Good Morning,

The stock carb is very pressure sensitive especially when they are old. I'm going to assume that you haven't rebuilt the carb lately and the float needle/seat is pretty worn. Your old pump was leaking through the diaphragm and down the weep hole, giving you a little less pressure to the carb. Change that out with a poorly made, brand new pump and the extra pressure pushes right past the float needle and down into the intake. Ultra-rich condition causes black smoke and plug fouling. Personally knowing what those pumps look like inside, it's impossible for the pump to be pulling oil from the head.

Glad its running better with the new pump, just felt like putting me 2 cents in <img src="/forums/images/graemlins/cheers.gif" alt="" />

On the rear main, was the seal just replaced or was a sleeve on the crank also installed? Those rear main leaks can be pretty pesky...