Issues with rallye wheels balancing?

Got a 72 Scamp with 14X6 rallye wheels. Wheels are powdercoated black. I have had a long time vibration on the front that I figured was wheel related. When they balance the wheels they take off the centers, which have a good amount of weight to them. I have them put on the sticky weights rather than the ones on the lip in order to protect the lip. Then we throw the centers back on and put them on the car. I got new tires yesterday and was hoping the vibration would go away. It did not. Could this simply be because the wheel is being balanced without the center on? Has anyone else run into this? Do they even make an adaptor to put on the balancer that would go over the center cap? Thanks for any recommendations. Was planning on running it soon without the centers and seeing if that made a difference.