I have the same bike. What is the re-build history on yours ? First, do a compression test. Second, check the valve clearance. If those 2 thing check out good, remove the carb. and clean it. Make sure the pilot jet is clear. My jetting is as follows and my bike runs perfect, Main: #160, Pilot: #50, Mixture screw 2 1/2 turns, needle clip position (std.) check your hot start plunger, make sure it is not staying open. Check the adaptor boot between the carb. and the head, make sure it's not cracked.
compression test is a waste of time. Autodecompression system on the cam will always open the valve when you kick it over.
Do a leak down test if you want to test the internals of the motor without teardown.
But clean the pilot. Those jetting specs are WAY fat for a yz450 on the pilot. stock is 42, we use to run 45s on them to keep the decel popping out of it.
More than likely the pilot is plugged, the fuel screw not adjusted or the idle needs to be turned up a bit.
You can turn up the idle all you want but until you can flow some fuel through the pilot to mix with the extra air you're letting in it still isn't gonna run.......
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