I have the same problem with a near-new (not quite 2 tanks of gas yet) BCS 852. The throttle doesn't have the same adjustment as the other cables. I took a wee bit of slack out of the other end of the cable, then tightened that same bolt (assuming you're referring to the wide slotted bolt just inside the throttle control. The best I can get is for there to be a little slack on either end of the throttle, but at least it stays up near full throttle now. It may be that the problem is ameliorated over time as the spring on the throttle near the carb relaxes a bit.
Any other ideas you have, I'm happy to hear. My brake on one side hasn't been working well, now isn't working at all, and is starting to make some noise. I need to fix that before I use it again. It's a cool machine, but I'm not wild about having to deal with these things on a brand new $5k machine. I'm hoping that it all settles in and becomes a very reliable piece of equipment that doesn't need constant tinkering.
