I just went through this problem a week ago, the throttle shaft was leaking from the body of the hydroulic governor. Purchased the pump rebuild
kit and replaced the o rings on the throttle and kill shafts, reassembled the govenor and reinstalled on the pump. No leaks motor
What was the reason for you taking the govenor off the pump? Thto measerue plunger you are referring to is the anti stall valve. The valve needed to be replaced in the govenor body at the same adjustment it came out at. I used a set of calipers to measure the height of the valve out of the govenor body.
If the anti stall valve is reassembled wrong or the adjustment is off slightly the motor
will run erraticly.
By the picture you have posted it appears that your govenor is identical to the one I just worked on, and it appears that the anti stall valve is screwed into the govenor way to far. The Perkins
shop manual goes into adjustment of the valve or as an alternative I still have the caliper set for the proper adjustment and could get this info to you.