I was trying to break in my new engine its a thunder tiger .28 out of a mgt and it ran well for about 10 seconds and then began to rev up high on its own then slow down again then I got scared and turned it off . I know it was not overheated because its cold out and I only ran for a few seconds so is this normal for breakin and if not does anyone know whats wrong?

What I mean is it was surging on its own like when u overheat it

its running lean, overheating isnt your problem, running lean is, you can be under temp and be causing engine damage from too much friction, here is what to do, read this guide, all of it, expecially the tuning parts, it will seriously help you tune and fix the problems
http://www.*********.org/cars_eng-tuning.htm

air leak? air bubbles in the fuel line?

air leaks can come from the backplates where air can get in, just seal it up with silicone, and also arounf the base of the carb, check for air bubbles in the fuel line too

Aside form attacking the air leak, reread you engine manual or instruction sheet to verify what glow plog you should be using. If the instructions specify to use a short glow plug, use the short ones only. The long plugs will significantly increase the compression ratios on some engines such that the engine will over rev and exhibit symptoms of an air leak.

By all means, take the precaution of warding off air leaks and RTV seal your engine anyways. This is always a good practice for all engines, new or used.

Another thing to check: if your engine still runs away after doing the above, dismount the engine, unbolt the heatsink and head, and verify that all the head shims that are supposed to be there are actually there. Consult your engine disassemply sheet to see that the factory installed the correct shims. If they are missing or there are not enough shims, you will have to contact the engine company to remedy this. They might even be good enough to replace the entire engine altogether!

Unless you re-tune for the cold weather (you need to richen) the engine will run dangerously lean and will exhibit an erratic idle. Warm it with a hairdryer before starting, re-tune for the conditions and if necessary wrap foil around the cooling head to allow the engine to get up to temp