very very strange problem

[LegioIgnatum]

I just got a 89 240sx(88800) with a rebuilt engine (soch unfortunately) that has 30000 on it for 1,800$

now to my problem the car rusn strong but there are 2 problems first one is with the 5th gear here is how it goes1st gear goes to 5000rpm no problem2nd gear goes to 5000rpm no problem3rd gear goes to 5000rpm no problem4th gear goes to 4500 i rpm no problem5th gear straight road the car can only go upto 3500 rpm and stops rebving higher when i push it harder it just stays in 3500rpm and car starts coughin and poping. it feels like it cant decide to go over or not.

I feel like the problem is in the exhaust system headers??muffler?? OH ALMOST forgot to mention i dont have a cat converter just a straight pipe and a stupid getto muffler (4.5' for a n/a car???)

the second problem is A BIG ONE the car leaks some or most the exhaust fumes into the car (yup the deadly CO) i can only drive with both windows and the sun top open.(this one just not on the 5th gear this happens all the time)

I believe both of these problems are related.What do you guys think? I am new to 240sx i now it is a really good car but got to do some work on it. Thanks for the time and attetion people

[ride4lame]

i would fix your exhaust holes and see what happens after that...that seems to be the most important issue at hand.