Why Your 2019 Honda Monkey ABS Is Burning More Fuel Than It Used To
Shop parts for a 2019 Honda Monkey ABS Motorcycle.
Your 2019 Monkey ABS is a 125cc air-cooled four-stroke single running Honda's PGM-FI fuel injection, driving through a chain final drive and lit by a fully electronic ignition. There is no carburetor to flood, no radiator or thermostat to stick, and no ignition points to wear. That is genuinely useful information, because it eliminates a whole category of advice you may have read online about choke plungers, float bowls, and jetting. None of that applies here. What remains is a shorter, more manageable list of suspects, and most of them can be checked in a driveway with basic tools.
One thing to keep in mind throughout: the Monkey is a small, light, low-displacement machine. On a big touring bike, a passenger or a stiff headwind is a rounding error. On a 125, it is a meaningful percentage of the load the engine is pulling, so fuel consumption moves more visibly when conditions change.
First, Make Sure the Drop Is Actually Happening
Before chasing a mechanical cause, confirm the change with real numbers. Fill the tank to a consistent level, note the odometer reading or reset the trip meter, ride normally, then refill at the same pump and the same fill level. Divide the distance ridden by the fuel you added. That gives you one tank's worth of data, which is not enough to draw a conclusion — a single tank can be skewed by wind, temperature, or how full you happened to fill it. Repeat the process over several tanks and compare the average to what you remember from before.
If you are judging by feel rather than math — you simply notice you are stopping for fuel more often — that is a valid observation, but back it up with a few tank-to-tank calculations so you know whether you are chasing a real trend or a run of cold mornings.
Also rule out a measurement error. If you have changed tire size, or swapped sprockets for different gearing, the speedometer and odometer no longer read accurately, and your calculated fuel economy will be wrong in either direction. A taller or shorter final drive ratio also changes the rpm the engine turns at a given road speed, which changes real fuel consumption too.
Conditions and Riding Habits That Quietly Cost Fuel
Cold weather is the most common non-mechanical explanation. A cold engine needs extra fuel to run smoothly, and it takes longer to reach its efficient operating temperature on a short ride than it does on a long one. Cold air is also denser, which means more of it enters the engine and more fuel is required to match. Winter-blend and ethanol-blended fuels carry slightly less energy per volume, so the same riding can return fewer miles per tank without anything being wrong.
Riding style matters just as much. Aggressive throttle from every stop, holding high speed on the highway, carrying a passenger or loaded luggage, and long idling warm-ups all increase consumption. Short trips are the worst combination: the engine spends most of the ride warming up and very little of it running efficiently. If your commute changed, or the season changed, that alone can explain a noticeable difference.
Tires, Brakes, and the Chain
Underinflated tires are the single most common and most easily corrected cause of poor fuel economy on any motorcycle. Check both tires cold with a reliable gauge, and check them more often than you think you need to — small tires lose pressure gradually and the loss is easy to miss. Low pressure increases rolling resistance, and on a light bike you will feel it as sluggish acceleration as well as higher fuel use.
A brake that is not fully releasing drags continuously and forces the engine to work against it. On a bike with a disc brake, that usually means a caliper piston or slide that is sticking; on a drum brake, it can be a shoe that is not retracting cleanly. After a normal ride, carefully feel near the brake components — an unusually hot wheel or hub points to a dragging brake.
Your Monkey is chain-driven, so the chain deserves attention. A dry, rusty, or overtightened chain absorbs power that would otherwise reach the rear wheel, and it wears the sprockets faster at the same time. Clean and lubricate it regularly, and set tension to the correct free play rather than running it guitar-string tight.
Air Intake and Spark
A clogged or heavily soiled air filter restricts airflow and makes the engine work harder for the same result. On a fuel-injected bike the system can compensate to a degree, but a badly restricted filter still hurts both performance and economy. Inspect the filter and replace it if it is dirty, damaged, or saturated with oil.
Spark plugs are the other half of that equation. A worn, fouled, or incorrectly gapped plug produces a weaker spark and incomplete combustion, which means fuel goes out the exhaust instead of making power. Pull the plug and read its color: a healthy plug has a light tan tint, while heavy black soot suggests the engine is running rich. Because your Monkey uses electronic ignition rather than contact points, there is no points gap or timing drift to worry about — that is one maintenance item you can cross off the list entirely.
Fuel Injection and Fuel System Issues
Fuel injection is reliable, but it is not immune to problems. A leaking injector dribbles fuel when it should be closed, creating a rich condition, a strong fuel smell, and sometimes hard starting. A dirty injector with a poor spray pattern delivers fuel unevenly and burns it less completely. Either way, the result is more fuel used for the same distance.
Temperature sensors feed the ECU information it uses to decide how much fuel to add. If a sensor reports that the engine is colder than it really is, the system keeps adding enrichment long after the engine has warmed up, and fuel economy suffers. If your bike is equipped with an oxygen sensor in the exhaust, a lazy or failed sensor can also push fueling rich, because the ECU loses its feedback on what is actually coming out of the engine.
Stored fault codes are worth checking even if no warning light is illuminated on your dash. A scan tool can reveal codes the system logged without ever lighting the display. Finally, look for actual leaks: fuel lines, fittings, and the tank area. A persistent fuel smell is a strong signal that something is seeping, and it is a problem worth fixing for reasons well beyond fuel economy.
Engine Condition and Modifications
As an engine accumulates miles, compression can drop through normal ring and valve wear, and a low-compression engine simply converts less of the fuel it burns into forward motion. Valve clearance that has drifted out of specification has a similar effect, since valves that do not seal or open properly disrupt the whole combustion process. Both are gradual, so the mileage loss creeps up on you rather than appearing overnight.
Modifications are another frequent culprit. An aftermarket exhaust or high-flow air filter installed without matching fuel tuning changes how the engine breathes, and the stock fuel map may no longer deliver the right amount of fuel for the new airflow. The result is often a rich-running condition, particularly in certain parts of the rev range, along with the extra fuel use that comes with it.
When to Hand It to a Mechanic
Most of the checks above are within reach of a rider with basic tools. But certain signs point to something deeper. A strong, persistent fuel smell, black smoke from the exhaust, spark plugs that come out heavily sooted, rough running that appeared alongside the mileage drop, or a warning light on the dash all justify professional diagnosis rather than more driveway guessing. So does a fuel economy drop that survives new tires at correct pressure, a clean air filter, fresh plugs, a properly adjusted chain, and a few tanks of careful measurement. At that point, the most efficient path is to describe exactly what you have observed — when the change started, how you measured it, and what you have already checked — and let a technician take it from there.
Related Shopping Categories
Shop Fuel System Parts for a 2019 Honda Monkey ABS Motorcycle.
Shop Air Filters for a 2019 Honda Monkey ABS Motorcycle.
Shop Tires for a 2019 Honda Monkey ABS Motorcycle.
Shop Brake Parts for a 2019 Honda Monkey ABS Motorcycle.
Shop Spark Plugs for a 2019 Honda Monkey ABS 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.
