How do I Tune a 2 Stroke Race Engine?

Written by tom lutzenberger
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How do I Tune a 2 Stroke Race Engine?
Water cooled cylinder heads add additional tuning benefits to engine cylinders. (chrome engine image by Dumitrescu Ciprian from

Many two-stroke engines involve fairly simple processes for tuning, including stock race engine versions. Typical tuning areas that allow for changes include the carburettor system, the cylinder system and the exhaust system. Additional work can push peak performance, but often involves risk and experience in making such changes (for example, porting, polishing, cutting a crankshaft assembly, etc.). For a mild boost to the cylinder, however, increasing the compression can produce beneficial results, as well as rejetting the carburettor and removing inaccurate stock carburettor jets.

Skill level:

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Things you need

  • Screwdriver
  • Socket wrench and sockets
  • Torque wrench
  • Rag
  • Crescent wrench
  • Different carburettor jet sizes

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  1. 1

    Use a socket wrench and appropriate socket to loosen the cylinder cap. Twist the socket by hand once the nut becomes loose enough and do the same for all the remaining hardware. Use a different socket to disconnect the spark plug after removing the ignition cap and wiring.

  2. 2

    Pull the cylinder cap off the stock cylinder. Wipe it off and stuff a rag in the cylinder with the piston recessed inward so no particles get inside. Take the cylinder to a scooter shop and have the cap squish -- the area that the fuel is compressed into -- milled so that the compression factor on the cap is increased This will create a hotter ignition for more power.

  3. 3

    Reinstall the milled cap back onto the engine with new washers and securing nuts on the cylinder studs. Use a torque wrench and appropriate socket to tighten the cap down, tightening each nut in an alternating pattern so the head won't warp under pressure. Tighten the nuts to the torque setting recommended for your particular engine. Insert a new spark plug and reattach the ignition wiring cap. Drain the existing regular fuel from the gas tank and replace it with racing fuel, which ignites at a hotter temperature.

  1. 1

    Use a screwdriver to disconnect the manifold hose and fuel line connected to your carburettor assembly. Carefully pull the carburettor off the engine assembly. Unscrew the top lid where the throttle cable enters the carburettor. Pull the throttle slide out and unhook the throttle cable.

  2. 2

    Use a crescent wrench to open the carburettor frame up, exposing the jets inside. Unscrew the old stock jets matching a factory engine formula and replace them with slightly leaner jets -- a smaller size allows less fuel. Rebolt the assembly together. Reconnect the carburettor throttle slide to the top. Reattach the fuel line and manifold hose.

  3. 3

    Test the engine and carburettor by running it through the gears at speed. After 20 minutes, pull over and remove the ignition cap off the spark plug. Remove the spark plug using a socket wrench. Examine the spark plug tip colour. Redo the jetting process for a larger jet size if the tip is chalky white, which indicates too little fuel. Replace the jet again with the same process if the tip is black or oily black, which indicates too much fuel. Complete the tuning process when the tip looks chocolate brown.

Tips and warnings

  • Many motorcycle stores sell racing fuel for motorcycle riders racing off-road.
  • Do not use normal gas fuel when using a modified cylinder cap for higher compression on your race engine. The ignition temperature will skyrocket faster with a lower octane and will overheat, causing the piston to melt inside the cylinder.

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