snapper 18hp lawn tractor model yt180h414k runs well but wont go fast please help

[taz67]

my mower is a 92 but in good shape i bought it from a freind but when i push the forward peddle it moves but when i push down harder i goes slower and gets jittery any solutions.

 

[Harry in Ky]

Could it be the hydro is low on fluid?

 

[taz67]

just change the fluid and filter because i thought it need it but still no diffrence

 

[mmtd]

do you have to bleed the system on those? may have some air in it. just a thought.or could their be a linkge adjustment on your peddles?