Why Your 2020 KTM RC 390 May Be Using More Fuel Than It Used To

Shop parts for a 2020 KTM RC 390 Motorcycle.

The 2020 KTM RC 390 is a liquid-cooled, fuel-injected, chain-driven four-stroke single, and that configuration matters a great deal when you start chasing a fuel economy complaint. Because it runs electronic fuel injection rather than a carburetor, most of the classic carburetor culprits – a choke left partly open, a stuck float, gummed jets – simply do not apply here. What does apply is a shorter list of injection-specific, ignition-specific, and mechanical causes, plus a handful of rider and environment factors that have nothing to do with the bike at all. Working through them in a sensible order will usually tell you whether you are looking at a cheap fix, a worn component, or something that deserves a technician's attention.

First, Confirm That the Change Is Real

Before assuming the bike has developed a problem, establish a baseline. Fill the tank to the same level each time, reset the trip meter or jot down the odometer reading, ride normally, then refill and divide the distance covered by the amount of fuel it took to refill. Two or three consecutive tanks give you a far more trustworthy picture than a single fill-up, because one tank can be skewed by a long highway run, a cold snap, or an unusually enthusiastic afternoon. If you have never tracked it closely, you may simply be noticing the problem now as reduced range – stopping for fuel more often than you used to on the same route.

One easy thing to rule out is measurement error rather than a genuine change. Fitting a different size rear tire or changing sprocket sizes alters the relationship between actual distance travelled and what the odometer reports. If the odometer under-reports distance, your calculated economy looks worse than reality. If you have recently changed gearing or tire sizing, factor that in before you start diagnosing the engine.

Riding Conditions and Habits That Quietly Cost Fuel

Cold weather hurts fuel economy in several ways at once. The engine takes longer to reach operating temperature, the ECU adds extra fuel while it is warming up, and cold, denser air increases aerodynamic drag. Winter-blend and ethanol-blended pump fuels can also return slightly worse economy than what you buy in the warmer months, and that is normal rather than a fault.

Your right hand and your route matter just as much. Sustained high-speed highway running, strong headwinds, carrying a passenger or loaded luggage, and repeated hard acceleration all burn noticeably more fuel than a steady cruise. Short trips where the engine never fully warms up are particularly thirsty, and so is a long idle warm-up in the driveway before you ride off. If your commute or riding style has changed recently, that alone can explain a drop that has nothing to do with the machine.

Rolling Resistance, Brakes, and the Drive Chain

Tire pressure is the single most common free fix and the easiest to overlook. Soft tires increase rolling resistance, and a slow leak that has crept up over a few weeks can make a real difference to how hard the bike has to work. Check pressures cold, and check them regularly rather than once a season.

A dragging front brake caliper is another frequent offender. If a piston is sticking or the caliper is not releasing cleanly, the pads stay in light contact with the rotor and the bike is effectively fighting the brake constantly. You may notice a hot rotor after a ride, or the bike rolling less freely when you push it. Because the RC 390 is chain-driven, the final drive also deserves a look: a dry, rusty, or badly overtightened chain soaks up power and drags on the drivetrain. A chain that is correctly lubricated and set with proper slack will free up measurable efficiency.

Air Intake and Ignition Health

A clogged or heavily soiled air filter restricts airflow and forces a richer effective mixture, which shows up as reduced economy along with slightly softer throttle response. Filters are inexpensive and quick to inspect, so this is worth doing early. On the ignition side, the spark plug is a small part with an outsized effect. A worn, fouled, or incorrectly gapped plug produces incomplete combustion, wasting fuel that never contributes to forward motion. Pulling the plug and reading its condition tells you a lot: a sooty, black appearance points toward an over-rich mixture, while normal wear suggests it is simply due for replacement. Because this bike uses a modern electronic ignition rather than contact breaker points, there is no points gap or timing drift to chase.

Fuel Injection Components That Can Run Rich

This is where a fuel-injected single differs most from an older carbureted machine. A failing oxygen sensor in the exhaust system is a genuine possibility on a closed-loop injection system; when it reports inaccurately, the ECU can richen the mixture unnecessarily. An engine or coolant temperature sensor that reads colder than the engine actually is will have the same effect, telling the computer the bike is still warming up when it is not.

A dirty, clogged, or leaking injector is another common cause, delivering more fuel than the engine needs or spraying poorly so that combustion is incomplete. Because the RC 390 uses a single throttle body and a single injector, there is no synchronization issue to worry about. Stored fault codes can also reveal a sensor problem the bike has already detected, so if a dashboard warning light is present, have the codes read. Finally, do not overlook a simple fuel leak – a weeping line, fitting, or seal wastes fuel continuously and often leaves a faint smell when the bike is parked.

Engine Condition, Cooling, and Modifications

As mileage accumulates, general engine wear reduces compression, and lower compression means less of the energy in each combustion event turns into useful work. A compression check is a straightforward way to confirm whether this is a factor. Since this is a liquid-cooled machine, a thermostat stuck open is also worth considering: an engine that never reaches full operating temperature runs in a permanently enriched state and will use more fuel than one that warms up properly.

Modifications deserve a mention as well. Fitting an aftermarket exhaust or a high-flow air filter without matching fuel injection tuning commonly leaves the engine running richer than intended. If the mileage drop began right after a performance part went on, that timing is your clue.

When to Bring It to a Mechanic

Some symptoms point toward something more involved than routine maintenance. A persistent strong fuel smell, black smoke from the exhaust, a sooty spark plug, rough or uneven running alongside the mileage drop, or a warning light on the dash all suggest a fueling or sensor fault that is best diagnosed with proper equipment. Likewise, if you have corrected tire pressures, cleaned or replaced the air filter, fitted a fresh spark plug, and serviced the chain, and the bike is still drinking noticeably more fuel than before, it is time to hand it over. Describe what you have observed – when the change started, whether it coincided with a modification or a service, and how the bike behaves otherwise – and a technician can work from your notes rather than starting from scratch.

Related Shopping Categories

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Shop Spark Plugs for a 2020 KTM RC 390 Motorcycle.

Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.