Hi, I have a 2007 big bear 250 that has a Carb issue that I'm stumped on. I bought it not running and found that the previous owner had replaced the intake boot with an after market one which was wrong and not making a seal on the Carb. I replaced it with one I got from the Yamaha dealership in town and while I had the Carb off I gave it a really good cleaning. When I got it all back together it fired right up, revved up good with the choke on and idled perfectly, but when you give it any amount of throttle it bogs down and dies. It will do this with the mixture screw all the way in or out. I took the Carb off and inspected everything but can't find a problem in it. I found an article about people mixing up the main and secondary jets and having similar issues but I triple checked that the larger jet was in the main jet emulsion tube and tightened down fine. If I remove the air filter and block half of the intake off it will rev up much better so I thought it was running very lean, but the spark plug is jet black. I'm at a loss, anyone have any ideas on what to try? I thought of adjusting the vacuum slide needle to a more rich mixture but with the bike being factory stock I wouldn't think it would need this amount of adjusting.