THose are super super high revving little thingies. Make sure your point are clean and not all worn out and floating at high rpm. Try re gapping them and messing about with the iggy.
And yeah, check your timing. also what gearing are you running.
I had one of those ...kits....( sorry i hated mine ) and it felt like it was broken till about 35 then hit the pipe up to about 50 but it was so miserable up to there it was silly. Maybe your geared so HIGH that you cant get to the point where it takes off?
worst case scnerio those case matches are real close to a metra 65...