Bearing in mind that I have done quite a few of these small 2 strokes, I have a couple of ideas for you.
I'm not exactly familiar with the mac II, but I presume the engine is the generic half crank Mac engine.
Try this first - Check the intake fuel line for cracks - if its sucking air, it will lean and cause the symptom you describe. Just had a Ryobi do the same thing not 2 hours ago. And check the fuel filter at the same time.
Take the carby off, and the carby adaptor (black plastic bit between the carby and engine). check the gaskets on either side line up with the small hole (which operates the metering diaphragm) and that the small hole is clear. and that the hole on the engine is also clear.
There is probably only one mixture adjustment screw on the carby (and the idle adjust). It probably will be a non tamper type. If you have an old limiting cap (from a homelite or ryobi) push it on the screw and it will allow you to turn it. Otherwise either dremel or hacksaw a flatblade head into the screw.
The last thing (after doing the above), take the exhaust off (held on by springs). Stick a piece of metal pipe (or a screwdriver) under the spring - push on both ends and lift the spring off. Don't try to use the end of the screwdriver to lever it off, slide the screwdriver through and use it as a handle - if that makes sense. Clean the carbon out of the muffler and check the exhaust port whilst you are there.
By the way, when you had the carby apart, did you remove the little metal mesh gauze filter and spray it with degreaser? they sometimes clog with invisible dirt that needs a good pressure to move it.
Best of luck - these small ones can be a bit of a bugger sometimes
Tyler
