Not an absolute, but you get the full affect if you change the carb to a pre-leanburn version. The vacuum signal is opposite from the std. ignition w/vacuum advance.

I have one car I kept the leanburn hooked up, and ran a dizzy with VA and changed the carb ( as the car had sat for years and the carb was varnished up ). What a difference.

I have actually had good experiences with lean burns. I've owned 20+ Magnums and a few Dobas over the years, and have not had much trouble. Some of the Magnums I've bought have had the change done already. I certainly don't change it back.