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.

it doesnt take much to cause the intakes to wear and get tight. If they have zero clearance they may stay open and not let it get compression. You will hyave to check them and decide if thats it.

Low hours mean nothing once you have a oil related problem, sucked dirt, or have had some other type of failure. The 08's oiling system is very suspect. Have you already shimmed and/or replaced the oil check spring and/or upgraded at least the pressure pump? If not, I'd suggest researching the topic, and then go ahead and check your valve clearances. It is very easy and takes almost no time. I'd also at least pull the rear cam caps and check for damage. This seems to be the first area that shows signs of oil failures. If any valves are out of spec, than that can cause starting issues, and would be a good reason to run a leak down test and/or tear it down to check the ring, which is also known to wear fast on this model.

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