D4 7U 17xxx running on 3 cylinders.

Check the intake and exhaust valve lash on that cylinder, and check the decompressor is not engaged when in the run position, just to rule out problems there. By the sounds of it, you're getting fuel to that cylinder. The other thing you can do is remove the injector and reconnect it to the fuel line and see if it sprays "adequately" (stay well back from it when you observe - you can direct it into a tin so it doesn't go everywhere)

It's either a cylinder problem (piston, rings, sleeve, valves, head gasket, precombustion chamber, rocker arm etc) or a pump problem (pump plunger mistimed to the rack, worn pump etc).

I would first of all open up side of injection pump housing and make sure the pump plunger for that cylinder is timed correctly. Look for two dots or a dot on the plunger gear quadrant and a line on the rack. Also watch it while the engine is running and see if that pump has a lot of fuel leaking compared to the others. The best check of the pump, assuming you see nothing wrong, is to swap with a known good pump from another cylinder. Then you see if the misfire follows the pump or stays with the cylinder. While the pump is off, a depth mic can be used to check the lifter height setting. If nothing else you could compare the depth with a "good" one next door to see if it is even close to right.

You can also install the injector line pointed out toward the track and attach the injector body or adapter piece to it. Then screw the injector tip or CAT calls them a valve on by hand or you can snug it up with pliers a little. The thread will not allow you to really torque it down but it shouldn't leak much and you can turn the engine over with the fuel on and watch the spray pattern out of the injector. Keep fingers away! The valve cracks at around 500 psi and can inject fuel under your skin which must be treated immediately to avoid gangrene. Injector should spray a nice cone shaped pattern of fine droplets and cut off cleanly with no dripping.

If that all checks out and you have a cylinder problem, then the only easy check is to pull the valve cover off and check valve clearance and make sure you dont have a broken rocker arm. I've seen a few and i can see how it might cause a popping noise.

First I want to say Thank You to those that have replied to the thread. I think you guys pretty much covered all the bases.

First is to check the timing on the injection pump plunger with the rack. Then work backwards from there removing the valve cover and looking for broken springs or rockers.

When i got the 7U to my place I took out the tool box (under the seat) and in the 1-2 inches of dirt in the box was an old plunger. Not knowing what it was I threw it away. Now I think someone has changed that plunger out and maybe the new one was put in out of time. A place to start.

This tractor had set 10+ years in covered storage so I really know very little about it. The owner had passed away and the widow knew very little about it. I had to do a lot of small stuff to get it going and if I get the miss out of the engine I will be happy.

I don't know when I will get to look at the timing. With corn harvest approaching it's all hands on deck.

I just want to say thanks to all of you for giving advice. I will check in when I get it going.