If it delivered correct mixture in the last two tests, Peter, it is very unlikely that the float level setting was wrong. The problem has therefore been an intermittent one. Dirt in jets and passages causes intermittent or permanent lean mixture, not rich. So does an intake leak. Intermittent rich mixture at idle only, only has one cause on my list of likely faults, and that is an intermittent float, needle or seat problem. It could be dirt between the needle and seat, or the needle happening to sit slightly askew in the seat due to its position in the float lever clip, or temporary binding of the float lever on its pivot. Most of those possibilities (all except the dirt) can usually be avoided by smoothly raising and lowering the float through its full movement half a dozen times before you put the float bowl on, then keeping the carburetor the right way up thereafter.
At the moment I don't suggest you do anything except run it a number of times before you consider it ready to deliver to a customer. Note that all of the possible causes I listed are likely to be self-curing anyway: the pronounced vibration of a single cylinder engine cures a multitude of ills.
