Tractor is pulling a 10ft covercrop disc, in heavy adobe, in 3rd gear, easy work for it, has a turbo and has not had any problem in 5 years of use.  So, yesterday, discing down a slight hill, doing fine, turn to go back up the hill, it is feeling very sluggish, loosing power, then starts black smoke ?  This morning, opened air cleaner to do daily cleaning, the whole outer air filter is coverd with soot?  It has a cross pipe from exhust and intake often used in very cold conditions up in Washington state.  I am at a loss what is causing this?  Can the rack be sticking at about 1/2 throttle, so as you turn up hill it is pulling too hard?  Something wrong, don't know what.  Any help greatly appreciated, have to get er going to finish farming this year.  Thank you, Tom.

 

Check the fuel air ratio control, if it has one installed, there is a rubber diaphragm inside the can crack or get a hole in it.

I believe the seals are shot on the turbo, order new cartridge

Thank you Rome, where would I look for this item, I am not famialiar with it and the turbo is aftermarket installed B4 I got the tractor. I am assuming that the air/fuel ratio control is for a turbo? Otherwise it should be in my owners manuel or parts book? Thanks again, Tom.

It's usually on the governor but I kinda doubt one was installed. Probably just the rack was set up. See if there is a tag on the center of the turbo, should be a Garrett, AiResearch or Holset turbo. A fuel/turbo shop should be able to match it. Might want to check with Zimmerman Tractor in Missouri, 573-378-7210,  he has installed many turbos on D4's on up on ag crawlers.

No air/fuel ratio controller on those Halton Tractor Turbo Conversions.
You mention a cross over tube from exhaust to intake. If the heat exchanger tube in the intake fails you have a direct route of exhaust into the intake.
If it has pony start you would also see huffing out the pony exhaust.
 
 

Thank you Old Magnet, it has direct start, no pony. The fuel pump is supposedly off a "grader" 318, don't know if that would have any effect or not! A small flex pipe connects the exhust and intake near the top of the stacks. Maybe 1inch dia.

That pipe acts as a suction tube, if the exhaust pipe has a cone in it to pull excess dust out of the bottom of the air filter housing.

If you have exhaust gas entering the inlet from the exhaust you need to check for obstruction in the exhaust stack, the other is that you have obstruction/blocked air inlet and is sucking air via that tube.

Wombat

For the aspirator or suction tube to work correctly There has to be a baffle to Create a Venturi affect for the exhaust to pull the dust out of pre-cleaner. If for any reason you lose the baffle or it gets a hole in it where it’s not creating the Venturi effect the exhaust gas will be sucked in the intake and your air filter will become black very fast.  
not sure if your exhaust has a baffle in it but this happened on my D8 and it created a lot of problems until it was corrected.  Skinner