Why Is My 1981 Honda CM400A - Hondamatic Motorcycle Getting Bad Gas Mileage?

Why Your 1981 Honda CM400A Hondamatic Is Using More Fuel Than It Should

The 1981 Honda CM400A is a machine with its own personality. It is a four-stroke, air-cooled parallel twin with carburetors, a chain final drive, and electronic CDI ignition – and of course, the two-speed Hondamatic transmission with its torque converter instead of a clutch lever. That last detail matters more than most riders expect when fuel economy is the question, because a torque converter works by design through a certain amount of slip, and the range you leave the transmission in changes how hard the engine works at any given road speed.

If your CM400A has started going through fuel faster than it used to, or you find yourself stopping for gas more often on roads you know well, the cause is almost always mechanical, behavioral, or a combination of both. Nothing about the bike's basic design changed on its own. Something in the way it breathes, sparks, rolls, or delivers fuel has.

Measure the Change Before You Chase It

Fuel economy complaints are easiest to solve when you have a real number to compare against, so start by establishing a baseline. Fill the tank to a consistent level, note the odometer reading or reset the trip meter, then ride normally for the full tank. At the next fill-up, record the miles covered and the amount of fuel it took to refill to the same level, then divide miles by fuel added. Repeat this for two or three tanks, because a single tank is easily skewed by a windy day, a lot of city riding, or one long idle session in the driveway.

One detail that fools a lot of riders: if you have changed tire sizes, or swapped sprockets on this chain-driven bike, the speedometer and odometer are no longer telling the truth. A smaller rear tire or a tooth change can make the odometer over-report distance, which makes fuel economy look better or worse than it actually is. Before you blame the engine, confirm the mileage you are calculating is real.

Riding Conditions and Hondamatic Habits

Some of the biggest swings in fuel use come from outside the engine. Cold weather thickens lubricants, lengthens warm-up time, and increases the fuel needed just to keep the engine running smoothly. Winter-blend and ethanol-blended gasoline also deliver slightly less energy per volume, so a seasonal drop in economy is normal. Headwinds, sustained highway speeds, a passenger, or loaded saddlebags all add drag and weight, and aggressive throttle use from every stop sign is one of the fastest ways to empty a tank.

The Hondamatic adds its own variable. Because the torque converter relies on fluid coupling, some slip is normal and expected, and it costs a small amount of efficiency all the time. What you control is the range selector. Cruising at steady road speed in the low range keeps engine rpm unnecessarily high, which burns extra fuel for no benefit. Short trips and long idle warm-ups compound the problem, since the engine spends most of its time cold and inefficient.

Tire Pressure, Dragging Brakes, and Chain Drag

Soft tires are the most common and most overlooked cause of poor fuel economy on any motorcycle. Underinflation increases rolling resistance, and on a bike this age, a slow leak from an old valve stem or a corroded rim can go unnoticed for weeks. Check pressures cold and inflate to the values you normally run.

Next, confirm the wheels spin freely. A front disc caliper that is sticking slightly, or a rear drum brake with a shoe that drags, will hold the bike back continuously and heat up the affected wheel. Feel the hubs and rotor after a short ride – an unusually hot wheel points to a dragging brake. On the CM400A, the chain also deserves attention: a dry, rusty, or overtightened chain robs power and adds friction, and worn sprockets multiply the loss. A correctly adjusted, properly lubricated chain is free efficiency.

Air Filter and Ignition Components

These carburetors meter fuel based on how much air passes through them, so a clogged or heavily oiled air filter richens the mixture across the board. If the filter has not been cleaned or replaced in a long time, that alone can account for a noticeable drop in economy. Worn or fouled spark plugs, or plugs with the wrong gap, cause incomplete combustion – fuel goes in but does not burn completely, and you pay for it at the pump. Plug caps and plug wires that are cracked or corroded create the same effect.

Ignition timing is worth a mention only to rule it out. Your CM400A uses electronic CDI ignition rather than contact breaker points, so there are no points to wear or gap, and drifting timing from point wear is not part of the picture on this bike.

Carburetor Problems That Quietly Run Rich

If everything above checks out, the fuel system itself becomes the prime suspect. A choke or enrichener circuit that sticks partly open, or a cable that no longer returns fully, keeps feeding extra fuel long after the engine is warm. This is easy to miss because the bike often still runs acceptably.

Float level is the next thing to look at. A float that has sunk, a needle and seat that no longer seals, or a float height that has drifted high will raise the fuel level in the bowl and richen the mixture at every throttle opening. That condition often comes with a faint fuel smell, fuel weeping from a vent, or a drop in oil level that does not match oil use. On this parallel twin, the two carburetors also need to work as a pair: when they are out of sync, one cylinder does more of the work than the other, and the engine as a whole runs less efficiently than it should.

Do not overlook the simple stuff either. Old fuel left sitting in the bowls leaves gum and varnish that partially block the jets and air passages, producing a rich, uneven mixture that gets worse the longer the bike sits. Finally, check for leaks anywhere in the system – fuel lines, the petcock, and the float bowl gaskets – since a small seep you never notice on the garage floor is still fuel you paid for.

Engine Wear, Modifications, and When to Get Help

An engine that has lost compression through worn rings or poorly sealing valves has to work harder for the same road speed, and fuel economy falls accordingly. Signs include harder starting, rough running, and oil consumption that has crept up over time. Aftermarket exhausts or high-flow air filters installed without matching carburetor jetting belong in the same conversation. Mismatched fueling can leave the engine running rich in some circuits and lean in others, and it rarely improves economy.

Bring the bike to a mechanic if you notice a strong fuel smell, black exhaust smoke, sooty spark plugs, a dashboard warning light if your bike has one, or rough running that appeared alongside the mileage drop. The same goes if your calculated economy stays poor after you have corrected tire pressure, cleaned the chain, and checked the brakes and filter. At that point the answer is inside the carburetors or the engine, and a professional diagnosis will save you far more than it costs.

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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.