Why Is My 1980 Honda CM400A - Hondamatic Motorcycle Getting Bad Gas Mileage?

Why Your 1980 Honda CM400A Hondamatic Is Burning More Fuel Than It Should

The 1980 Honda CM400A Hondamatic is an unusual machine in a way that matters here. It is an air-cooled, four-stroke parallel twin fed by a pair of carburetors, sending power to the rear wheel through a chain — and between the engine and that chain sits Honda's two-speed Hondamatic transmission, which replaces the clutch lever with a torque converter. That automatic makes the bike remarkably easy to ride, but it also changes how the bike consumes fuel and changes which causes are most likely when consumption starts climbing.

If you are stopping for fuel more often than you used to, or your tank-to-tank math has gotten worse, the items below cover the realistic culprits for this specific bike, roughly in the order a rider can work through them.

First, Confirm the Drop Is Genuine

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 divide the miles ridden by the fuel added. One tank is not enough to judge anything — a short fill, a long warm-up, or a windy day can skew a single result. Track three or four tanks before you conclude the bike has a problem.

Also consider whether your numbers are being distorted. A change in tire size or profile alters how far the bike actually travels per revolution, and on a chain-driven bike like this one, swapping sprockets does the same thing. Either change throws off the speedometer and odometer, which makes your calculated economy look worse than reality even when the engine is running perfectly.

The Hondamatic Itself Costs You Fuel

A torque converter transfers power by slipping, and that slip is energy turned into heat rather than forward motion. It is most pronounced pulling away from a stop and at low speeds, which is exactly where a lot of CM400A riding happens. The two-speed gearbox compounds the effect: at steady highway speed the engine turns faster than it would in a comparable manual-shift twin, and higher rpm at the same road speed means more fuel burned per mile.

How you use the selector matters too. Riders who leave the bike in the lower range around town, or who never let it shift up on open roads, will always use more fuel than someone who lets the transmission do its job. If the torque converter's fluid is low or long overdue for a change, the converter can slip more than it should, and that extra slip shows up as wasted fuel and extra heat.

Riding Conditions and Habits

Cold weather is a common and completely mechanical explanation. A cold engine needs a richer mixture, takes longer to reach full operating temperature, and denser cold air requires more fuel to match. Winter-blend and ethanol-blended fuels can also reduce the energy you get per gallon. Headwinds, a passenger, loaded saddlebags, and sustained high-speed running all increase the load the engine has to pull.

Habits matter just as much. Short trips where the engine never fully warms, long idling warm-ups in the driveway, and aggressive throttle from every stop all burn fuel without covering distance. The Hondamatic makes hard launches effortless, which is precisely why they are easy to fall into.

Rolling Resistance: Tires, Brakes, and the Chain

Check tire pressures cold, front and rear, and check them often. A tire that is only a few pounds low adds drag at every speed and is the single easiest thing to rule out. Next, feel the wheels after a ride: a front disc caliper with sticky pistons, or a rear drum brake with a misadjusted linkage or a tired return spring, can drag enough to noticeably cost fuel.

Because this bike is chain-driven, the chain deserves attention. A dry, rusty, or overtightened chain robs power, and worn sprockets do the same. Set the slack correctly, keep the chain lubricated, and replace the chain and sprockets together when they are worn.

Air Intake and Ignition

A clogged or dirty air filter richens the mixture on a carbureted engine, and a rich mixture is wasted fuel. Worn, fouled, or incorrectly gapped spark plugs cause incomplete combustion, so some of the fuel you paid for leaves unburned. The CM400A uses an electronic ignition rather than contact breaker points, so you are not chasing points wear or timing drift, but a weak coil, a failing pickup, or a corroded plug cap can still hurt efficiency. Reading the plug color is a quick way to see whether the engine is running rich.

Carburetor Problems That Cause a Rich Condition

The most common carburetor culprit is the choke, or enrichener, not fully releasing. A cable that sticks or a lever left partly engaged keeps the mixture rich long after the engine is warm. A float level set too high, or a worn needle and seat that lets fuel past, produces the same result and can also cause fuel to dribble into the intake.

Fuel left sitting in the carburetors for months leaves varnish and gum that restrict passages and upset the mixture. The CV-style carburetors on this bike also rely on rubber slide diaphragms, and a torn or hardened diaphragm makes the engine run poorly and rich. Because this is a two-cylinder with two carburetors, they also need to be synchronized — if one cylinder is doing more than its share of the work, fuel economy suffers. Finally, look for leaks: a weeping float bowl gasket, a cracked fuel line, or a dripping petcock wastes fuel steadily and is a genuine hazard.

Engine Condition and Running Temperature

Low compression from worn rings or poorly sealing valves reduces efficiency, meaning you need more throttle for the same speed. Valve clearance that has drifted out of spec affects how well the engine breathes. This engine is air-cooled, so there is no thermostat to fail, but it still needs to reach proper operating temperature to run efficiently — and packed dirt or debris in the cooling fins can push temperatures the other way and hurt performance.

Modifications That Upset the Mixture

An aftermarket exhaust or a high-flow air filter installed without matching carburetor jetting leaves the mixture wrong across much of the rev range. The bike may still run, but it often runs richer than it needs to at part throttle and makes less power, so you end up using more throttle to hold the same speed. If the intake or exhaust has been changed, the carburetor settings should be revisited.

When to Hand It to a Mechanic

Bring the bike in if you smell raw fuel, see black smoke from the exhaust, find sooty black spark plugs, or notice rough running alongside the mileage drop. The same goes for a fuel economy decline that persists after you have checked tire pressure, the chain, the air filter, and the choke. Describe what you have observed — how many tanks you tracked, when the drop started, and whether the bike runs differently — and a good technician can narrow it down quickly.

Related Shopping Categories

Shop Fuel System Parts for a 1980 Honda CM400A - Hondamatic Motorcycle.

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Shop Tires for a 1980 Honda CM400A - Hondamatic Motorcycle.

Shop Brake Parts for a 1980 Honda CM400A - Hondamatic Motorcycle.

Shop Spark Plugs for a 1980 Honda CM400A - Hondamatic 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.