Quote:
Originally Posted by gsxr750
exhaust valves are only open after the mixture has been ignited and the gases expelled out the exhaust after combustion, no air is ever sucked into the exhaust ports.
Vacuum is only created in the intake port , not the exhaust port
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Cat material is fairly regularly sucked into the engine when they go bad. You should try searching these things before posting about them.
I think you misunderstand the way the valves are timed. Overlap is very common.