I have a PiPosition (early batch version? with raspberry pi soldered to PCB). Just got around to testing it. Upon boot, a coin is inserted and the game begins normally. The car does not move until the accelerator is touched. Once the accelerator is touched the car starts to accelerate but will immediately stop if the accelerator is continually pressed. By letting off the accelerator fully the car will go into full acceleration. Reversing the on/off (while the game is still running) in the MAME analog control menu will immediately reverse the accelerator and the game will control normally until game end. If a new game is started the car will be in full acceleration mode with no contact on the pedal. Unfortunately, this issue will repeat regardless of saving the settings or rebooting. In other words, reversing the on/off toggle in the analog control menu doesn't fix the issue. I have even tried swapping the red/black wires on the pot, it made no difference.
If anyone has any questions or ideas concerning this issue, please let me know. Thanks!