Assuming your engine has a conventional centrifugal advance mechanism, +/- 2 degrees is their typical tolerance range, so retarding the initial timing 2 degrees may not make much difference at higher engine speed.

Suggest you retard the initial timing about 6 degrees from spec. The initial timing spec is on the "tuneup/emission lablel", which should be located somewhere in the engine compartment.

The other way to reduce the timing at part load is to disconnect the vacuum advance, assuming that vacuum advance is active during the test, which is may or may not be depending on the details of the emission control system and test conditions.

Duke