If it is only retarding two deg or less with RPM change, it is likely your cam end play moving forward and rearward changing the gear mesh and backlash of your helical cut dist gear altering your timing setting. Less end play settings give less timing change at your dist. A crank trigger pick up eliminates this issue that most all distributor triggered igns that are rotated by the cam have due to necessary cam end play. The retard can be virtually near zero to 1/2 degree with minimum recommend cam end play settings.and proper gear backlash set ups.
Bruce