I've found that there's an insert (KTM part # 77005081065) for the off road 450FW's. It's called a throttle element and it's basically a spark screen that can drop right into the 250 WC's silencer at the end cap. Perfect fit and it should get you passed for events requiring spark arrestors. Only $15 and takes about 16 seconds to load. Does make bike run very rich though. Before I experiment with jetting has anyone gone this route already and determined a jetting fix?