air going in exhaust coming out carb

I have a 2005 YFZ 450 that wont start, when turning over air is being sucked into the exhaust. Ive checked the timing and the cams. does anyone know what would cause this?

To some extent, this is normal. The exhaust valves don't close until about 30-40 degrees after TDC on the intake stroke, and since the throttle is closed, the engine will, at cranking speeds, draw some air through the exhaust.

The intake also remains open into the beginning of the compression stroke, so some air coming out of the carb is normal too.

If any of this is happening in excess, your cam timing is off, or you have bent or leaking valves. Re-check the timing, and not whether the engine has the normal amount of compression.

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