Ok heres my situation. I have a 08 450 XC-F that has fairly low hours. I raced a hare scramble that was very dusty. When i cleaned the bike i found a small amount of dust that made it through the filter. After getting everything all cleaned up i tried to start the bike( that was running great when i last turned it off , 4th place in my first B class race) and it would not start. I cleaned the carb and checked the spark, all ok. I asked a reputable mechanic and he said that even a small amount of dirt cold tighten the valves. My question is how tight would the valves have to be to cause the bike to not start? It will pop and thats all i can get it to do. Everything else is working as it should. Thanks for any help you guys can give me.