What is it on?
Where is it?
Why do I need to clean it?
I've got one on my small Onan (2800 Microlite), and also my Husqvarna chainsaw. It's a Forest Service requirement on both, AFAIK. I've no idea how many models of Onan have them; I think every chainsaw I've owned has has one. My Onan's is located in the muffler outlet, before the tailpipe. It's a piece of fine-mesh screen wire, sometimes in a holder of some sort. The Onan's is a PITA to remove & replace.
When it gets close to being plugged up, you'll notice a lack of power that may drive you nuts trying to trouble shoot. My Onan would run about a half-load of electric heaters, but start to struggle, sputter and die with more. It could be re-started almost immediately. The Husqy just plain wouldn't start.
Jim, "Mo' coffee!"
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison (aka 'Loafer's Glory') www.tigervehicles.com