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I have noticed a small box on the bottom of the muffler on a Mountfield SV150 lawnmower engine.

Exhaust with inlet box

Inside is a reed valve set up as inlet only and a small filter grill.

What is the purpose of this part? I understand these are catalytic mufflers, could it be to prevent some pressure differential? Thank you

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  • I allows fresh air (oxygen) into the cat during hard deceleration. – Moab Jul 3 '20 at 14:48
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It is usually to add air to the hot exhaust gas to make sure that the catalyst can cause the reaxtions to breakdown the combusted products sufficiently.

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