I am dealing with Tractor that will not achieve rpm sluggish off idle too. In troubleshooting I replaced fuel filter no suction side leak air filter ruled out, found if I cracked the inj pump caser drain the rpm rose and engine opoperated normally. this also happened when either loosening the timing adj set nut or the timing window cover. moving the timing adj set screw has no effectc.the return line is not clogged and there is no air in the system. Do I have a inj pump internal seal leaking ?