Help! 2006 yz450f big bore problems

So im trying to help out my buddy who is having trouble with his 478cc big bore kit. i guess his issues are with tuning it properly. it is back firing and will only idle on choke. 

 

ill try and get more info but does anyone have any advice as to why this is?? 

 

Thanks!

Sounds like a standard case of the pilot jet being clogged. Happens very easily if the carb sits and dries out for a while, and it may even look open. A dried film only .001" thick will change a #45 jet effectively into a #38, way too small to work. Visit the topic, "Common Threads", and look for "Cleaning Jets and Carb Passages".

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