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Hi all, I have a new to me 2006 crf 450. It's in pretty good shape. had it out to the desert for a few days. it ran great until the end of the last day. fired right up first kick, idled and ran great for three days. After a hard ride through the sand one afternoon, it just did not want to idle or start again on a kick without using the decomp lever. We were pretty much done for the day, so I took it home.

Same, thing today so it's not a hot start issue, it will eventually fire, and run, seemingly ok. But it won't idle, and won't start without serious kicking and use of the decomp. Hot or cold.

I am going to go in and check valve clearnace, and other ideas on what it could be?

Welcome to the site !!

I am a little confused by your explanation , you mentioned it would not start unless you used the "decompressor" lever , .....there is not a decompressor lever on the bike , it is a Hot Start lever , which leans the mixture ..... with that said , by your description it sounds like you may have sucked some sand past the air filter and possibly have a little bit in the bottom of the bowl , what I would try first is to remove the drain plug on the bottom of the carb bowl and see if there is in fact dirt at the bottom of the plug ... if so I would also suspect that your pilot jet is clogged or partially clogged , so it needs to be removed and either cleaned , or replaced .. try not to use any carb cleaner inside the carb body , as there are O-Rings that will be damaged , but try to clear everything with compressed air , including the snorkel to the carb from the airbox , (you should either lift the subframe or remove it to clean the airbox on the carb side past the filter , also clear the carb throat .......... , and good call on checking the valves as well , dirt is a killer of valves and if it gets past the filter , then its just a matter of time before the valves get eaten .... hopefully the cleaning solves the issue

Yeah I am thinking filter caked in sand making the mixture rich. Hope the filter stopped all the sand. If not you will see it on inside of filter (toss if that bad) and on the clean side of the housing. Clean well and good luck.
Also check the carby vent hoses aren’t clogged either.