I bought a 2005 yfz 450 about a month ago. The performance mods that I know are done are a White brothers pipe, Jetted(unknown size), the air box lid is cut, and a UNI air filter. It didn't idle at all and it back fires and shoots flames. I fixed the idle problem by tightening this silver thumb screw on the bottom of the carb because it fell out. I turned it forever and it didn't tighten but it stays in place now.

But, my problem is that it still backfires and wants to choke out in low rpms.(which could because i have big tires on it). I read on other forums that I probably need to adjust my air/fuel screw, but i dont know where it is at. So, can someone give me some detailed directions on where it is at, and how to adjust it so my bike runs properly.

 

That big thumb wheel you were turning is probably the air/fuel screw. If I remember right, there should have been a spring attached to it. It resides on the underside of the carb, toward the engine side. If it IS the thumbwheel, then you have an aftermarket screw.a popular mod. First, make sure you have all the parts to it. You can go to one of the online microfiches to check as internally the aftermarket stuff is identical to stock. The proper way to adjust it is with the engine brought up to operating temp, to turn it all the way in(gently bottom it out.) Now start tuening it the other way until you notice a rise in RPM. Just out from that point is where it should be set at. If you never notice an rpm change(and you have a lean condition i.e. popping and backfiring on decel), the pilot needs to be changed out for a larger size. Repeat that procedure again. HMF has a nice jetting tutorial on their website in the forum section if you need it.

 

Thanks for the info,

Two questions:
1. I'm not sure what you mean by online Microfiches.

2. How many turns do you think it would take to bottom the screw out. When I found the screw, it was half way out of the carb sitting on the starter. So, I screwed it in and turned it at least 15 times and it never tightened up and stop going into the carb any farther. Right now my bike is idling high, but at least it idles now.