[LWJ]

I am shooting from ancient memory here.

Let's assume high RPM means that the jetting is correct. Which is good.

The low RPM is idle, I assume? Is the mixture determined by the float height at idle? Many (most, I believe) carbs had this function.

If I were in your situation, I would double check the timing. You could be correct about the advance.

But, what I would also do is lean the crap out of it at the float. It may not start well and won't run the best. BUT, you have a pretty good chance of passing.

After you pass you may make further adjustments.

Also, remember that this advice is worth exactly what you paid for it. Very very little. I am not an expert and it has been years since I dinked around with a down draft carb.