From what you've said, the interference only happens when receivers have a 27.175MHz crystal. This would suggest some sort of harmonic mixing. Without knowing the specifics of your receiver we can't really make any comment. A spectrum analyser would be of help here! Otherwise, change the crystal of the 8051 and see what frequency receiver it interferes with. After a few tests you may see a pattern evolving. This would give you the clue as to the frequency involved. The next job would be to attenuate the radiation of that frequency or to choose a frequency that won't interfere. If you're pushed for a solution - put the micro in a tin box and use feedthrough capacitors for each signal going in or out! Remember - any one of those wires could be acting as an antenna to couple the unwanted interference.

I see you're still playing with the kart timer - what conclusion did you come to with the software changes we suggested some time ago?