ruben,
1. Where the carby is you have 4 screws holding the plastic housing on (that the carby screws on). Make sure that they are tight and see if you can see the gasket between the plastic and crankcase.
2. If that is ok go to the muffler and remove it. Look for scratches on the piston and cylinder. If you have scratches then you will have trouble starting.
3. Make sure that the exhaust port is clean if it is blocked up with carbon it won't start because of the back pressure.
4. Also check that you have movement in the rings(just use a small screwdriver through the exhaust port).
5. Try running the machine without the exhaust and it should start.
If you do all this stuff and it runs without the exhaust the muffler could be blocked up.
Hope that this is of some help.
Regards,
Bruce
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