adjust variable mechanism -- how/??

38
' 12.5 hp yard machine how do you adjust the speed mech.
mine will not go any faster / or into any (fake) gear besides 3rd.?? does reverse and neutral fine, BACKS UP FASTER THAN GOES FORWARD.. this was my mothers little used,stayed in her garage forever,
I adjusted shifter due to not wanting to go into neutral,, will I have to drop mower deck to adjust?
got to believe something simple stupid,

mows well, just slowwww!!!
don't have manual, is there one onlinea???