I have an '05 z400, basically stock except for a Yoshi exhaust. It handles great on the trail, but on long straights or flat out on dirt roads it gets really lose to the point of having to back out of it sometimes. I've been riding on and off for years and consider myself to be a pretty experienced rider, but have just gotten back into the ATV thing after riding street bikes for the last few years. I've never done much in the way of suspension adjustment. I'm 6'1 and about 220 lbs. Do I need to stiffen the dampening or tighten the preload from the stock set up? Or a combination of the two ? Or am I even close to what the solution might be ? Any advice would be appreciated.

 

Have you checked your tire pressure? to make sure that both left and right tire have the same pressure?

 

Here is a good resource for tuning your suspension. Keep in mind, stock suspension sucks compared to aftermarket. I remember a pro motocrosser being asked "if you had a stock motorcycle off the showroom floor and could only change one thing, what would it be" his answer was suspension. ATV's suspension, with 4 wheels, is even more of an impact.
anyway, here is the link:
http://off-road.com/atv/suspension/shockadj.htm