Tell them nothing Dave! Next thing they will try to fix'em themselves.
You say it like it's a bad thing.
No...it is not a bad thing. Most of times i do encourage my customers to have a go at fixing minor problems themselves that way they do not have to rely on me all the time. As long as they have some understanding things mechanical.
Unfortunatly over the years i have seen so much back yard butcherings that makes me cry in disgust especially if it is done to valuable muscle cars.
Some people simply do not deserve to own them.
But hey.... they keep me in business
PS: Dave's Mustang was an easy one. Cheap electronic Bosch look alike dizzy causing havoc to timing plus some fine tuning fixed all his problems. So.... there you have it... no more secrets.