I just rebuilt a 89 lt250sk and now i cant get it to idle. It will kick in one try when its cold but you have to hold it wide open and kick it when its hot. It starts easily but it wont idle at all. It has all the top end in the world and runs great. except that for some reason even if im just sitting there it will just start revving to the high rpms. I have almost hit trees and all kinds of **** because of this. the throttle isnt stuck it will just revving up. I can adjust my mixture screw and idle set screw all i want and it makes no difference. I was told that I may have the timing retarded a little.

 

I would start with the carb, if it starts easily i wouldn't be too quick to mess with the timing. It's set in the engine and unless the chain is way stretched it's probably ok - that's if your tensioner is working (it's automatic), and it was set correct to begin with.
The other issues sound familuar to some of my old carb issues, first is the slide in correct and not backwards? it's silly but often overlooked. You said it was just rebuilt, what exactly was done? that revving could come from an air leak causing a lean condition. What is it jet at, did you go through the carb? and what accessories is on the bike? Also double check the choke, it can get sticky and stick on, or worse keep turning off when you need it.
If you went through the carb, or have not at all, I would pull it off and go through it. In my opinion they are over engineered pos and they get finicky if not assembled correct. Pay attention to the needle setting, including the spacers and set the adjustments to stock to start off.
Here's a look at what I am going through with my two 250's quads including a pretty good guide to putting the carb together.
double suzuki lt250s rebuild

 

It's the carb, you've done something wrong with your rebuild and or your reinstall.
Leave the timing alone.
Pull the carb back off and recheck, it may even be the throttle cable is in the wrong slot or something like that.

 

I am having the same issue with my 89 lt250 quadsport. Walked out to start it today and started cold after 2 kicks. Tried to adjust the idle and air screw with no change. It runs great until I get into neutral and just barely touch the throttle. That is when the RPM's go way up. They will come down after a while or if I put it in gear and pull forward the R's drop right away. I dont understand. I had the exhaust pipe red hot and adjusted the air screw out to solve the red hot pipe issue. I am lost, it seems to be something simple but I cant figure it out. I am not even sure how much I should screw out the air screw? Any help anyone can give would be great. I cleaned the carb and adjusted both valves today also.

Thanks.

 

The screw you're turning is just for idle, it only effects air/fuel mixture at idle, once you open the throttle plate it does nothing. Your main jet takes over at this point.

 

First double check your carb, it's easy to assemble it wrong and not know it, the settings should not be too far off from the stock settings (not counting your local adjustments), also double check the cable routing.

If that checks out it's either way lean, or the timing is off. I fought a similar problem for years.... no really years. Always with the same issue - you adjust the carb, jet higher, jet lower, check timing. sometimes you got the bike to started good, sometimes not. Always the same result, way low on power, the pipe would get red hot, and it would stall if left running long enough.
I ended up getting a new (used) carb - that took care of the high idle issue. Just for the heck of it I also double checked the whole engine over. I found the key for the flywheel was broken, putting the timing off just so, this was the red pipe issue.