There could be several reasons why your dirt bike turns off when you give it half throttle and doesn't turn back on. Here are a few possible causes:
Fuel delivery issues: One common reason for this problem is a fuel delivery issue. It could be due to a clogged fuel filter, a malfunctioning fuel pump, or a blockage in the carburetor. Insufficient fuel reaching the engine can cause it to stall when you demand more power.
Air intake problems: If there is a restriction or blockage in the air intake system, it can lead to an imbalance in the air-fuel mixture, resulting in the engine stalling. Check for any obstructions in the air filter or intake system.
Ignition problems: Faulty ignition components, such as a malfunctioning spark plug, ignition coil, or CDI (Capacitor Discharge Ignition) unit, can cause the engine to stall under load. Inspect these components and ensure they are functioning correctly.
Electrical issues: Loose or damaged electrical connections, including wiring harnesses, kill switches, or ignition switches, can lead to intermittent power loss and engine stalling.
Engine overheating: If the engine is overheating, it may shut down when you apply more throttle. This could be due to inadequate cooling, low coolant levels, or a malfunctioning radiator or cooling system.
Mechanical problems: Issues such as a worn-out piston, cylinder, valves, or a damaged clutch can also cause the engine to lose power and shut off. These types of problems often require professional inspection and repair.
Other factors: There could be other factors contributing to the issue, such as a dirty or contaminated fuel system, improper carburetor settings, or a malfunctioning power valve (if applicable to your dirt bike model).
To diagnose and fix the problem, it is advisable to take your dirt bike to a qualified mechanic or motorcycle technician. They can perform a thorough inspection, run diagnostics, and address the specific issue causing the engine to stall.