Howdy Chris and all:
Of course you can run with more initial advance. The question really should be what will I gain/lose by running more/less initial advance. I'm assuming your '66 is using the OEM Load-a-Matic distributor, which only has a vacuum advance mechanism built into it. 1st question- is your distributor vacuum advance working as designed? You will need to apply vacuum to the hose coming out of the vacuum cannister. Watch to see if the advance mechanism arm moves. If it does good. If not consider how to fix it. If the answer to 1st question is yes, 2nd questions- How do your plugs look? If they are nice and light tan in color, good, try more degrees of advance. Take a drive of about 10 miles at various speeds and loads listening carefully for knock. If you hear knock, go home and reduce advance 2 degrees and try this again. If you don't hear knock check the plugs anyway. Look for peppering and/or glazing. If you see concerns try a plug with a colder heat range, and/or possibly a different brand of gas. If no concerns there consider whether the change improved anything. Better gas mileage? Better acceleration? Be as objective as you can be. If you cannot discern a change for the better, go back to 14 degrees and be happy.
It is hard to be objective when you want something to happen. I recommend a timed acceleration test to add to the assessment. A timed 20 mph to 50mph in 2nd gear test with before the change and after the change will tell if power is improved. Honest mileage testing is another useful bit of data.
Simply advancing the initial timing may do nothing more than generate more heat. There is a point of diminishing returns. No point in exceeding that point- if you get my point.
A later distributor with centrifugal advance would be a nice addition to your engines overall performance. Electronic ignition would be even better.
Enjoy the trip.
Adios, David
