Okey Dokey!!
When looking at the rear of the engine, the carb goes in the upper right hand corner.
And also..very important..the disk can go on the crank pin in two different positions 180 degrees apart. You want the opening in the disk to just start to open as the piston becomes visible in the exhaust port as it goes up. That is, as you turn the engine in the direction of rotation (let's say CW since we are looking at the rear), the disk opening should start to open just a little after BDC.
Just be careful when putting the rear housing on and make sure the disk engages the crank properly..it is possible to miss to one side and it will look like it's working...until you turn it the other way.
Oh yeah..one more thing!! One of the screws is just a little longer than the others..be sure to put it next to the carb...if you put one of the short ones in that hole it will strip very easily (yeah, I did that once!)