I am not sure about u'r simulator, but u'r program works depends on the value at A. I made a slight change in u'r program.
check it.

org 30H
mov a,#00h
mov tmod,#01h
tt:
MOV TH0,#0
MOV TL0,#0
setb tr0
JNB TF0,$
CLR TR0
CLR TF0
inc a
cjne a,#140,tt
end