The choke on could be reason for the black plugs.
Is this SxS used in frequently where condensation can be forming in the fuel system? Then if so, your carb make be gummed up and needs a thorough cleaning.
I know I am hawking a product I sell but AMSOIL recently came out with a fuel system cleaner for small engines that is pretty darn good. I have sold a few bottles to local boater and got some very good comments about it. One of my riding pals has a 2001 Polaris 500 that he was having trouble starting after letting is sit since last fall with the same fuel in it. He says it cured his problems.
I am not guaranteeing your problem is fuel but according to a local repair shop, they are seeing a lot of problems with fuel because of the ethanol being blended in just about all fuels. The problem is really bad with boats, he has taken apart carbs on the outboards to find lots of gunk in them after sitting for as little as a week or two with ethanol blended fuels. He said they are having so much problems at the local Marina that they are spec'ing bids for fuel must not contain ethanol and they have no one who can supply the because its Michigan law, all fuel has to the at least 10% ethanol. Ethanol in fuel has a tendency to absorb water and separate from the gasoline, sinking to the bottom of the gas tank, where it quickly degrades and creates gum, varnish and other insoluble debris that can plug fuel flow passages and negatively affect engine performance.
Here is a link to the product ---->Quickshot SE
See if some atv's etc are not operated in a similar fashion the boats..
Reasons Boat Engines Have More Problems with Ethanol Gas:
Boaters, often store gas in tanks longer than recommended for E10 (90 days).
Cars, unlike boats, usually replace fuel every week or two, which will successfully prevent the possibility of water-contamination/phase separation.
Boat engines live in a water environment - Alcohol gas loves to absorb water.
Ethanol E10 gas can absorb large amounts of water into the fuel tank, MTBE in conventional gasoline did not.