Welcome to the site !!
I have a few questions for you
Is this a bike that ran fine before the rebuild ?? , in other words , did it start fine cold before ?? ,
Is this a recent purchase ?? , if so , was it jetted correctly for the altitude your riding at ??
If its flooding that easy , then i would look at Needle position , Float Adjustment , (does it drip gas ??) and jet sizes first
Have you tried adjusting the Fuel Screw (remember its a Fuel Screw not a Air Screw) it should start/run easily within (*1-1/4 - 2-3/4*) turns out from lightly seated , Out is Richer , In is Leaner , idle should be around 1500-1800 and you should be able to turn the fuel screw till the idle raises then starts to drop off , then close the screw till the bike wants to die , counting turns each way , the highest idle should be between the settings i listed (* *) preferably in the middle , this would indicate your jetting is correct , then you need to look at internal parts in the carb if jetting specs out , Floating Valve Seal (on the slide) could be bad , accelerator pump timing (it needs to squirt and just miss the slide , not hit it) , or float level , or needle/seat sticking (you would notice the bike dripping gas when sitting with fuel valve on)
Jetting as follows:
Jetting Guide for Main Jet:(pilot 45) CRF450R/X
Sea level....165
2000 ft.......162
4000 ft.......160
6000 ft.......158
8000 ft.......155
Temperature/Elevation:
One main jet size (up/down) for every 2,000 feet or 25 degrees in temp.
Try this things , and report back with findings , if no positive results , go to the Jetting part of the forum and look for
leardriver , find a post with that name , click on the name , it will take you to his Profile , then PM(Private Message) him and he will hopefully be able to figure out your issue , as he is the resident carb guru here .....hope that helped some