Not sure I understand the problem. How can the kart's receiver trigger the 'crossing line' event?

It is also important to know the receiver's intermediate frequency (IF). This used to be 455kHz, and can be calculated from the ACTUAL receiver crystal frequency (if it is given on the can) and the required receive frequency (27.175). If the IF is 455, I can't see any way the receiver would respond to 22 or 33MHz (the second and third harmonics of the cpu clock). So called 'image' reception would require an IF of ~2.5 or 3MHz.

I wonder if some dominant repetetive action is occuring on the board at a relatively lower frequency that is generating a strong harmonic on 27.175, 27.175+2.IF or 27.175-2.IF? Hard to imagine - even a single cycle loop is <1MHz...
Dave