I have a 2012 yz250f that I recently purchased. I noticed after purchasing it my had a 450 muffler on it. It was a Yamaha Canada demo bike for pros to try apparently and it has Lots of gytr goodies on it so I imagine the carb isn't stock either. Anyhow I race this bike in enduro and harescrambles and need to be below 94db so swaped muffler to the 250 complete with db dawg. Tried it out and it had a real noticeable stumble on +- 1/8-1/4 throttle. It feels like it is starving for fuel which I have seemed to have confirmed as it's better with the choke on. Here is the problem though, shouldn't the db dawg and longer pipe make it richer? How can this combo have leaned out the bike? It rocks when your hammering on it but down low putting around it sucks. It's better when the engine is hot hot hot like after racing or riding for an hour or two. I'm probably about 400ft above sea level and current temps are 10-20celcuis. I think that's about 55-70F. Thoughts? I'm assuming it fuel or air screw but I'm totally baffled with what appears to be a lean condition when conventional wisdom says it should be fat.