I never had a problem tuning my pro .15 until the compression got so low when warm it would just quit for no reason. With the engine cold, it would scream. Though I had the MIP pipe on it which helped give it more power but it always tuned well until it was just flat-out worn out. I have a 2.5R with a worn out sleeve - It'll start and run fine from cold but once it gets hot it gets really boggy and overheats and is a real pain to restart when warm not to mention you can't get a "perfect tune" with it no matter how hard you try. When I run into a hard-to-tune engine I eliminate air leaks. Check the fuel tank, make sure all of the screws in the engine are tight (carb, head, and backplate), replace fuel lines & glow plug, and try some fresh fuel. If you have a gallon jug of fuel you've been running, go buy a quart of either the same thing or a different brand. Fuel will go bad if left in a partially full container for a long time. If there are no leaks, and you've tried new fuel & line, and a new plug and you still cant get it tuned then I would think the piston and sleeve might be worn out. Without hearing the engine run, thats the best I can suggest. If you have a way of taking a video and posting a link to it, someone might be able to better diagnose the problem. Sometimes just hearing what its doing can make a huge difference on determining the problem and fixing it. Not all guys that work at hobby stores can tune an engine OR explain how to do it. The place I bought my nitro sport from was the only store I knew of at the time and took the info they gave me as gospel. I later found out they didnt really know what they were doing and gave me the wrong advice which made me send my engine in to exchange it and I didnt really need to do it since it only had 7 tanks through it. (I was an ultra n0ob.) <shrug> Don't know what else to tell you.

That could be a possibility but didnt it have the same problem before it started to get cold? I rarely have to put anything on the heads of my engines - they temp and run just fine without it as long as you dont flip the car and stick the engine in the snow (I run mine with the body on the car in winter to keep some of the heat in). Without a temp gun/gauge, its hard to judge if the engine is getting hot enough. Best thing to do here is put 5 or 6 drops of water or spit on the head by the glow plug. if it doesnt sizzle at all or takes a long time (more than 5-6 seconds) to burn off, its not getting warm enough and you'll need a sock on the head. I think a good time to have it burn off at optimum temps is 2-4 seconds or so. This is just an estimate.