If your 2015 YZ 250 two-stroke motorcycle won't start despite having a fresh top end, a new cylinder, a new clutch, spark, air, and a clean carburetor, there are a few possible reasons for the issue. Here are some troubleshooting steps you can take:
Fuel: Ensure that you have fresh fuel in the tank and that it is flowing properly to the carburetor. Check the fuel line, petcock, and fuel filter for any blockages or restrictions. If the fuel is old, drain it and replace it with fresh fuel.
Carburetor: Even though you mentioned that the carburetor is clean, it's worth double-checking to ensure that all jets and passages are clear. Pay close attention to the pilot jet, main jet, and idle circuit. If any of these are clogged or improperly adjusted, it can prevent the engine from starting.
Fuel mixture: Confirm that you have the correct fuel-to-oil mixture ratio for your two-stroke engine. An incorrect ratio can cause starting issues or poor performance. Refer to your owner's manual or the manufacturer's guidelines for the correct mixture.
Compression: While you mentioned a fresh top end and a new cylinder, it's still worth checking the compression of the engine. Use a compression tester to measure the compression levels. If the compression is low, it could indicate a problem with the piston rings, cylinder head gasket, or other internal components.
Ignition system: Even though you have spark, ensure that it is strong and properly timed. Check the spark plug condition, spark plug gap, and ignition timing. Weak or improperly timed spark can hinder starting.
Air filter: Although you mentioned that the air is clean, inspect the air filter to make sure it is not excessively dirty or clogged. A blocked air filter can restrict airflow to the engine, causing starting difficulties.
Reed valves: Examine the reed valves to see if they are in good condition and functioning properly. Damaged or worn-out reed valves can affect engine performance and starting.
If you've checked all these potential issues and the motorcycle still won't start, it may be beneficial to seek assistance from a qualified mechanic who can diagnose the problem more precisely.