I've got a 1998 454 Arctic Cat 4x4 that had gas in the water. I drained, cleaned and refilled the tank. I took the carburetor apart, cleaned every passage, replaced jets and needles, checked float level and checked the diaphragm for holes. I reinstalled everything and started it up. It purrs like a kitten on idle. When you give it gas, though, it will pop, back fire, stutter and either generally runs like crap or dies I assume that it is running rich - the spark plug is black with carbon after just a minute or two of running. I went through the carburetor again, three times, swapping jets (there were three different sizes in the kit) and wound up putting the old jets back in. I have moved the main jet needle e-ring up and down. I adjusted the float. I disconnected the exhaust (to eliminate that, even though you can feel pressure coming through). I have tried two aftermarket carburetors. I am at my wits end and out of ideas. Please tell me someone else knows what is wrong here...

 

Look in the carb when its sputtering and see if the slide is going all the way up. If it is half way and and not going all the way up replace it. Those diaphragms get just a little bit stiff and they won't work properly. They will feel soft.

Fixitup

 

I will check that, but one would think that the two new carburetors I tried would have taken care of any issue with the diaphragm?