Any way to adjust the "Park" setting (Wiper)?

[soule64]

For three days the wiper motor has been kicking my butt. I have searched online and checked service manuals, but no luck. I am using a '78 NOS switch with a rebuilt '78 wiper motor in a '76 SWB with a new Painless chasis harness. Everything works great except for the park function - the motor WILL park itself, but it stops 90 degrees short of where it should be, putting the wipers in the middle of the windshield. Since it still wants to travel down when re-applying power to the motor, changing the postion of the wiper arms on the output fittings won't help. I can't change the position of the cam(?) on the motor shaft without drilling the shaft hole (on the cam), but doing that would eliminate the beveled area which keeps the arm from slipping around the motor shaft. So, back to my question - is there any way to adjust the wiper motor "park" timing?
Also, can anyone tell me was the bolt in the picture is used for? I thought it might have something to do with parking the wipers, but turning it in either direction has no effect. It does change the RPMs of the motor slightly, so perhaps it is used to set the speed of the wipers?
I sure would appreciate any help if anyone has ever run into this before. Thanks.