Slow to Rev up

I have a Massey 135 and when I first crank tractor it will not rev up only a slow idle then maybe 15 min. it will rev up and run find, as I use it more in the day its easier to rev up maybe less than 5 min or rev up immediately, but the next day slow again. I usely crank it let it idle to it breaks free. All of a sudden it will automately rev up, any ideas

I think it is a 60s model

Fuel filter clogged, or obstruction in tank?

um.. gas or diesel? makes a difference!