Re: Weekly Shounen Jump [2009] - Discussion & TOC Talk v.2
Since Play Ball started with Issue 27 of 1973 (probably could go back earlier, but it was getting frustrating telling which series were sports and which weren't) the only point where there's no sports series in jump is from when Wrestling with Momoko ended in Issue 39 of 1997 to when Rookies began in Issue 10 of 1998.
I'll try to help you out a bit by linking all the sports series that would prove there was at least one (mostly major but could try minor if needed) with overlap, it won't be every sports series ever, just enough to prove there always was one:
-Chichi no Tamashii (baseball): 1968 (issue 1 no less)-1971
-Samurai Giants (baseball): 1971-1974
-Play Ball (baseball): 1973-1978
-Ring ni Kakero (boxing): 1977-1981
-Captain Tsubasa (soccer): 1981-1988
-Kenritsu Umisora Kōkō Yakyū Buin Yamashita Tarō-kun (baseball): 1987-1990
-Ace! (baseball): 1990-1991
-Slam Dunk (basketball): 1990-1996
-Captain Tsubasa World Youth (soccer): 1994-1997 (Wrestling with Momoko ended a week later so it shouldn't really be part)
-Nothing: Issue 40, 1997-Issue 9, 1998
-Rookies (baseball): 1998-2003
-Eyeshield 21 (American football): 2002-2009
-Kuroko no Basket (basketball): 2009
Yeah...just that one short period in '97-'98 is the only time without one sports manga in Jump.