OK, I got my ST-15 going today its got one tank through is at idle. I got it running while I had my finger on the brake (forgot to mess with the throttle trim) and it was running pretty good. It cut out a few times but it started right back up. Then one time it cut out and locked the whole engine up. I removed the glow plug because I thought I had some biult up pressure, it did a little ppsstt and that was it. And it still is locked up. I dont think I threw a rod or any thing because it was just idling. I dont know if it has do with the temperature out side and the stuff being more brittle (25 degrees), but I dont think that would be to cold. PLEASE HELP ME.

25 degrees is too cold to break in an engine. are you sure that the engine isnt just flooded? have you tried to turn it over wit the plug out?

Yeah I had the glow plug out and it is still siezed up. It wont even budge.

well it sounds like it is in fact seized. you have tried to really yank the pull start right? if you cant get it free try to take the engine apart(starting with taking the head off) and make sure that its not just a metal shaving or something. if it is seized and not able to be fixed without buying a new piston and/or sleeve, email the company that makes the engine and tell them what happened. they should replace it.

i broke my rc in -15 weather here in Canada[>:]

I was thinking it might be something in the pull start because I was pulling on it until I thought it was going to break and it turned over a couple times.

I pulled the motor and took of the pull start, muffler, and head and it broke free.