What jet size should i use in my 400ex?

For a Honda 400EX, a common jet size is typically between 152 to 158, depending on modifications and altitude. If your 400EX has a stock engine and exhaust, starting with a 155 main jet is a good baseline. Adjustments may be necessary based on factors like aftermarket exhaust or air filter changes, so it's best to test and tune for optimal performance. Always consult your owner's manual or a mechanic for specific guidance related to your setup.