I'm not sure how useful this little tidbit might be...
Years ago my family had a motorhome that would run on either gasoline or propane. Since I am equating propane with vapor, and the Pinto is/was using a vapor carb, I am remembering being told that the timing curve for gasoline was different than the timing curve for propane.
At first (during accelleration), there would be about the same degree of timing advance, but after a certain point the advance for propane would remain flat, whereas with the gas it would continue to advance the timing.
Not sure, but maybe this tidbit might help with the Pinto. (As long as it doesn't get rear-ended anyway
)