No compression?
 

Hello im fairly new to to the hobby and have a nitro rc with a sh.18 engine. i tuned it well and was running it in my backyard. then from running well it went to not accelerating properly until it shut off. Now I cant get it to start at all. the pull cord pulls as if i took out the glow plug. So does the fly wheel. any ides how to fix this? thanks in advance.

Valve seat maybe? Glow plug tight? Head gasket? Piston rings went?

was it new or used?

also will it start again after the thing cools down completely?

If used, and restarts when cooled and runs for 5-10 mins prolly the pinch is bad. I've had this many times when I bought used Nitro RC's... if this is the case easier, and cheaper to replace the engine with a brand new one :(
If you take the glowplug out and turn the flywheel by hand there should be resistance when the piston hits the bottom of the shaft. If there is none more then likely it needs a piston/sleeve, but like I said usually its cheaper, and easier to get a new engine.

its a new car not used. when i turn the fly wheel i feel some resistance but not much i will try to start it again hopfully it will fire up.

Does the piston still move up and down when you turn the flywheel?

Yes i think. I have now regained compression but everytime its about to fire it makes a noise inside the engine as if metal hit metal and doesnt start. Normally however i can pull thee cord and nothing bad happens.

can you take a video of it
hard to diagnose a problem from descriptions.

can you post a video without a youtubd account? if you can and you tell me how i will do so once the weather gets better.

Why not just get a YouTube account? It's free and doesn't take very long. Or you can use a photo hosting site like photobucket to upload and link videos.