At a certain weight 116kg+, its not happening, you're also not gonna do enough to notice the gains. Have big juiced up dudes done them? Of course, 6-7 and after that, its half a pull every time. You can youtube Thor "doing" some...they're never gonna be full pull ups.
It easier to do the bench/standing press, and push ups at that weight. I used to be a pull up machine at 180, but I didn't see much gains, and still felt weak. At 255 I feel strong, lift heavy, and can do 50 push ups in one set. Pull ups are great if you're thin/lean.
