Most Common Lawn Mower Problem

Fuel from a Carburettor Float Bowl.

The Most Common Lawn Mower Problem We See in Dorset: Water in the Fuel

By far the most common lawn mower problem we saw over the last year in lawn mowers and other machines coming into our workshop in Yeovil, Sherborne, Dorchester, Bridport and across Dorset was water contamination in the fuel.

It often causes poor running, hunting, stalling, loss of power, or a machine that simply will not start. This can be particularly frustrating, as many machines were running perfectly the last time they were used.

Why does water get into lawn mower fuel?

Condensation on a window.

Water can enter fuel in several ways, and it’s not always due to obvious mistakes. Modern petrol, especially E10, is hygroscopic, meaning it absorbs moisture from the air. Over time, particularly in damp conditions common across Dorset and Somerset, this moisture builds up and separates as water. We frequently see this in mowers that have been left standing between cuts or over winter. Poorly sealed fuel caps, fuel tanks exposed to rain, and condensation inside half-empty tanks are also common contributors.

If you have condensation inside your house or car windows, you probably have condensation inside your fuel tank. You can help to prevent this by keeping your tank very full - no room for damp air, or completely empty. When condensation runs down the side of your tank, if it gets under fuel, it can not evaporate, but if the tank is emplty, it might evaporate and leave the tank dry again.

Jerry cans are often the real cause

In many cases, the issue starts before the fuel even reaches the mower. Old petrol sitting in a jerry can is one of the biggest causes of water contamination we see from customers around Yeovil, Sherborne and Bridport. Fuel cans stored in sheds or garages that experience temperature changes are especially prone to condensation. Plastic cans can allow moist air in over time, while metal cans may develop internal rust or debris that ends up in the machine.

Occasionally, after resolving problems related to water or dirt in the machine's tank or carburettor, customers report the next day that their machine still isn't functioning correctly. This is usually because they have refilled the mower with stale or contaminated fuel. You can take old fuel to the local council tip, where they offer a free disposal service.

How to avoid water in your mower fuel

Prevention comes down to fresh fuel and correct storage. Only buy the amount of petrol you expect to use within a few weeks, keep jerry cans tightly sealed, and store them somewhere cool and dry. Avoid leaving fuel in mowers for long periods, particularly during the wetter months we often experience in Dorset. If a mower won’t be used for a while, drain the tank and carburettor, or run it dry. Many of our customers choose Aspen fuel, which does not absorb water, stores for years without degrading, and significantly reduces fuel-related problems. We keep Aspen fuel in stock at our workshop.

Common signs of water-contaminated fuel

Typical symptoms include difficult starting, uneven running or “hunting”, stalling under load, and loss of power. Sometimes the fuel will appear cloudy, very pale yellow, or visibly separated in the tank or fuel bowl. If you notice any of these signs, it’s best to stop using the mower rather than risk drawing water and debris into the carburettor.

What to do if you suspect water in your fuel

If you think water has entered the fuel system, stop using the machine as soon as possible. Continuing to run it can cause further damage and increase repair costs. The old fuel should be safely disposed of, the tank drained and flushed if necessary, and any jerry cans cleaned before refilling with fresh petrol. You can often drain the water and old fuel from the Carb using a 10mm spanner on the drain carb plug "B". Some carburettors only have the plug "A". Be careful with this, as the whole bowl will come off the bottom of the carb if you overundo this. In many cases, the carburettor will need cleaning to remove water and residue, which is a very common repair we carry out for customers from Dorchester, Yeovil, Sherborne and Bridport.

We’re here to help

We are almost exactly halfway between Yeovil and Dorchester.

If your mower isn’t running as it should and you suspect a fuel issue, we’re happy to help. Water in fuel is extremely common, but caught early, it’s usually straightforward to fix. If you’re based between Yeovil and Dorchester, feel free to get in touch or drop your mower into our workshop before the problem gets worse.