My '03 Honda CRF450 suddenly became hard to start this past weekend. After 20 minutes of cranking it finally sputtered then fired off. It ran perfect on the track during my first Moto. But, I encountered the same starting problem after allowing the engine to fully cool following the first Moto. I did eventually get it started again and once fired up it ran like a top. In the mean time I have change the oil, completely drained and replaced the fuel, installed new NGK plug and cleaned air filter element. Now it won't start at all - actually it only makes a loud "pop" from time to time. I have confirmed spark is present by placing the plug beside the cylinder head. At this point I have no idea what to trouble shoot next in order to narrow down as to what the culprit to my strating problem may be. Has anyone encountered starting problems such as these with their 4-stroke (Honda). If so, what was your remedy to the problem? Note: My bike has very low hours (25 +/-) and valves have never been checked or adjusted.