I am new to this site so please bear with me. I have an O.S. 12TZ 5 port. I am having a problem with this engine. I can't get it to tune correctly. I broke the engine in as I do all my engines. I leaned it out a little from the factory settings and just drove it around. The engine never got over 200 degrees during break-in. I have about eight tanks through it so far. I use Sidewinder 20% nitro with 12% oil formula. The problem is it is very touchy on the bottom end settings, much more than any other engine I have owned. The engine will start and idle forever, but once it gets up to temperature, it will not pull. I am no novice when it comes to tuning engines, so this confuses me. Three other guys that I race with that also use this engine have tried to tune it with no luck either. I have completely disassembled this engine and found nothing out of the ordinary. I have owned many O.S. engines in the past and found them to be easy to tune, and if taken care of properly; they will last along time. This 12TZ is a headache. I know that this is not much information to go on, but does anyone have any suggestions. My next option is to tune it with my small sledge hammer!!