I have recently embarked on rebuilding a MTD / ACE lawn tractor that has been abused, it was left to sit in the woods with bad gas in it for about a year. The tractor has a Briggs 287707 14.5hp ohv engine in it and a nikki carb. After a thorough cleaning and dumping the gas in the carb and tank as well as lightly cleaning the carb, the engine started. It however will not run with ought the choke 5/6 the way out on full throttle, and all the way out on low throttle. I have read through countless other posts on this forum as well as others directing to thoroughly clean the carb soaking overnight etc. I have done this twice now, I soaked all parts in carb cleaner and scrubbed all deposits off it looks like new. In fact it almost is new this mower is only four years old! The gaskets in the carb are like new and I cleaned all passages using a piece of copper wire. I don't think I have ever seen such a clean and good looking carb I'm actually pretty proud except that is still doesn't work. There has to be something else that is causing this issue. When it is at full throttle and the choke is all the way out it starts to billow black smoke, when it is less than 5/6 out it just surges and will keep running but just continuing to surge up and down. If the throttle is lower and the choke is pushed in the engine will just die. It also seems there is no way to adjust this carb? Please help