Are you sure it doesn't start with JYA?? Yamaha VIN # from 1984 - 2005 at least, started with JYA.
Either your number is wrong, or it's not a YZ, possibly a different country/region, or... ??
You found that number on the frame, right? Did you find one on the engine? The Engine # should start with 39X or 43N
A 1984 YZ 250 VIN number should look something like: JYA39X007AE00XXXX
(where "X" are variable numbers)
The first number indicates where it was made: J = Japan
The Next 2 numbers indicate the mfg: YA - Yamaha
The 10th digit tells what year it is...
1980 = A 1981 = B 1982 = C 1983 = D 1984 = E 1985 = F
1986 = G 1987 = H 1988 = J 1989 = K 1990 = L 1991 = M
1992 = N 1993 = P 1994 = R 1995 = S 1996 = T 1997 = V
1998 = W 1999 = X 2000 = Y 2001 = 1 2002 = 2 2003 = 3
2004 = 4 2005 = 5 2006 = 6 2007 = 7 2008 = 8 2009 = 9
2010 = A 2011 = B 2012 = C 2013 = D 2014 = E 2015 = F
2016 = G
The 11th digit is the plant code (which facility it was built in)
The last 6 digits are the sequential serial number of the specific bike.
Bottom line, your number is way-off for a Yamaha YZ 250. Might want to investigate further. Good luck.