05 Warrior not running correctly.

I have a friend that has an 05 Warrior and he has some issues with one of the cylinders not firing all the time. He can run it one day and it runs fine and the next day the number one cylinder is not firing and it has no power. The engine also seems to use way more gas when it is running this way. He has gone through the carbs and also had the local dealer sync the carbs. The dealership said syncing the carbs would fix it but it didn't and apparently now they don't know what's wrong.
When the sled is not running right the number one spark plug gets black right away. Every time they check there seems to be spark and the compression is good.
I'm just wondering if anyone has heard of a similar issue. Thanks in advance for any help.
Terry.