Pony motor not running fast enough 212 Grader

If the pony overheats, check that there isn't air trapped in the top cover of the pony impeding coolant flow. There is an air bleed petcock on the right side of the pony motor generally under the governor or in that area. Early models didn't have one but can be drilled for one if you want.

Then you may have a pony that is full of scale and sediment so coolant cannot circulate from right side to left across the base of the motor. There is a restriction in the casting sand core there under a soft plug that is visible when the engine is removed and turned upside down. Usual procedure is to remove both cylinder heads and use wire, cable chucked in drill, pressure washer, compressed air, in short about anything short of jackhammer and dynamite to get the stuff broken up and flushed out. If you do not establish flow across the engine from right to left, the soft "freeze" plug can be removed to allow direct access to the choke point. Unfortunately you cannot get a direct shot from either cylinder deck surface ports to put a probe etc into the spot you need to reach. Finally do not forget the heads themselves, they can be filled with the crud as well.

I guess in summary, the pony should not overheat and if it does you need to address the cooling system.

I had a 212 once that was slow taking off and kicking out the pony pinion latch until I learned that once the diesel started puffing black smoke and the pony sounded like it wasn't working hard any longer, if I disengaged the pinion CLUTCH lever, the diesel would immediately come up to speed and kick the pinion out. If I didn't disengage the clutch, the diesel and pinion would continue on together indefinitely. I never addressed the problem while I had it other than disengaging the clutch.

Further to cc's post, your pony cylinder head gaskets are reusable so have no qualms about pulling the heads off. And then, to reach those inaccessible parts of the coolant jacket, get an old speedo cable inner, munch up the end of it so it'll flog around a bit, and then chuck it in your drill and feed it in there while running a hose into the radiator inlet to continually flush the crud out. Also remove the coolant drain cock just below the pony oil drain and feed your speedo cable in there, up and to the back