Tuning the air-to-fuel ratio on a Yamaha motorcycle's carburettor is a process called "rejetting". Rejetting a carburettor involves the replacement of the carburettor's fuel nozzles, or jets, to increase or decrease the amount of fuel introduced into the air flow within the carburettor. This is done for a number of reasons, either to compensate for differences in altitudes, to increase fuel economy, or to increase the motorcycle's performance. While the actual process of rejetting a carburettor is simple enough, several test rides and rejetting sessions will be required to fine tune the jets for maximum effect.
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Things you need
- Replacement main and pilot jets
- Aerosol carburettor cleaning spray
Turn the fuel valve to the "off" position and disconnect the fuel line from the carburettor. Allow any remaining fuel within the line to drain into a container. Loosen the hose clamps that secure the carburettor to the air box and to the motor's intake manifold. Remove the carburettor from the motorcycle and disconnect the throttle cable from the throttle valve.
Place the carburettor on a clean workspace. Remove the drain bolt on the bottom of the carburettor's float bowl with a screwdriver. Allow any fuel remaining in the float bowl to drain into a container. Remove the float bowl bolts with a screwdriver and pull the float bowl off of the carburettor. Rest the carburettor on its top cap,positioning it for easier access to the jets on the underside of the carburettor.
Locate the main jet in the centre of the carburettor and unscrew it with a small flat screwdriver. Locate the pilot, or slow, jet, just off to the side of the main jet, and remove it with a small flat screwdriver. Place the jets in individual plastic bags for later use. Spray the carburettor with an aerosol carburettor cleaning spray to remove any debris or build up from the jet mounting holes.
Insert a new main and pilot jet into their respective positions, tightening the jets with a small flat screwdriver until they are seated against the carburettor. Place the float bowl onto the carburettor and tighten its mounting bolts with a screwdriver. Insert and tighten the drain bolt with a screwdriver.
Flip the carburettor over, right side up. Using a screwdriver, remove the bolts that secure the top cap to the carburettor's body. Remove the top cap and pull the spring and throttle slide or diaphragm out of the carburettor. Push the jet needle out of the slide or diaphragm.
Adjust the jet needle's "height" by repositioning the clip on the end of the jet needle. The clip rests on a set of grooves cut into the end of the jet needle. Moving the clip to a different groove will raise or lower the jet needle, changing the amount of fuel emitted by the main jet when the throttle is opened. Moving the clip towards the bottom of the jet needle will increase the amount of fuel earlier, enrichening the fuel mixture. Moving the clip towards the top will decrease, or lean out, the fuel mixture. Remove the clip from the jet needle using a pair of needle nose pliers and place it on your desired position on the jet needle.
Insert the jet needle and its holder into the throttle slide or diaphragm. Place the slider or diaphragm back into the carburettor. Reinsert the spring and attach the top cover. Tighten the top cover's bolts with a screwdriver.
Reinstall the carburettor onto the motorcycle, tightening the hose clamps that secure the carburettor to the motor's intake manifold and the air box. Reconnect the fuel line to the carburettor.
Test ride the motorcycle and take note of any flat spots in acceleration or power loss. Rejet the carburettor as needed until the motorcycle runs smoothly at all speeds.
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