MSD dist or MP (mallory or old style) dist?. 2 commmon SB slots are stamped 11 & 17 (22/34 on crank) on older electronic dists. not normal at all for the box to change the timing. it might "take out" (improperly delay) timing at higher RPMs a common complaint of the new (er) orange boxes but not at idle. is your idle speed changed which it would some if the initial was changed 4 deg. I'd open up the dist & see what you see for starters. what box did you have & what did you swap in?