400 with 450 carb can't start

I got a 99 yz400f with a 450 carb. Can't get it to start. Need some help doing so please.

Assuming it would start OK before the carb went on, verify that the carb is filling the float bowl, that the accelerator pump works, and that the starter jet and the whole pilot circuit including the pilot jet and the end of the pilot screw are clean and open.  You'll have to pull the carb to check the condition of the jets.  Read the link that follows.  It's possible that the carb you bought had sat for some time unused, and the pilot may be blocked or partially blocked by varnish.

I bought the bike as is. Person said it started before, but couldn't start it when I showed up. Bowl fills up with fuel. I put a new diaphram for the accelerator pump. Going the change out all the jets. When I kick it sometimes it back fires.

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