Ambac's suck to get right in our opinion...takes a shop familiar with them to make sure they run right.
If it black smokes during cranking, I'd say you have a pump issue still...AC's should white smoke on startup until they warm up with the timing they run.
I'm not the pump expert, but I do know we have the most problems with the Ambac's....not that it's a lot of problems, but for the number of tractors that used them, we have more problems than the old Roosa's
Chris