D4 suddenly locked up, help

To disengage the starter pinion, pull the front floorboard cover. This will expose the little round plate on the left side of the bell housing. Remove the little plate and the pinion will be underneath. Reach in with a screwdriver and "pop" the pinion fingers loose, this should release the starter pinion from the main engine flywheel. This is also where you adjust the "kickout" of the pinion.

If the tractor rolls freely w/o the engine clutch engaged, I'd say the tranny is ok. You said you'd pulled the injectors and the engine is still either locked up or uncommonly tight. Sorry, but I'm out of ideas there. .maybe someone else on this board might have some notions. .

Good luck,
Paul

It's early, I got my coffee......but I'm still a little "fuzzy" but here goes😉 😉
Your dealing with two separate issues:
The pony pinion needs to be disengaged as Ol Grump describes.

With fuel to the injector pump vents, you have fuel in the supply chamber but flow is not getting to the injector pumps and injectors for a couple of possible reasons. 1. The rack is stuck or 2. One or more lifters are stuck in their bore which prevents the pumping action.

Also what are you getting for transfer pump fuel pressure?

Take the side inspection cover off the injector pump and work the throttle to see what is or is not working. Usually a little nudge and your favorite penetrant will get the rack going (it is spring operated by the governor, not direct linkage)

A little prying and pen. lube of the lifters unusually gets them going again.

My biggest concern is that I could turn the main over by hand and then immediately following could not. Is there something that would cause this to happen? I was pulling on it pretty good with the D2 and don't think it would be that tight if it only had some slightly rusty rings or liners. Is there something mechanical that would stop it from turning like that? Would the rack being froze up create that much resistance? When I try to turn the main with a prybar on the crank pulley, it feels like it has a small amount of play. The closest comparision that I can think of is the small amount of play in an automotive differential when you wiggle the pinion back and forth against the ring gear teeth. I am a pretty good size guy in my mid 20s and even with the decompressors open I cannot budge the engine. To pressurize the fuel system I hose clamped part of an inner tube with the valve stem over the fuel tank opening and charged it with a couple of psi. I have no idea what pressure the transfer pump is putting out as I have never had either the pony or the main running.

First off you need to get the starter pinion disengaged. So long as it is coupled you are trying to turn the pony through the pinion/flywheel gear ratio when attempting to spin the main. It would be at its highest ratio when the pony is coming up on compression. When it leaks down you are good to roll it over until the next compression stroke.

Once you are uncoupled, roll both engines over to see if there is a resistance point.

The last time I remember trying to pull start a D4 with a engaged pony the D2 could barely even pull it.

Wild Guess - but could it be "hydraulic ing" on its own oil or some other liquid thats getting into a cylinder somehow ?? A hydraulic lock would explain how its not there, there and so solid (pun intended) . . . better than loose metal in a cylinder but you would think that using the decompression lever would clear it . . . could the decompression gear be playing up ??

Cheers - Foster

One more thought on the engine "locking" up. .could it have happened that the air cleaner was off the engine long enough that little critters may have set up housekeeping in the engine?? Built a nest maybe in one of the cylinders? I wouldn't have believed if but a D4 7J engine I'm tearing/parting out had the water pump off and was sitting in a field for a few years before I got it. .and critters had set up house in the water passages of the head and block!!! You should have seen the stuff that I blew out with an air hose. .small pine cones, seed pods, fiberglass insulation, grass. .all SORTS of junk.

Just a thought. . .

Paul

The pony could be locked from fuel. A D2 pony was locked at a show unloading even though it had been running the day before and the fuel had been turned off. Bad shut off valve. (also a poor float valve) With pinion ingaged, this would make it look like a solid stop. This condition was developed over night. Open both drain valves in the pony heads and see if anything drains out.

Just a thought, Bob

You might pull the valve cover and check for a stuck or worse bent valve . We had an off brand that had a stuck valve that ended up broken off and lodeged in the head of the piston when the owner just pulled bigger and bigger tractors untill it broke the head of the valve off !! Good luck Scotty

I bought a 3-53 Detroit that sat for 10 years. I had to use a pry bar on the flywheel after removing the starter. The leverage that is possible between the flywheel teeth and and the flywheel housing can be pretty impressive. Material that gets in through an open valve as previously mentioned can stop a piston near TDC. I had a D318 once like this. It's possible that an engine is free at first but when the pistons move up into the heavy rust on the liners that they seize. Phil😊