engine races then dies

air leak tighten all air intake and carb bolts

Yep sound like a classic air leak , there are a number of threads about this problem.
The gist of it is to look over whole motor(with covers off) look for misshapen gaskets, fuel/oil residue ,gaps in case ,and or seals out of place .
I even had one motor were there was a leak from stator mount
http://db.tt/i8q5OoC. <--- pic link
So clean up motor real well and when you find leak will tell you how to fix it ,generaly a new seal or some lock tight in the right place will do the trick.
Hope you fig it out good luck

Sounds just like mine, I had a small leak up top too, but my big leak that I couldn't find was the bottom crankcase bolt was gone and a piece of the gasket was hanging out! It was running just like you describe. I cleaned it up real good and then coated where the missing gasket came from with SealAll and let it dry for 4 hours. It was much better and ran good. I came home and foound a tiny little bit of oil on the bottom, so I cleaned it again and went over it with more SealAll. That is some good stuff! Going to let it dry until tommorow before trying it again, but I'm happy with how it's driving now, except I don't know how much damage I may have already done from running it lean like that and letting sand get in through the leak. Looking at replacing it, but ride it till it blows for now!