what is wrong with my van if it is losing power?

my van seems to running at 85% and every now and then it loses power the van is still running but it gives up and I can not drive when it loses power it makes more noise as if it has slipped or somting I have checked the injectors and they seem ok they are atomising I thought it could be somthing with the btreathing but the butterfly valve is working ok on the cam so maybe it is the fuel pump but then why black smoke as if too rich