i just put an os 18 on my car and ive tried all day to get it to run. I can here the engine hitting, but it wont turn over and when it does which is rare the engine races then dies. could i have an airleak, and if i do how do i find out where it is?

Is this your first nitro?

Check the following: glow plug, glow igniter, fuel lines, fuel, and engine getting fuel. If you are saying that it starts, revs real high, and dies, then it may be too lean from factory or you are not getting enough fuel into it. How are you priming the motor? I suggest taking the air filter off and plugging the carburetor with your thumb and THEN priming it until the fuel reaches the nipple.

No its my second, i had a slide carb on it and i switch it out for a rotary. it does kinda seem like its not gettin fuel. i ussually put my finger over the muffer and prime it that way, sometimes it will start for a second then die. its the same setup as my last engine and it ran fine.

Check your glow plug heater , i had this problem when trying to run my os 18cvr for the first time the battery was defective and it was quite cold outside !
Did you convert the slide/rotary linkage proprely ?