How to change mech advance on this distributor?

Ya good idea. $100 440 Source distributor. Get what you pay for I guess. MSD ready to run looking real good right now!
Well since you have it OUT and APART, you can get it exactly where you want it. That's certainly what I'd do. You said you need 14, right? Then just limit the distributor to 7 degrees and there you go. You can use a school protractor and get it dead perfect.