I have a 2007 450 kingquad and the belt has slippage after i am hard on the brake and then accelerate. Has anyone else had this problem or solution?

 

This can occur when the belt position/speed is out of wack. What I mean is that it was in too-hi-a-gear range and prevented from resetting itself because of the rapid slow down by use of brakes. The sheaves had not re-set.
When you accellerate again, the belt has to get back to where it should be for the speed at that time and can slip for a moment. There is nothing you can do about it so ignor the situation. It soon comes right and no damage occurs.

 

I concur.
I would get a little slippage before my clutch kit when I would go through some water and slow down fast. The belt would make a horrible noise...
Since my clutch kit most of that has gone away.

I still however have a little chirp if I use 2 low for a steep decent and when at the bottom, stopped put it into 2 high and start out.

The MFG says change out the belt around 1200 miles or so if you check your book. There is a slim chance the belt is stretched so it doesn't sit low enough in the primary sheave.
That said a belt could last 5k miles or more.
If you are getting the now and then chirping as described in this and the prior post dont worry about it and blame it on animals off the trail if someone asks.

 

From the Nyroc.com website. Very good explanation.

When you throttle up to take off, you get a sound from the CVT like a dog barking, and the machine lurches forward as though you had popped a manual clutch. How come?

Well, it might be due to the way you last came to a stop. It's a mini version of what you hear when a big plane comes in for a landing and the stationary rubber tires touch down at over 100 MPH onto the tarmac. Same reason for that sound too: moving hard surface contacts stopped rubber and has to get the rubber up to speed.
The King Quad (and other brands of constant tension CVT) bark happens when you lock up the back brake until you come to a stop, or when you come to a stop while still on the throttle, such as when you become stuck in wet snow. It can also happen more readily when you come to a quick stop while going uphill.

When this type of situation stops the ATV, the whole rear drive train stops suddenly while the CVT is still partway into its operating range, and the belt will remain stuck somewhere down the rear clutch instead of being able to return all the way to the outside (greatest reduction) location, as it normally does when the rear clutch can turn enough to fully close.
The front clutch however has nothing to prevent IT from returning to its fully open/smallest pulley size, and it does this as soon as you let off the throttle. This results in some slack in the belt, in a system that's supposed to be constant tension.
Therefor when you put it into another gear, or start out again, or rev up again before the drive train can turn, the front clutch will spin inside that slack, throwing the weights out and closing the sheaves, and the belt gives a bark as the spinning front clutch grabs the belt again.

Anytime you stop the drive train by locking up the rear wheel when the CVT is somewhere within its range of gear reduction and then you let the motor drop rpm more than the CVT is 'set' for, you get slack in the belt, and when you rev it up again you'll get a bark when the front clutch spins up again, the weights fly out, and the clutch grabs the belt with all the spinning mass of the crank and flywheel behind it.

This can happen when you come to a stop while the machine is still under throttle load, such as when plowing snow and shoving a big bladeful into a snowbank, or when becoming mired in snow or thick mud. Same thing, the belt doesn't get a chance to go back to the top of the rear clutch.

That bark is pretty much harmless, but that's why you hear it.
If you like to skid to a stop and want to avoid the bark, then whenever you stop like that you can either back off and let it roll a bit just before coming to a full stop, or otherwise you can put it in neutral and give it a brief little rev before letting it go back to idle and putting it in a gear. This lets the clutches reset themselves and it will be business as usual when you take off. If pushing snow up then before going into reverse just go into neutral and give it a tiny blip of throttle. That's all it should take to reset the belt.

Note: In some cases a bark can mean there might be something mechanically wrong with the rear clutch, not letting it do its job of backshifting to compensate for what the front clutch is doing, and keeping slack out of the system. Dirt or glaze can cause that.

Another cause can be a worn belt or not enough secondary spring pressure or load. Worn belt goes deeper into the rear clutch and may not come all the way back by the time the CVT stops spinning when you come to a stop. As your belt wears it is more prone to bark.
This video shows how it can happen, even without being under load. I'll bet that belt is old.

 

The above needs to be stickey. Its asked all the time!