call DA--
the DA pipe (what is that?) can be one of many -also if the (?) pipe is setup incorrectly it may not want to run at full rev-if th prop is a mismatch in load .
there are many pipes and cans and headers etc., and judging on the number of comments here on this type of problem--I would gues that the combo you have is not matched.
The engine will run as strong as any 50 -if the exhaust is matched to the prop LOAD (forget size)
on the outside chance there is a carb problem -- first get the prop to YOUR exhaust setup sorted out - then make carb fixes as may be needed.