I have a 2007 Yamaha Big Bear 400 YFM400FWN I have cleaned the fuel tank, installed a new fuel filter, fuel hoses, and a brand new carb with a Dynojet Q413 kit adjusted to Dynojet’s specs. Now before the carb replacement it would do the exact same thing, and still after just having installed the Dynojet kit it still does this. It idles fine and starts right up, but when I give it throttle, if I give it just a little it will rev a little then it will almost bog out if I don’t let off, and then if i don’t wait a minute and hit the throttle again it will start to choke itself out again. now when I start to open the choke up It starts to rev and rev and rev higher and higher the more I open the choke. Then close it and it goes back to idle. I’ve tried everything I know to do, and I’ve played with the air flow screw on the carb with no change. nothings worked so far so I’m needing a little help. Thank you in advance.

 

I am interested to hear responses, as I have the same problem. Brand new carb, new spark plug, new air filter. Starts and idles fine, bogs and backfires above 1/2 throttle. If I restrict the air intake port above the airbox, it runs great. Checked for vacuum leaks and tried 3 different main jets...no change. Help needed!

 

If they are cheapo Chinese carbs, it is possible they have a built in fault. I have fitted Chinese ones to Honda 350s and Suzuki LT50s and both have strange faults built in, the 50s have too small a hole for the outer throttle cable to sit properly. Easily fixed, but why? The 350s dribbled fuel from leaky excess fuel devices in the float chamber.