Q: How Do You Tell if a Carburetor Is Rich or Lean? A: One way to tell for sure is by “reading” the spark plugs. If the plug tip is white, the mixture is lean. If it’s brown or black, it’s rich.
Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns. If your carburettor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube.
* Here, if you turn the screw towards the engine (CLOCKWISE) the carb sends in more fuel. This is also referred to as “running rich”. Whereas, if you turn the screw towards the air filter (COUNTERCLOCKWISE), the carb sends in more air. This is referred to as “running lean”.
But just for reference (for those who haven’t experienced both) a lean bog is a hollow “BOOOOGGGGGGGG” sound. A rich bog is a sputter stutter.
On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.
Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn’t have enough gasoline can cause a backfire, too. A “lean” mixture is one that doesn’t have enough fuel, and too much air.
If the coil is vertical and the screw is at the bottom, you need to turn the screw counterclockwise if you’re looking at it from above. Turn the screw 1-2 times to make minor adjustments, or 3-5 times to dramatically increase the idle RPM.
An air screw adjusts how much air is being delivered thru the pilot circuit: in is rich (less air) and out is lean (more air). A fuel screw adjust how much fuel (or air/fuel mixture) is being delivered from the pilot circuit. In is lean (less fuel) and out is rich (more fuel).
If a motorcycle or ATV only runs with the choke on, it’s because the richer “choke on” mixture is actually closer to the engine’s normal operating fuel mixture than the leaner “choke off” mixture. So when the choke is turned off, the engine gets too little fuel and too much air for it to run and it stalls.
A faulty oxygen sensor sending the wrong signal to the engine control module can result in a rich fuel condition. Symptoms include black-colored exhaust, fouled spark plugs, and poor engine performance.
This can be caused by a failing fuel pump releasing too much gasoline into the engine, a problem with your carburetor’s adjustment, leaking fuel injectors or ones stuck in the open position, a faulty MAF or oxygen sensor, a faulty engine control unit, and a clogged air filter.
Regardless of whether or not the engine is running too rich or too lean, bring it down to a very lean mixture by turning both screws a quarter-turn at a time, counter-clockwise, then slowly bringing them back up to an equal and smooth mixture.
TLDR – Consistently running rich can damage the internal engines in a motorcycle. Unburnt fuel from a rich mixture can stick and get deposited into cylinder walls, pistons and exhaust valves. Over time, these deposits can cause wear and tear damages to engines and exhausts.
If you have codes stored in the engine computer for lean bank 1 and 2 is caused by the entire engine is running lean. This is caused by not enough fuel getting into the engine or too much unmetered air getting into the engine and the computer cannot compensate anymore to run it normal.
Problems with air fuel ratio sensors are common. Often a sensor gets contaminated or simply fails. In some cars, the heating element inside the sensor fails causing the malfunction. For example, in many Toyota and Honda cars the code P0135 may be caused by a failed heating element inside the sensor.
Lean or Rich Mixtures
When an air/fuel mixture has too much fuel, it is rich. When there is not enough fuel, it is lean.
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