Remember while working on the Fuel System that Gasoline is Extremely Flammable and Extreme Care Should be Exercised so as to Not be Around, or Create any Sparks or Flame while Working with the Fuel System.
How to Disassemble and Clean A Dirt Bike's Carburetor
If you're certain the carburetor's not working the way it's supposed to, the steps below will detail exactly what you need to know to properly overhaul the carburetor on most popular dirt bikes, and although most carburetor adjustments, or parts which need cleaning can be accessed without removal of the carburetor from the motorcycle simply by loosening the intake boot clamps and rotating the carburetor to access the float bowl, the following steps will detail the procedures for a complete carburetor disassembly and cleaning.
- Beginning with the servicing of a carburetor, it's important to know that when working on the intake tract, it's strongly advised that you wash the dirt bike beforehand.
- With a clean motorcycle you'll need to remove the carburetor from the motorcycle and if you're not in a hurry to get this done, it never hurts to remove the seat, gas tank and subframe (if removable) so as to allow non-restricted access to the carburetor, as this will also allow for inspection of the cable and wire routing under the fuel tank to ensure these are not chaffing, but be sure to check out the article on ensuring the throttle closes properly.
- With the Carburetor Accessible, the first step to carburetor disassembly will be to remove the carburetor top and slide from the main body on 2 strokes, or the cables from the pulleys (or linkage) on a 4 stroke. On a 2 stroke this will be possible by either unthreading the top, or by removal of 2 screws which will allow removal of the top along with the spring, slide and needle as an assembly. On a 4 stroke there may be a plastic cover covering the cable attachment points or linkage. If so this will need to be removed to ensure proper disassembly / reassembly and adjustments of the throttle cables
- With the top of the carburetor removed on a 2 stroke, or the cables removed from a 4 stroke, ensure that the vent and overflow hoses are clear, then disconnect the fuel line, loosen the intake to carburetor clamp(s) and remove the carburetor from the motorcycle.
- Next, you'll want to drain the fuel from the float bowl so as to prevent a mess during disassembly and if you're reading this, you're probably having carburetor troubles, or have a concern regarding contaminated fuel so you'll want to be sure to drain the fuel currently in the float bowl into a clean glass jar for inspection.
The best way of draining the fuel is to either unthread the sediment bowl at the bottom of the carburetor's float bowl, loosen the drain screw on the float bowl, or turn the carburetor upside down allowing the fuel to run out of the overflow tubes.
Note: If water is present in the fuel, the water will be able to be seen below the fuel in the jar, as there will be a distinct separation of the fuel and water in the jar if allowed to settle for a few minutes.
- Once the fuel has been drained, find a clean work area, a #2 Phillips screwdriver, an assortment of Torx drivers, a small flat blade, and a can of carburetor cleaner and let's get right into how to clean a dirt bike's carburetor.
Although there are vast differences between 2 stroke and 4 stroke carburetors, the following tips are merely intended to cover the basics of rebuilding a carburetor on a 2 or 4 stroke motorcycle. If you need more information that's specific to the motorcycle you're working on, be sure to pick up an OEM Service Manual specific to the motorcycle being serviced for specific details on disassembly, reassembly and adjustments of the carburetor.
- With a clean work area, begin by removing all the vent and overflow tubes, replacing them if they look stained, cracked, smashed, hard or burnt while also taking note of the routing of the hoses, as well as the position of any brackets that may be attached to the carburetor.
- Now remove any external sensors or solenoids, as well as the choke mechanism by unthreading the plunger assembly from the carburetor body and lifting this out.
- Next, you'll want to thread the air screw on 2 strokes, or the slow speed fuel screw on 4 strokes in until lightly seated while taking careful note of the turns, then remove the screw and spring while paying close attention for the presence of any small, thin flat washers, or o-rings under or over the spring.
- Now remove the screws securing the float bowl and let's get into inspecting the inner parts of the carburetor.
Note: If the float bowl seems stuck, first remove the sediment bowl, then use the handle of the screwdriver and lightly rap on the float bowl from the side. You'll find this will frequently, and easily break the bond of the bowl gasket without damaging the gasket.
- With the float bowl removed, remove the float hinge pin, the float and the needle inspecting all these parts for damage while paying close attention to the float and the viton tip of the needle for any damage. A damaged float can either absorb weight and sink, or if hollow, these floats occasionally fill with fuel causing the float to sink. Additionally, pay close attention to the viton tip of the fuel inlet needle as these occasionally develop pitting but either scenarios often result in a flooding condition that's only correctable by replacing the float or needle and seat as none of these parts are repairable.
- Next, remove the plastic baffle (if present), then remove the main jet (in center), then remove the pilot jet (may be down a hole requiring a thin flat blade screwdriver to access)
These jets are the stars of the show so be absolutely sure the orifices in these jets are clear by using carburetor cleaner and compressed air, ensuring you can see daylight through their orifices
Note: If you have trouble with getting the center of the pilot jet clear, pull a wire from a wire brush and use this very carefully to clear the passage, just don't get too aggressive otherwise you may affect the jetting, resulting in an overly rich fuel mixture.
- Once the carburetor is completely disassembled, use an aerosol carburetor cleaner (available at any auto parts store) with compressed air
Things to keep in mind when Rebuilding a Motorcycle Carburetor
- When using compressed air and chemicals it's strongly advised to use eye and ear protection.
- Ideally, you should replace all the gaskets and o-rings within the carburetor at disassembly.
- During cleaning, be sure to use the straw that came with the carburetor cleaner to force the solvent through all the passages of the carburetor while looking for a flow of the cleaner at the other end of the passage while always following with compressed air until it's absolutely certain that the passage is clear.
- Should there be fuel staining, float bowl discoloration, or other irregularities within the inner parts of the carburetor, it's critical that all this be removed either soaking the carburetor in a soak type of cleaner intended for this purpose and readily available at auto parts stores, or by using something such as scotchbrite, carburetor cleaner and your finger, otherwise the carburetor will be prone to future jetting problems.
- Upon reassembly, if you're unsure of the correct float height (and the float has not absorbed fuel), once the float is installed with the needle seated and the hinge pin in place, you'll notice when holding the carburetor upside down with the float bowl removed that the float has a horizontal line on the float at a seam or molding line. If necessary, adjust the tang on the float to make this line parallel with the surface where the float bowl seals.
Once you know how to rebuild the carburetor, you'll likely want to maintain the performance that comes with a carb that's not clogged up anywhere, and it's not hard as long as you stay on top of the Air Filter Maintenance and ensure that the intake tract is sealed and not allowing unfiltered air to be introduced into the intake resulting in future engine damage or carburetor problems.
If you keep getting trash in the carburetor wreaking havoc with your jetting, but you're sure the inside of the fuel tank is clean, remove the petcock and check the pickup tube for any tears in the screen, or cracks in the plastic which may be allowing unfiltered fuel to reach the carburetor. Should any damage or defects be noticed here, this is only correctable by replacement of this tube with an OEM Replacement.