New Exhaust System = Carb Jetting?

When you upgrade your bike with a new exhaust system, air box or hi-flow air filter it may upset the air to fuel ratio creating a mixture that is too rich or too lean.

  • A rich ratio is one where there is too much fuel in relation to the air flow. Signs that your air/fuel mix is too rich are black smoke in the exhaust, blackened and fouled spark plugs and a poor idle.

  • A lean ratio means you are not getting a sufficient amount of fuel in relation to your flow of air. Your mix may be too lean if you are getting excessive backfiring on deceleration, blue-ing of the exhaust pipe or spark plugs that seem not to be functioning, but look very clean. You may also get a hesitation or "dead spot" during acceleration.
  • Blue-ing of exhaust pipes is a telltale sign of a lean fuel mixture.

    However, some replacement pipes have very thin walls. They are more prone to turn blue because of extreme temperatures passing through a bend. There are products, made by Kreem, to shield the pipes from the inside. It involves coating the inside of the pipes with a liquid barrier. We will cover this on another page.

    Adjusting the fuel to oxygen ratio

    There are several ways you can adjust your carb jetting problems but, generally if a bike is running lean you increase the jet size. The larger the jet, the higher its number.

    If a bike is running rich, you need to decrease the jet size. Click here to read more about Carb Jets.

    A few manufacturers' carbs give you the ability is make minor adjustments without opening the carburetor housing. Some minor air/fuel ratio problems can be alleviated by adjusting the air fuel mixture screw on the carburetors.

    Please read the entire topic before you begin to avoid unnecessary work.

    Road or drag racing bikes may require very drastic and specialized technical alterations but still follow the basic guideline below.




    Before You Begin

    When making alterations to your carb jetting or any OEM settings or equipment please make some notes to record your original settings before you begin to make changes. You could decide that you do not like the sound of your new pipes and want to put the stock exhaust back on.

  • Read the entire page before you start taking things apart.
  • Take precautions for fire prevention (extinguisher handy, no smoking, splash guard or safety glasses).
  • Get ready to mark down current information: jet sizes, needle position and clip location, etc.
  • Record how the bike starts, idles, accelerates, decelerates and responds to throttle in low, mid and top RPM range. This step is important - do not skip it.

  • Now that you are ready to get down to work, let's make sure you have the tools and supplies that you will need.

  • safety glasses or splash guard
  • rags
  • fire extinguisher (water does not work on a gas fire)
  • notepad & extra pens
  • digital camera (not necessary, but really helpful memory aid)
  • gasoline resistant drain pan
  • tray for small parts (one for each carb)
  • assorted sizes of screw drivers
  • dental or wire picks (Harbor Freight sells good ones)
  • magnifying glass
  • telescoping magnetic tool (magnet on a stick)
  • carburetor cleaner, SuperTech carb cleaner, sold at WalMart works well. Some other brands can dissolve rubber and plastic parts found in the carbs.
  • spray lubricant
  • All set? Good. Now go outside.

    Be sure your bike is facing toward the road or unobstructed exit. If you have a fire issue you can push it out of the way easily and you won't need to turn it around to take it for a test ride.

    Your working environment when dealing with fuel is very important.

    Carb jetting or any work with gasoline is best done outside for many reasons. Fresh air and great light are two. Not burning down your garage if you have a flare up is another.


    Get Started

    Let's begin our carb tuning with a question. How does the bike start?

    Start the motorcycle cold with the choke on and see if there are any signs that it is loading up with fuel. Decrease the choke to lessen the fuel. Allow the bike to warm up to riding temperature, about two minutes.

    How does the bike respond to a standing throttle range? Is it smooth in the low RPM range from idle to about 1200? How is it in the mid-range 1200 to 3000 and the top-end of 3000 to near redline?

    Record your findings. This step is important do not skip it.


    Correcting Problems

    Pilot or Slow Jet

    Problems experienced at low RPMs are best corrected by pilot or slow jet replacement. First loosen the float bowl drain bolt and empty the bowl. Be sure that the fuel is going down the OEM drain hoses into the drain pan on the floor. Do not allow the fuel to run down the engine. This can ruin block paint. Or worse, fuel can spill on to the starter motor or hot pipes, causing a fire when the bike is cranked.

    When the bowl is empty, remove the float bowl from the carb housing. Identify the pilot jet. It is usually off center of the main jet. Pilot jets are generally down in a tunnel and have a regular screwdriver slot. They can be covered by a rubber plug or splash shield. It is best to use a high quality screw driver that fits snugly into the slot in the jet for this next part. Five minutes of preparation to find the right screwdriver is better than days drilling jets out or searching for replacement carbs.

    Once you have located the pilot (slow) jet place the screwdriver into the slot and lightly strike the handle with a small hammer. This will jar the brass jet from the aluminum housing. Take your time and do this right. If you strip this jet it may be over for your carb.

    Once the jet has been removed, use a magnifying glass to read the jet size. Remove the rest of the pilot jets and record the pilot jet sizes one at a time. Do not assume that they are all the same size. Some bikes run different sizes for inner and outer or front and rear cylinders. The inner and rear cylinders can run hotter and may require more fuel than the others.

    There is no definitive answer on what size to change your jet to. In has been my experience that a bike that has had an aftermarket air filter or exhaust upgrade will occasionally need an increase in pilot jet size.

    For example if a bike has a new exhaust system added and now will not hold an idle and / or stumbles at the low end of the RPM range and originally had a size 42 pilot jet, a size 45 or 48 pilot jet would be installed. This change will only fix the problems in the RPM area below about 1200. If your bike responds positively, you are moving in the right direction in correcting the air to fuel ratio.

    Fluid Needle

    Once the bottom end or low RPM area is running correctly, it is time to address the mid-range RPM, also known as the acceleration portion of riding. Problems in this RPM range are corrected by adjusting or changing the fluid needle located in the top of the carburetor.

    There are several different types of carburetors. Each is a little different. If you are not familiar with your machine's carb type please review Carb Type page.


    In common CV or constant velocity carbs the top of the carb houses the barrel, air diaphragm or air slide. This area can be confusing and frustrating to those unfamiliar with it. Slowly, take the top off the carb and it should expose a long spring and the air slide. Remove the slide and spring by picking them straight up. Do not turn the carb over and shake them out. Do not turn the slide upside down, parts may fall out.

    Old style carburetors, known as mechanical carbs, will not have springs or air diaphragms. They do have the fluid needle and the procedure below is similar.

    Inside the slide is a fluid needle, needle clip and retainer. These items may vary in shape and there may be additional parts depending on your carburetor style.

    Fluid needles come in two types, adjustable and non-adjustable. If your needles can be adjusted, there will be a clip on the needle and possibly a washer under the clip. The lower the needle hangs into the carb, the less fuel you get. By adding a washer under the clip or needle head, you raise the needle up out of the carb, thereby increasing fuel supply in the mid RPM range. Carburetors with adjustable fluid needles have notches to lower the clip (raise the needle) and increase fuel supplied.

    Carburetors with non-adjustable fluid needles benefit from the purchase of a jet kit (vs. buying individual jets) because most kits come with a replacement, adjustable needle.

    You will find that moving the clip a single notch down the needle will eliminate a stumble or hesitation in the mid-range and give you a nice, smooth acceleration.

    While you are in there, always inspect the rubber portion of the air diaphragm or slide for tears, pin holes or rot. Now would be the time to replace them.

    If you are installing a jet kit, it will include instructions and may require a hole to be drilled in the aluminum portion of the diaphragm. They usually supply the drill bit.


    Make sure that you test ride the bike after each carb jetting modification. Do not think you can change all the components at once and then test ride. If things are not as you want them to be, it may not be clear which component needs more adjustment.

    The exception to this would be if you have purchased a quality carb jet kit specifically designed for your make and model motorcycle. They have already done the troubleshooting part for you. You should be safe installing the complete kit before your test ride.

    More Carb Jetting on page 2.


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