computer system are fuel ?

i have a friend that has a 1996 eagle talon with a neon engine swap that he came up with on his own and he wants to know if anyone has any idea why his car only revs at 4 grand and it is hard to start when it has it ran for awhile like a a week are so .................

and he has put all new sensors on it and he has know idea what it is ?????

if anyone has an idea what it might be let me know


thanks for the help