i have a 2004 polaris sporstman 500 HO and it started running poorly this spring. it started sputtering ahd acting up around 1000 rpm's it has now gotten so bad it will only idle and soon as you try to go it stalls sputters and blows dark smoke i"ve been told by a couple guys it"s probably the cam but it's only got a 3000 miles and 3 years old, can somebody tell me i f this is common or what else could it be. ( the carb has been cleaned twice)

 

Sounds like there is more fuel in the fuel to air mix running rich.
A good tune up may be the charm. A can of carb cleaner may help.
Start with a new set of spark plugs, they may be getting fowled up by now, clean the air filter, check the valve lifter and gap adj if needed.
Sometime you can smell a burnt cam smells like a welding shop when you open the lifter inspection covers
Make sure there is nothing restricting air flow thru the carb, see if the slider in the carb is not sticking in an open position.

 

have the carburetor taken apart and cleaned out. fresh new spark plug

 

Oh one other thing loosen the gas cap sometimes the vent gets clogged preventing a positive flow of the fuel.

If it idles for a few minutes then the idle drops off by it self, if the vent is clogged the fuel that is being pumped, by the fragile diaphragm, has a volume that needs to be replaced by the air from the vent to prevent a slight vacuum developed in the tank which may overwhelm the capacity of the fuel pump. Starving the engine of fuel leaning the mix and killing the performance.

Worth a try.