separated ethanol-water mixture can actually freeze in winter, potentially causing physical damage to fuel lines and carburettor passages.
Modern Formulations
Today's petrol is formulated for modern cars with sophisticated fuel injection systems, not for small engines with simple carburettors. The additives and lighter hydrocarbon fractions that make modern fuel work brilliantly in cars make it less stable during storage. It simply doesn't keep as well as the petrol of decades past.
The upshot? What might have been a minor nuisance twenty years ago is now a major problem that catches out thousands of equipment owners every spring.
What Actually Happens Inside Your Engine
Let's walk through exactly what's happening inside your equipment while it sits in the shed all winter.Your Briggs & Stratton engine has a carburettor with tiny jets and passages—some as small as 0.5mm across. These precisely control fuel flow into the engine. When fresh fuel evaporates from these passages, it leaves behind those heavier compounds we mentioned. This varnish doesn't just coat surfaces; it actively blocks these critical jets.
Come spring, you pull the starter cord. The engine turns over—that's fine, nothing's seized—but it won't fire. Or maybe it catches briefly then dies. What's happening is this:
You keep trying. Pull, pull, pull. Maybe you're flooding it now, washing raw fuel past the piston rings and diluting what oil is in the sump. Eventually, you give up and call a repair shop.
At the workshop, they pull the carburettor apart. Inside, it's coated with a brown or amber varnish. The jets are blocked. The float bowl has a layer of sticky residue. If it's been left really badly, the float needle might be stuck, or the diaphragms (if it's a diaphragm carb) may have degraded.
A standard carburettor clean and service: £60-80. A complete carburettor rebuild if components are damaged: £100-150. A new carburettor if the old one's beyond saving: £80-120 for parts alone.
All completely avoidable.
The Dead-Simple Solution (Takes Ten Minutes)
Right, let's talk prevention. You have two options, and honestly, both work. The key is choosing one and actually doing it before you put your equipment away for winter.Option 1: Run It Dry
This is the method we recommend for most people because it's foolproof and costs nothing. Here's what you do, ideally in late October or whenever you're done mowing for the season:
Job done
That's it. The entire process takes ten minutes at most, and five of those are just waiting for the engine to run out of fuel. Your carburettor and fuel system are now empty—there's no fuel to go stale, nothing to form varnish, and nothing to gum up those tiny jets.
Come spring, you'll add fresh fuel, give the primer bulb a few pushes (if fitted), and your mower will start on the second or third pull like it should.
Option 2: Fill It With Treated Fuel
Some people prefer to store equipment with a full tank, particularly for generators or equipment they might need to use during winter. If that's you, here's the method:
Add fuel stabilizer to your fuel can before filling your equipment (following the product's instructions)
Run the engine for five minutes to ensure treated fuel reaches the carburettor
Fill the tank completely to minimize air space (less air means less moisture and oxidation)
Store in a dry location
Fuel stabilizer works by preventing oxidation and absorbing moisture. A decent bottle costs £8-12 and treats multiple tanks. Popular brands include Briggs & Stratton's own fuel treatment, STA-BIL, or similar products.
The catch? Fuel stabilizer only works if you add it before the fuel degrades. Adding it to already-stale fuel achieves nothing—you can't reverse fuel degradation. And even with stabilizer, you shouldn't store fuel for more than 6-8 months.
Personally, we still prefer running equipment dry. It's free, it's definitive, and there's zero chance of something going wrong.
What About the Fuel in the Can?
Here's a question we get asked constantly: "What do I do with the fuel left in my can?"If it's less than a month old, you can use it in your car—just pour it in before your next fill-up. Your car's fuel tank is so much larger that the old fuel will be diluted and used up long before it causes problems.
If it's older or you're not sure, dispose of it properly. Your local household waste recycling centre will have a facility for old fuel and oil. Never pour it down drains or onto the ground—it's both illegal and environmentally damaging.
For next season, buy fuel as you need it rather than storing large quantities. A 5-litre can is plenty for most homeowners. You'll use it within a few weeks during the cutting season, so it'll always be fresh.
When Prevention's Too Late: Spring Rescue
Let's say you're reading this in April, and you've already made the mistake. Your mower won't start, and you're pretty sure there's six-month-old fuel lurking in there. What now?The DIY Approach:
If you're handy, you can often sort this yourself:
This works for mild cases. If the varnish build-up is minimal, a thorough clean might be enough.
When to Call the Professionals:
If your equipment still won't run after cleaning, or if you're not confident working on carburettors, that's when to bring it to us. We have ultrasonic cleaners and proper solvents that can shift varnish from places you can't reach with a spray can. We can also test fuel flow rates and make sure everything's calibrated correctly.
The earlier you bring it in, the better. Come May, every repair shop is drowning in mowers with the exact same problem. Book in March or early April, and you'll get it back quickly and avoid the seasonal rush.
The Economics of Prevention
Let's be completely mercenary about this for a moment.We're not trying to frighten you. We're just pointing out that ten minutes in October saves you money, time, and considerable frustration in April. It's possibly the best return on investment you'll ever get for ten minutes of your time.
Why This Matters for Briggs & Stratton Engines Specifically
Briggs & Stratton engines are brilliantly engineered and incredibly reliable. They're designed to run for decades with basic maintenance. But they're designed to run on fresh fuel.The carburettors fitted to most Briggs engines are beautifully simple, which is part of their charm—fewer things to go wrong. But that simplicity means there's no high-pressure fuel pump forcing fuel through blocked jets, no electronic compensation for poor fuel quality. When those jets block, the engine simply won't run.
The good news? That same simplicity means prevention is straightforward. Running the carburettor dry takes minutes and guarantees it'll be clear in spring. There's no complex procedure, no special tools, just running the engine until it stops.
The Bottom Line
Every single spring, we see dozens of mowers, generators, and other equipment suffering from fuel-related problems. Brand new machines that should be giving decades of service. Barely-used equipment that cost hundreds of pounds. All undone by the same simple mistake: fuel left in the tank over winter.The frustrating thing—from our perspective—is how completely avoidable it all is. Ten minutes. That's all it takes. Ten minutes running your equipment dry before storage, and you'll never have this problem.