Why is my top speed 70 mph?

that's not a lot of info to go on...........it could be alot of different things performance-wise, timing, carburetion, compression could be bad on the motor, timing chain could be worn...................u say the car has a newer motor, is this what the guy told U? did you have this verified by a mechanic?

i would probably start with a compression test............

explain "newer"
higgs