Also, you didn't indicate if you were running points and condensor, or a
breakerless ignition. If you run a breakerless ignition with a magnetic (not
optical) sensor, then this type of ignition introduces an inherent retard as
rpm's increase. So, you really need to "curve" your distributor on a
distributor machine with the ignition system in place to have an accurate idea
of what your timing is really doing at various rpms. This can also turn up
timing problems due to erratic cam wear and/or bushings in the distributor.
Another possible solution if you're using points, is to install a second set
retarded from the main set. (Again, you need a distributor machine to set
this up.) Then, with a switch, you can manually switch from the retarded set
to the main set for starting and idling.
Myles H. Kitchen
1965 Lotus (formerly ignition engineer) Cortina Mk1 #128