My 2011 450 grizzly idles fine but when i hit the throttle i reach 15 mph and it starts couching and sputtering and will not go any faster

 

First check the obvious= Spark plug, fuel filter plugged if it has one, fuel line pinched, bad gas, if it has a carb, clean it, blow out jets, plugged air filter, plugged muffler. I'm sure I missed a bunch. If efi, maybe partially plugged injector?