Backfiring long post please read need help

Ok I'll try not to ramble here but I bought this 07 yz450f from a friend and when I bought it it had a cracked water pump cover and had sat through a Alaskan winter and summer uncovered and nothing done to winterize it so I changed the fuel pump cover new plug fired her up and now when I braap the throttle wide open it's like it governs out not sure if that's part of the bikes design never had a 4 stroke but when I let off the throttle it back fires and kicks a 6 inch orange flame out the exhaust and it is smoking quite a bit also I already cleaned the carburetor really good please any ideas on how to handle this or what's causing it maybe I should just rip it up and down the road to clean it out?

Silly question but did you flush tank and put in fresh fuel? If so and it's blowing smoke, you might have to rebuild the top end. Does the smoke smell like oil?

First, the bike has a rev limiter.  The limit is set to 11,500 RPM, but in neutral, the limit is lowered to 6,500 to keep some people from blowing up the engine for fun.


Secondly, excessive backfire from the exhaust is caused either by an air leak into the exhaust system (always check for this first), or by a lean idle mixture.  Lean idle can be caused by a partially blocked pilot jet, or simply a misadjusted pilot screw. 



Ok so my bike idle is set way high now and I wanna adjust it to were I just get a nice smooth idle I'm sure and do you think I will have to mess with the fuel skrew at all when adjusting the idle and were is the pilot skrew located and how do you recommend to adjust it I really appriciate the advice guys this is my first 4 stroke so I have a lot to learn

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