Why Is My 1970 Honda CL175 - Scrambler Motorcycle Getting Bad Gas Mileage?

Why Your 1970 Honda CL175 Scrambler Is Using More Fuel Than It Used To

The CL175 Scrambler is a simple machine by modern standards: an air-cooled, four-stroke parallel twin fed by carburetors, shifted through a chain final drive, and sparked by a contact-breaker ignition. That simplicity is part of its charm, but it also means fuel economy is governed by mechanical condition in a way that a modern, computer-managed bike is not. There is no oxygen sensor trimming the mixture, no engine control module compensating for a worn plug or a dirty filter. If something drifts out of spec on your CL175, the engine simply burns more fuel, and you feel it at the pump long before anything else tells you something is wrong.

First, Confirm the Change Is Real

Before chasing a mechanical cause, make sure the bike is genuinely using more fuel and not just appearing to. The reliable method is tank-to-tank measurement: fill the tank to a consistent level, note the odometer reading or reset the trip meter, ride normally for the full tank, then refill to the same level and divide the miles ridden by the amount of fuel it took to refill. A single tank proves very little, because a windy day, a long highway stretch, or a tank that was not filled to the same point can skew the result. Track three or four consecutive tanks before drawing conclusions.

It is also worth checking whether the odometer itself is telling the truth. A change in rear tire size, or a swap to a different sprocket combination, alters the relationship between wheel rotation and indicated mileage. If the odometer now reads low, your calculated fuel economy will look worse than it actually is. On a chain-driven bike like the CL175, sprocket changes are common and easy to overlook, especially if a previous owner made them.

Riding Conditions and Habits That Quietly Burn Fuel

Not every drop in economy is a fault. Cold weather thickens the air and lengthens warm-up, and many riders leave the choke engaged longer than necessary in chilly conditions. Fuel blends change seasonally as well, and ethanol-blended fuel contains less energy per volume than straight gasoline, so the same riding produces slightly worse results. Headwinds, sustained high-speed running, a passenger, or loaded saddlebags all increase the work the engine must do.

Riding style matters just as much. Frequent hard acceleration, short trips that never let the engine reach full operating temperature, and long idling warm-ups in the driveway all consume fuel without covering distance. If your routine has changed — a longer commute, more stop-and-go traffic, a new passenger — the mileage drop may be explained entirely by how and where you are riding.

Rolling Resistance and Drag: Tires, Brakes, and the Chain

Underinflated tires are the single most common and most easily corrected cause of poor fuel economy on any motorcycle. Low pressure increases the contact patch and the energy required to roll, and it also wears the tire faster. Check pressure cold, with a reliable gauge, and check it often.

A brake that drags is another frequent culprit. On the CL175, the front brake is a drum, and a rear drum brake is also fitted. Worn or poorly adjusted shoes, a sticky cable, or a return spring that has lost tension can leave a shoe lightly contacting the drum, creating constant drag that the engine has to overcome. Feel the hubs after a ride; if one is noticeably warmer than the other, investigate.

Because the CL175 drives through a chain, the chain itself deserves attention. A dry, rusty, or overtightened chain robs power and forces more throttle for the same speed. Lubricate it regularly and set tension so there is a small amount of free play at the midpoint of the lower run.

Air Intake, Spark Plugs, and Ignition Timing

An engine needs the right mix of air, fuel, and spark. A clogged or heavily oiled air filter restricts airflow and upsets the mixture, and on a carbureted bike that often means a richer, thirstier running condition. Inspect the filter, clean or replace it as its type allows, and do not over-oil a reusable element.

Spark plugs tell you a great deal. Worn, fouled, or incorrectly gapped plugs produce incomplete combustion, which wastes fuel and dulls throttle response. Pull the plugs and read their color; a healthy plug shows a light tan deposit, while heavy black soot points toward a rich mixture or weak spark. Because the CL175 uses contact breaker points, the ignition system also depends on mechanical adjustment. Points wear over time, which changes dwell and timing, and timing that has drifted retards or advances combustion in a way that costs both power and efficiency. Setting the points gap and timing correctly is one of the highest-value tune-up steps on this bike.

Carburetor and Fuel Delivery Problems

Carburetor issues are the most likely mechanical cause of a sudden economy drop on a CL175. A choke that sticks partially closed, or a cable that does not fully release, enriches the mixture constantly. A float that has sunk, a float height set too high, or a worn needle and seat can allow too much fuel into the bowl and into the engine. Jets that have gummed up from fuel sitting in the bowls over a winter can also disrupt the mixture, sometimes in ways that make the engine run rich at certain throttle positions.

Because this is a twin with a carburetor feeding each cylinder, the two must be synchronized. If one side is opening earlier or further than the other, the engine runs unevenly and the rider compensates with more throttle. Finally, check for leaks: a cracked fuel line, a weeping bowl gasket, or a petcock that seeps will waste fuel whether the engine is running or not.

Engine Wear and Aftermarket Changes

As mileage accumulates, compression falls. Worn rings, tired valve sealing, and cylinder wear all reduce the efficiency with which the engine converts fuel into motion, and no amount of carburetor tuning will restore it. A compression check is the straightforward way to assess this.

Modifications deserve a mention too. Fitting an aftermarket exhaust or a freer-flowing air filter without matching carburetor jetting commonly leaves the engine running rich, which feels fine at the seat but shows up as reduced range. If a previous owner changed the intake or exhaust, the jetting should be reviewed to match.

When to Hand It to a Mechanic

Some symptoms point to problems beyond routine tuning. A strong, persistent fuel smell, black smoke from the exhaust, spark plugs that turn sooty again quickly after cleaning, or rough running that accompanies the mileage drop all suggest a deeper issue. So does a fuel economy decline that continues after you have confirmed tire pressure, freed up the brakes, serviced the chain, replaced the plugs, and checked the air filter. At that point, describing what you have already ruled out — and how many tanks you have measured — will help a mechanic move straight to the likely cause rather than starting from scratch.

Related Shopping Categories

Shop Fuel System Parts for a 1970 Honda CL175 - Scrambler Motorcycle.

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Shop Brake Parts for a 1970 Honda CL175 - Scrambler Motorcycle.

Shop Spark Plugs for a 1970 Honda CL175 - Scrambler 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.