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Carburetors What does it mean to “re-jet” the carburetors?

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Carburetors supply a mixture of gasoline and air to the engine. The gas comes out of little things called “jets” (usually a pilot jet, needle jet and main jet). If you modify your airbox or exhaust to flow more freely, you get more air going through your system. This often means that there is not enough gasoline, so the engine runs badly. “Rejetting” means replacing or modifying some of these jets so as to supply gasoline differently than stock, usually more gasoline than stock. If done correctly, this allows your engine to generate more power, run a little cooler, start and run smoother and get worse mileage.

Posted on Nov 20, 2008

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SOURCE: re jet the carburetors

The November 10, 2008 solution is correct but a bit more should be added. > > Forget modifing the jets. The effort will be a waste at best. Instead, just go to the dealer and tell him the problem. Jets are a VERY low cost item. If in a high altitude state like Colorado a smaller jet may be needed since less oxygen requires a thinner fuel mix. If at sea level, a larger jet may be appropriate. The only jet needing replaced at any time is the main jet. The needle jet is a variable flow jet and the flow is controlled by the throttle.

Posted on Mar 11, 2009

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    • 3 Remove the stock jets from the right and left side of the carburetor with the screwdriver. The jets are the gold pins that are in clear view.
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    • 1 Remove the seat by unscrewing the bolts found on the four corners of the seat. Remove the clamps from the top of the air box using a pair of pliers to access and remove the air filter. The air filter is removed by unscrewing the wing nut and sliding the filter out of the box. Loosen the hose clamp that connects the carburetor to the intake manifold, using a screwdriver.
    • 2 Loosen the fuel delivery hose clamp with a screwdriver and pull the carburetor out of the Blaster. Before troubleshooting the carburetor, it's important to note whether you've made a significant change in altitude or a change in your exhaust/intake setup. If you've made an altitude change, installed an aftermarket exhaust pipe or modified your air box, the jets will need to be changed. Refer to your manual for specific re-jetting instructions based on your altitude. Your exhaust kit will provide a recommended jetting setup. Do-it-yourself modifications like removing an air box will require you to experiment with the jet setup until you find the right one.
    • 3 Disassemble the carburetor by unscrewing the bowl screws from the bottom of the carburetor. With the bowl removed, the float, pilot jet and main jet will be exposed. To remove the pilot and main jet, unscrew them from their respective holes. The main jet will sit in the exact center of the bowl, while the smaller pilot jet will be set just below the main jet.
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    • 4 Inspect the pilot jet for blockage, slide a thin wire through the pilot jet's hole, spray it with carburetor spray and blow the center hole with compressed air. Repeat this process with the main jet. If there is any blockage or wear, replace the jets. If you have made altitude changes or exhaust/intake modifications, insert jets that correspond to your changes as recommended in your manual. Note that this may take some experimentation with different jets to get the correct jet setup, as there are many jet sizes for different scenarios.
    • 5 Inspect the bowl, floats and ports for any signs of debris or gunk. If debris or gunk is found, remove it using a spray carburetor cleaner, soft wire brush, or thin wire (to insert into small orifices). If no gunk or debris is found, you've ruled this out as a possible cause for the starting issues, and can focus on the replacement of carburetor components.
    • 6 Look for abnormal wearing on the jets, float valve, gaskets, O-rings and springs. If you have not changed altitude or modified your bike in any way, then it is best to inspect these components. Any one of these components, or a combination, may be contributing to your starting issues. You may choose to replace individual parts such as a new float valve, though replacing all the interior components with a carburetor "rebuild kit" is advised. Rebuild kits can be ordered online or through your dealer, and will ensure your carburetor is fully repaired.
    • 7 Slide the gasket, spring, spring holder and needle jet assembly back onto the carburetor cap and insert the assembly into the top of the carburetor. Screw the main jet into the bottom of the needle jet assembly by accessing it through the bowl area. Screw the pilot jet back into its hole below the main jet. Clip the float valve onto its hinge within the bowl a insert the bowl gasket. Screw the bowl to the bottom of the carburetor to complete the re-assembly process.
    • 8 Re-attach the carburetor to the intake manifold using the hose clamp and screwdriver. Slide the fuel feeder line onto the brass receiver port on the side of the carburetor and slide the air box boot over the carburetor's intake port, which should be facing toward the back of the bike. Re-install the air filter with its wing nut, slide the clips over the air box top to secure it to the box and slide the seat into place. Re-install the seat screws to complete the re-assembly process.
    • 9 Prime the carburetor by twisting the throttle once and start the Blaster.

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I just rejetted ( at least I think I did ) my carburetor on my vstar 1100 custom. The problem is that the Cobra Carb Jet Kit 92-2170 only had main jets.... five of them. Does this mean that only the main...

In many case when you rejet your carburetors you are only changing the mains and the needles(to get rid of the acceleration bog) If they give you several size jets to try, be sure you are putting the right sizes in the right spots. Yamaha typically will run one carburetor leaner than the other. Depending on how the cycle is running , I have known people to match the jets in both carburetors. Check the color of your plugs to double check if you have set the cycle up correctly.

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1 Answer

1985 yamaha maxim 700, Runs good through first half of throttle but when you try to get past that it just boggs down. Carbs have been cleaned. Spark plugs are jet black after a quick ride? Any help is...

Hello, from your description, it sounds like some of the jets of your carburetors are in the wrong places. Your bike has four carburetors, and each one of them has a main fuel jet and an idle fuel jet inside the bowl. The main jet goes in the very middle of the bowl and is the one that screws into the needle jet. This main jet has a relatively large hole through the middle. The pilot jet goes off to the side of, and screws right into the metal of the carburetor body. This jet has a very small hole through the middle -- so small that it may be hard to see through it.

These two jets look similar at a glance, and they have identical threads which makes it easy to get them in the wrong place. You mention that your carburetors have been cleaned, so there is a good chance that whoever cleaned them accidentally switched these jets in some or all of your carburetors.

If these jets are switched you will notice the following symptoms:
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  2. Your spark plug electrodes will be very dark black after even a little running.
  3. It will feel like you have little power from the middle of the throttle all the way to wide open throttle (WOT).
  4. If you run like this for a while you will start to get backfiring, and the bike will start to run on three and then two cylinders, and perhaps eventually not at all.
But don't worry, fixing this problem is easy. It will be especially easy if you are the one who switched the jets in the first place. You just have to take your carburetor rack off of the bike, and remove the four bowls from the bottom. Look at the main and pilot jets in each of the four carburetors. The main jet should have a biiger hole and will have a tiny stamped number that should be around 120 or 130 on the top. The idle jet should have a small hole and a tiny stamped number around 15 or 20 in the top. Be sure that the jets are in the right places, and put your carburetors back together. It is also a good idea to get new spark plugs after you correct the problems, or thoroughly clean your old ones as carbon buildup on the spark plugs may cause a weak spark.

In the attached picture the main jet is toward the bottom, and the idle jet is on the top.

Good Luck,

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Re jet the carburetors

Carburetors supply a mixture of gasoline and air to the engine. The gas comes out of little things called “jets” (usually a pilot jet needle jet and main jet). If you modify your airbox or exhaust to flow more freely you get more air going through your system. This often means that there is not enough gasoline so the engine runs badly. “Rejetting” means replacing or modifying some of these jets so as to supply gasoline differently than stock, usually more gasoline than stock. If done correctly, this allows your engine to generate more power, run a little cooler, start and run smoother and get worse mileage. ,,,

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