Final answer:
The sentence requires the word 'not' to indicate that the Jims liked motor racing but not baseball.
Step-by-step explanation:
The sentence is incomplete and requires an appropriate word to complete it. Based on the given information about baseball's popularity and its significance in American culture, one possible word that could complete the sentence is 'not.'
Both Jims liked motor racing, but not baseball.
This word indicates that while the Jims enjoyed motor racing, they did not enjoy baseball.