The Fox 35 is an iron piston motor so be sure to run it on fuel with around 25-28% percent castor oil. Some folks claim to get by on less but most seem to have good success with this ratio. Using some of the modern all synthetic blends with total oil content below 20% will probably result in a shortened engine life. Also if it has set around for years make sure the rod isn't frozen on the wrist-pin. It will run in this condition but the wrist-pin holes in the piston will wear out quickly. This is easy to check on a Fox 35. Just remove the backplate and see if you can slide the rod back and forth on the wrist-pin.