1995 763 blows black smoke only when i use arm down function

[cherry510colo]

Great machine. 3000 hours and it only smokes when I do the arms down function. I can do arms up while combining bucket tilt and driving the machine and there wont be a puff of black smoke. But I can be stationary and once i put the pedal all the way forward, black smoke comes out of the exhaust, the engine bogs down, and everything is fine when I let go of the pedal. I can roll the pedal forward about 1/2 way and the arms work good and the machine doesn't smoke. But the moment I roll that pedal all the way forward and the arms start to go down, the black smoke comes right out and the engine bogs down. Arms speed and power feel about right even with the smoke coming out of the exhaust.
No leaks. Filters are clean. good fuel.
Anyone see this before??

I've been watching lots of YouTube videos on hydraulic spools and relief valves. not sure if I'm barking up the wrong tree though.