08 July, 2011
How to Tune a Bike for Top Speed
Most bicycles and motorcycles can be modified for top speed. You can change the sprocket ratio on a bicycle, but this is associated with gearing and not tuning. As for motorcycles, you have a variety of options with regard to tuning the engine for top speed. Dirt bikes and sport bikes run at high rpm at top speed and road cruisers run at slower rpm at top speed. For this reason, it’s important to have the service manual for your bike on hand.
Increase the size of the carburetor main jet for more top speed. Typically, stepping up 3 mm in size provides the best results without the need to change other internal components, such as the needle valve and idle jet.
Follow the procedures in your service manual and remove the single or multiple carburetors. Notice the first step is always turning off the fuel supply petcock before you disconnect the fuel lines, air filters and the throttle linkages.
Place the carburetor or carburetors on a workbench. Remove the float bowl from each to access the main jet at the underside of the carburetor body. Identify the jet by the size that’s stamped on the face of the jet.
Remove a main jet using a large screwdriver. Install a larger jet and tighten it securely with the screwdriver. Squeeze a small bead of RTV sealant on the rim of the each bowl and reattach the bowls to the carburetors.
Reinstall the carburetor or multiple carburetors at the intake manifold. Connect the linkage and the fuel lines and reconnect the air filters. Turn the fuel supply petcock on and tighten the fuel line clamps a second time as extra measure.
Advance the ignition timing for more top speed. Remove the timing cover at the side of the engine crankcase to access the timing rotor. Locate the viewing window and the timing indicator at the top of the rotor.
Refer to the timing procedures in your service manual and attach timing light wires to the appropriate spark plug and ground as described. Start the motorcycle and allow the engine to idle.
Loosen the timing adjustment screw at the base of the rotor with a small screwdriver. Direct the timing light at the viewing window and note the position of the timing mark on the rotor in relation to the indicator at the viewing window.
Rotate the rotor by hand and advance the timing by two degrees. Typically, two degrees positions the timing mark slightly left of center at the indicator. Tighten the adjustment screw and disconnect the timing light.
Turn off the engine. Reattach the timing cover at the side of the crankcase.
Adjust the fuel/air mixture at the carburetors to properly tune the engine for the new main jet and advanced ignition timing.
Start the motorcycle and allow the engine to warm up to normal operating temperature.
Locate the brass fuel/air mixture screw on the side of each carburetor. Typically, a screwdriver with a long stem works best for accessing the mixture screws.
Listen to the engine as you turn a fuel/air mixture screw each way. Find the setting where the engine runs at the fastest speed while idling. Repeat this on each carburetor.
Refer to your service manual and adjust the throttle linkage for the quickest response when you twist the throttle grip.
Install new bowl gaskets or manifold gaskets if these are torn or damaged during the tuning.
Install new air filter cartridges and fuel filters for improved performance.
Refer to your service manual or talk with a racing mechanic about tuning fuel-injected engines for top speed.
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