What Does It Mean When A Motorcycle Is Running Lean?

Motorcycles have evolved into complex, computer-controlled machines in the 140 years since William Daimler built his wood-frame Reitwagen. WIlliam Harley and Arthur Davidson followed with their Model 1 around two decades later, but that was also essentially a bicycle with a motor mounted to it. Motorcycles and motorcycling took big leaps forward in the 1970s with the debut of powerful bikes like the Honda CB 750 and CR 750 and the engineering breakthrough that was the Kawasaki Z1.

Just about all modern motorcycles are fuel-injected, but those '70s icons and most older models relied on carburetors to deliver a mixture of gasoline and air to the engine's cylinders. While a fuel injection system uses sensors and a computer to deliver an optimal mixture as conditions change, a carburetor requires more attention and adjustment to keep your bike running at its best. 

If the carburetor is delivering too much air or not enough fuel, this is known as running lean. The opposite condition, where too much fuel or not enough air is in the mixture, is called running rich. If your engine is running lean, it may be hard to start, accelerate poorly, or stall at idle. Your engine and exhaust might be hotter than usual, and if you remove the spark plugs, you might notice that they are unusually clean.

A lean condition can be a simple case of a poorly adjusted carburetor or a change in altitude, or can result from restricted or failing fuel system components. Carburetors can be finicky beasts that require a delicate hand to troubleshoot and adjust, but we'll hit the basics here. Carburetors have adjustment screws, which will change the amount of air or fuel in the mixture. The video below is a good quick primer as to how to identify which screw changes airflow and which adjusts fuel flow, but it's best to hunt down the factory service manual for your particular bike or carburetor.

If adjusting your carburetor doesn't correct the lean condition, you might have clogged jets, a bad accelerator pump, or a bad fuel pump. There could also be a leak somewhere in the fuel delivery system or carburetor, letting fuel escape or air enter. A fuel system cleaner can help clear up minor obstructions, and fuel system pressure and volume tests will tell you if the fuel pump is working properly. A change in altitude often requires the installation of new jets, as the difference in air density can be significant. Your carb may run lean as you descend and the air gets thicker.