Went through this recently with my 86 Voyager - it was very difficult to get air in the rear tire with the straight schrader valve so close to the wheel hub. As you've probably discovered, the valves come in different diameters. In my case, I ended up using a 90 degree valve stem for a VN1500 (PART# 16126-1191) which was the right fit (may or may not be the right fit for your application).
According to Parzilla....
This Kawasaki 16126-1191 VALVE,TIRE fits the following models and components:
Kawasaki Motorcycle 1987 Voyager XII - ZG1200-B1 TIRE
Kawasaki Motorcycle 1987 Vulcan 88 - VN1500-A1 TIRE
Kawasaki Motorcycle 1988 Vulcan 88 - VN1500-A2 TIRE
Kawasaki Motorcycle 1988 Voyager XII - ZG1200-B2 TIRE
Kawasaki Motorcycle 1989 Concours (Canada Only) - ZG1000-A4 TIRE
Kawasaki Motorcycle 1989 Voyager XII - ZG1200-B3 TIRE
Kawasaki Motorcycle 1991 Voyager XII - ZG1200-B5 TIRE
Kawasaki Motorcycle 1991 Vulcan 88 - VN1500-A5 TIRE
Kawasaki Motorcycle 1990 Vulcan 88 - VN1500-A4 TIRE
Kawasaki Motorcycle 1990 Voyager XII - ZG1200-B4 TIRE
Kawasaki Motorcycle 1991 Concours - ZG1000-A6 TIRE
Kawasaki Motorcycle 1989 Vulcan 88 - VN1500-A3 TIRE
Kawasaki Motorcycle 1990 Concours - ZG1000-A5 TIRE
Kawasaki Motorcycle 1999 Voyager XII - ZG1200-B13 Tires
Kawasaki Motorcycle 1999 Vulcan 1500 Nomad - VN1500-G1 Tires
Kawasaki Motorcycle 1999 Vulcan 1500 Nomad - VN1500-G1A Tires
Kawasaki Motorcycle 1999 Vulcan 1500 - VN1500-A13 Tires
Kawasaki Motorcycle 2001 Vulcan 1500 Nomad Fi - VN1500-L2 Tires
Kawasaki Motorcycle 2001 Vulcan 1500 Nomad - VN1500-G3 Tires
Kawasaki Motorcycle 2001 Voyager XII - ZG1200-B15 Tires
Kawasaki Motorcycle 2000 Voyager XII - ZG1200-B14 Tires
Kawasaki Motorcycle 2000 Vulcan 1500 Nomad Fi - VN1500-L1 Tires
Kawasaki Motorcycle 2000 Vulcan 1500 Nomad - VN1500-G2A Tires
Kawasaki Motorcycle 1998 Voyager XII - ZG1200-B12 Tires
Kawasaki Motorcycle 1998 Vulcan 1500 - VN1500-A12 Tires
Kawasaki Motorcycle 1995 Vulcan 88 - VN1500-A9 Tires
Kawasaki Motorcycle 1995 Voyager XII - ZG1200-B9 Tires
Kawasaki Motorcycle 1997 Voyager XII - ZG1200-B11 Tires
Kawasaki Motorcycle 1997 Vulcan 1500 - VN1500-A11 Tires
Kawasaki Motorcycle 1996 Voyager XII - ZG1200-B10 Tires
Kawasaki Motorcycle 1996 Vulcan 1500 - VN1500-A10 Tires
Kawasaki Motorcycle 1994 Vulcan 88 - VN1500-A8 Tires
Kawasaki Motorcycle 1994 Voyager XII - ZG1200-B8 Tires
Kawasaki Motorcycle 1988 Concours - ZG1000-A3 Tires
Kawasaki Motorcycle 1987 Concours - ZG1000-A2 Tires
Kawasaki Motorcycle 1986 Concours - ZG1000-A1 Tires
Kawasaki Motorcycle 2004 Vulcan 1500 Nomad Fi - VN1500-L5 Tires
Kawasaki Motorcycle 2004 Vulcan 1600 Mean Streak - VN1600-B1 Tires
Kawasaki Motorcycle 2003 Voyager XII - ZG1200-B17 Tires
Kawasaki Motorcycle 2003 Vulcan 1500 Mean Streak - VN1500-P2 Tires
Kawasaki Motorcycle 2003 Vulcan 1500 Nomad Fi - VN1500-L4 Tires
Kawasaki Motorcycle 2002 Voyager XII - ZG1200-B16 Tires
Kawasaki Motorcycle 2002 Vulcan 1500 Nomad Fi - VN1500-L3 Tires
Kawasaki Motorcycle 2002 Vulcan 1500 Mean Streak - VN1500-P1 Tires
Kawasaki Motorcycle 2008 Vulcan 1600 Mean Streak - VN1600B8FA Tires
Kawasaki Motorcycle 2008 Vulcan 1600 Mean Streak - VN1600B8F Tires
Kawasaki Motorcycle 2007 Vulcan 1600 Mean Streak - VN1600B7FA Tires
Kawasaki Motorcycle 2007 Vulcan 1600 Mean Streak - VN1600B7F Tires
Kawasaki Motorcycle 2006 Vulcan 1600 Mean Streak - VN1600F6F Tires
Kawasaki Motorcycle 2006 Vulcan 1600 Mean Streak - VN1600B6F Tires
Kawasaki Motorcycle 2005 Vulcan 1600 Mean Streak - VN1600-B2 Tires
Kawasaki Motorcycle 1993 Concours - ZG1000-A8 Tires
Kawasaki Motorcycle 1992 Vulcan 88 - VN1500-A6 Tires
Kawasaki Motorcycle 1992 Voyager XII - ZG1200-B6 Tires
Kawasaki Motorcycle 1993 Vulcan 88 - VN1500-A7 Tires
Kawasaki Motorcycle 1993 Voyager XII - ZG1200-B7 Tires
Kawasaki Motorcycle 1992 Concours - ZG1000-A7 Tires