Chris hit on most of this in his post but really 91 octane is pretty low, all in all its simply too low as thats why the dcs light is coming on. More octane will make the dcs not bother you. The higher the octane is the richer the sled runs WITHOUT touching the jetting. Octane is the fuels ability to not burn, so it slows down the flamefront and burns not as violent.
So.. you have a couple options to solve the problem.
1.) use higher octane fuel then 91, like 93-94. If this is not feasable because of your location , you can always get some race gas and mix a little bit of that to a tank of the 91, this will raise the octane of the whole tank. I am talking about 1-2 gallons of racing gas say like 104+ octane stuff to the rest 91.
2.) if thats not possible or you dont want to be bothered with it, you have a other option to make the sled run richer via the jetting set up, will yield off detonation and make the light not bother you.
First, raise the carb needles to 3.5, this is the 4th groove down from top of the needle and then theres 2 small plastic shims, 1 shim on top of the clip, 1 shim on bottom of the clip. You can also install 157.5 mains in the carbs. This will richen it up and "cool" off the pistons with more fuel and hence make the detonation not occur. If all your going to use is 91 this is your only real way to go.
The vipers were set up lean from the factory, and they have higher compression in the mag and center cylinders, along with a cdi box with lots of ignition timing. Todays low quality fuel is just not cutting it. these problems were not very noticeable until the last few years as the fuel has gotten cheaper quality(they dont mind still charging us for the higher prices though...
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