I have isolated my issue to the secondaries when I did some runs this summer at various stages of my rebuild, I only had the high idle after I gave it enough throttle to cause the secondaries to open up. If they stayed closed, it would return to the normal idle. I would try and test your carb the same way. Do a run with just the primaries open and then return to idle. If it idles normally, I would say that you found your problem.