I've been fighting a rich mid-range hesitation with my 450 since I installed Keyster carb kits. After comparing the old slide needles to the Keyster ones I found that the taper of the needle was quite different and the needle was quite a bit narrower at the tip. Since the kits don't come with the needle jets to match the needles I decided to re-install the old needles. That has solved the issue. The gaskets, jets and needle/seat assemblies seem to be OK but I would be very careful to compare the old and new parts in these kits from now on.