How to Adjust the Carburetor Mixture Ratio?
3 Answers
Check the tightness of the connections related to the carburetor to ensure normal air intake without any air leaks or intake blockages. Turn the carburetor mixture screw clockwise all the way, then turn it counterclockwise 1.5 turns. Start the engine and let it warm up for 10 minutes to reach normal operating temperature. Adjust the carburetor idle speed screw so that the engine runs at the lowest possible speed without stalling. Repeatedly adjust the carburetor mixture screw left and right to achieve the highest engine speed.
I've tuned quite a few carburetors myself, and the key to this job is patience. First, start the car and let it warm up for ten minutes—the engine needs to reach normal temperature for proper tuning. Locate the small spring-loaded screw on the side of the carburetor—that's the air-fuel mixture screw—and turn it slowly with a flathead screwdriver. Turning it clockwise enriches the mixture, while counterclockwise leans it out. Listen to the engine as you adjust; the moment the RPM suddenly spikes, stop—that's the sweet spot. Then, adjust the larger idle screw nearby to bring the RPM back down to around 1500. If the RPM starts jumping around, you'll have to start over. Test-driving is crucial after tuning—take it for a spin to check if acceleration is smooth and whether black smoke comes out of the exhaust. The screw directions may vary by carburetor brand, so it's best to check the manual beforehand. If tuned right, you can squeeze out dozens more kilometers per tank.
Just helped a friend adjust the air-fuel mixture on his old motorcycle a couple of days ago, and found this job really tests both your ears and touch. You've got to warm up the engine until the fan kicks in before starting. The mixture screw is hidden at the bottom of the carburetor, and on some models you need to remove the casing to reach it. I usually turn it just 1/8 of a turn at a time, immediately listening to the engine's response. When the RPM reaches its peak and stabilizes, that's when the fuel-air ratio is just right. Remember to remove the air filter before adjusting, otherwise dust getting sucked in would cause more trouble. After finishing the adjustment, don't rush to wrap up - test several cold starts repeatedly to check if the ignition is crisp. If the mixture is too lean, cold starts become particularly difficult, and if too rich, it's easy to flood the spark plug. It's best to do the whole process in a well-ventilated area - that gasoline smell can really give you a headache.
