I've been recently having trouble starting the 400ex when the engine is cold. I let it sit for 2 weeks and bam. It starts after about 5 cranks. I checked the carb today. It wasn't dirty at all. I don't know what the problem can be. Is it time for a valve adjustment?

 

Make sure you have a full charge on the battery and a fresh spark plug. 400EX's are a real pain to start when it starts to get cold, especially if any modifications were made to them. You may also want to try one or two sizes bigger on the main jet to allow more fuel at idle speed. I did all of the above to mine and it starts better, although it still isn't great.