By the UK Generator Guide — Home Petrol Generator Reviews & Advice Team · Updated May 2026 · Independent, reader-supported

How to Maintain a Petrol Generator UK: Full Service Checklist

A petrol generator is one of those purchases you hope you'll never need but absolutely must have working when it matters. Whether you're relying on it for emergency backup power or using it regularly on site, proper maintenance is the difference between a dependable piece of kit and an expensive paperweight. The good news is that keeping a generator in top condition doesn't require specialist knowledge—just a straightforward routine and a few hours a year.

Why Regular Maintenance Matters

Generators sit unused for weeks or months, then suddenly need to perform flawlessly when the grid fails or you need power on a remote job. The longer a generator sits idle, the more fuel gums up in the carburettor and fuel lines, and the more likely you are to face a no-start situation precisely when you need it most. Oil oxidises, spark plugs foul, and filters clog. A simple maintenance schedule prevents all of that.

Oil Changes: The Single Most Important Task

Engine oil is the lifeblood of any petrol engine. Dirty oil loses its ability to protect moving parts, and generators accumulate sludge faster than you'd expect because they often sit idle.

Change the oil every 100 hours of operation or at least once a year, whichever comes first. For most home users, that means a yearly change even if you've barely run the thing.

Before draining, run the generator for 5–10 minutes so the oil warms up slightly—this helps old oil flow more freely. Let it cool for a few minutes (you don't want to scald yourself), then locate the oil drain plug at the base of the engine. Place a container underneath, unscrew the drain plug, and let the old oil drain completely. This usually takes 5–10 minutes.

Once drained, replace the oil drain plug (don't overtighten—hand-tight plus a quarter turn is enough). Then refill with the correct grade oil for your engine. Most small generators use SAE 10W-30, but check your manual. Fill until the oil reaches the maximum line on the dipstick, not above it.

Spark Plug Replacement

A fouled spark plug is often why a generator won't start after sitting for a few months. Spark plugs are cheap and easy to replace.

Remove the spark plug wire by twisting it gently at the connector, then unscrew the spark plug with a spark plug socket. Have a look at it: if it's heavily carbonised, cracked, or the gap looks too wide, it's time for a replacement. A new spark plug costs just a couple of quid and takes two minutes to fit.

When installing a new plug, hand-thread it first to avoid cross-threading the cylinder, then tighten it firmly with the socket. Don't wrench it—just snug. Reattach the wire. Some people swap plugs annually as part of their winter storage routine, which is sensible peace of mind.

Air Filter Cleaning and Replacement

The air filter prevents dust and debris from entering the engine. On small generators, it's usually a foam or paper element—quick to inspect and cheap to replace.

Pop off the filter cover (usually held by clips or a bolt), then examine the filter. If it's visibly choked with dust and dirt, it restricts airflow and makes cold starts harder. Foam filters can be gently rinsed in clean petrol or water and allowed to dry completely before reinstalling. Paper filters can't be cleaned effectively and need replacing when dirty.

Replace the filter every 100 hours or so, or whenever it looks clogged. A clean filter improves starting and fuel economy.

Carburettor Maintenance

The carburettor is where fuel and air mix before combustion. When a generator sits idle, fuel in the carburettor evaporates, leaving sticky varnish that blocks tiny passages. This is usually why generators won't start after months of storage.

The simplest fix is fuel stabiliser. Before storing your generator for winter or extended periods, add a fuel stabiliser to the petrol tank (follow the product's dosage), then run the generator for 15 minutes so the stabilised fuel works through the entire system. This prevents varnish buildup.

If your generator is reluctant to start despite a new spark plug, carburettor cleaner sprayed into the air intake while attempting to start often works. Spray a 3-second burst, wait a few seconds, and try again. Do this a few times if needed. For stubborn cases, some people remove the carburettor bowl (the flat drain plug at the bottom of the carburettor) and rinse it out, but that's rarely necessary if you use fuel stabiliser preventatively.

Other Checks Worth Your Time

Fuel system: Check fuel lines for cracks or splits. Petrol perishes plastic fuel lines over time. If yours are brittle or cracked, replace them before they leak.

Cooling fins: The metal cooling fins on the engine block accumulate dust and restrict airflow. Use a soft brush to clean them gently.

Loose bolts: Vibration during operation shakes bolts loose. A quick visual scan and tightening of obvious fasteners prevents parts dropping off.

Fuel freshness: Petrol degrades after 3–6 months. Before winter storage, either drain the tank completely or add fuel stabiliser.

Creating Your Maintenance Schedule

Keep a logbook. Note down when you've changed oil, replaced the spark plug, cleaned the filter, and how many hours you've run the machine. This takes 30 seconds and eliminates guesswork later.

Before each use, check the oil level and fuel supply. Before seasonal storage, add fuel stabiliser and run the engine briefly. Once a year (or every 100 hours), perform a full service: fresh oil, clean or replace the air filter, inspect the spark plug, and check all external connections.

The Bottom Line

A petrol generator kept in good nick will start reliably and run smoothly for years. The maintenance cost is minimal—just a few quid on oil and filters, plus a couple of hours of your time annually. Compare that to buying a new generator because you neglected the old one, and it's money genuinely well spent.