What Is The Main Jet On A Carburetor For?

Why do you jet a carburetor?

In simple terms jetting your dirt bike’s carburetor is determining the proper air-to-fuel ratio the engine receives. Running the correct ratio of fuel and air makes your dirt bike run optimally. Installing jets of different sizes helps increase/decrease the fuel/air ration depending on what your bike requires.

What happens if main jet is too small?

Be careful: most engine jams occur when the main jet is too small. A lean fuel/air mixture will cause the engine to overheat fast.

How do you adjust the main jet on a carburetor?

Carb Quick Guide

  1. Verify carburetor is set to stock settings:
  2. Start bike, bring to operating temperature.
  3. Set idle speed adjusting screw, clockwise to increase rpm, counter-clockwise to decrease rpm.
  4. Adjust idle mixture by turning idle mixture screw slowly clockwise until the engine runs poorly.

Where is the main jet on a carburetor?

The main jet is located at the bottom of the carburetor inside the float bowl. It’s the larger of the two jets, and it is installed in the needle jet—that part we will be covering next week. If this guy is plugged up, you’re in for a full carb cleaning ritual.

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What is the pilot jet in a carburetor?

The pilot jet handles the mixture from idle to 15 to 20 percent throttle. Air comes in through the front of the carburetor drawing fuel up through the pilot with a vacuum that is created as the air flows through the pilot circuit.

What does changing the pilot jet do?

Changing the main jet size will adjust the top end of the operating range from half to full throttle. Adjusting the needle jet size, jet needle, and needle clip position will affect the mid range operation. This leaves the pilot jet size and the pilot screw for the idle and low throttle conditions.

How do I know if my jet is too small?

If the bike surges or hunts, the pilot jet is probably too small (lean). Other indications that the pilot circuit is lean are popping or spitting through the carburetor when the throttle is opened and popping or backfiring through the exhaust when the throttle is shut.

How do I know my main jet size?

Consult your manual first to find the stock size. Also pay attention to the numbers on the side of your old jets, which you’ll be able to see in the steps to come. A larger jet number means a bigger hole and more fuel. A smaller jet number has smaller holes and should be used if your engine needs to run more lean.

How do I know if my carburetor is rich or lean?

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.

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How do you adjust a carburetor air fuel mixture?

Turn the screw clockwise until the engine starts to sound rough.

  1. Tightening the screw weakens the air and fuel mixture and decreases the amount of fuel flowing to the engine.
  2. Tightening the screw is also called making the fuel mixture leaner, which lowers the RPMs at which the engine idles.

How do you fix carburetor rich air/fuel mixture?

Once the engine has arrived at operating temperature, go back to the carburetor and adjust the air fuel mixture screw, or screws. Tightening the screw increases the amount of fuel, while loosening decreases the amount of fuel.

Can you drill out carb jets?

By drilling out your stock main jet to larger sizes you will be able to narrow down what jet size your carburetor needs. You will need a numbered drill bit set. If it’s lean and you need to increase the size of your main jet, drill the jet out with the next size larger drill bit, which in this case is the #60 bit.

Does main jet affect idle?

The main jet does not effect the jetting for starting and idling. It plays no part on low RPM or mid RPM jetting either. The main jet is very important to your machines overall tuning, but should never be over emphasized at the expense of needle tuning or other facets of your carburetion tuning.

Are all carb jets the same?

Yeah, most Keihin jets are interchangeable and most Mikuni jets are interchangeable (though the two brands are not interchangeable with eachother). Any motorcycle dealer should be able to get jets for your bike even if they do not sell the brand of bike you have.

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