I am working on a Yamaha YFB250. I'd say around a 98-99, not sure on the exact year. Where would I find the air/fuel screw? The only one I can see is the idle set screw.

 

Should be on the bottom of the carb. Thats your pilot air/fuel mix. If it has a separate mixture screw it should be next to your thumbscrew for idle. Really not sure on that particular bike since I don't have one.

 

So that is not a drain at the bottom of the carb? I thought that was to drain the bowl. I am going to have to get a photo of it and have someone point the correct one out. I can't find an exact replica of that carb online.

 

Yeah there is a drain screw that will be pointing out to the side. When you have the carb off and turn it upside down you will see the drain and screw and then down in a hole you should see another screw head. Thats the pilot.

 

I found it today. Didn't help the problem though. It idles like a brand new engine, but try and tap the gas and it's like I just hit the kill switch on it.

 

If you have it adjust correctly can be a couple different things. Too much fuel or not enough. I would bet not enough due to the age. Not being familiar with that carbs settings usually if you turn the pilot all the way down(go easy don't force) then back out about a turn and a half you should be close to where it will run. Something else that could be an issue on the older bikes is the slide diaphragm. Take the 4 screws off the top of the carb and carefully remove it and use a flashlight to shine through the rubber to see if its tore or has a hole. Tear it down again and make sure none of the jets are clogged and float height is correct.