1999 Mitsubishi Galant • 125,000 miles

I just bought a neatly used engine and gear to replace my old engine that was smoking and gear that was leaking oil regularly.
The engine is very good, cool and silent and the gear works fine too, it changes gears fast and without hindrance.
However, I noticed my car does not accelerate as fast as it usually does with my old engine and gear, with my old engine and gear, if I engage the gear to "Drive" and I step my foot hard on the throttle, my car can accelerate fast to 60kmh in an average of 4 seconds, the tyres spins so fast and it moves so fast too, but with the new engine and gear, if I step my foot hard on the throttle, the car's tyres doesn't spin as fast as it does then and it now takes an average of 9 seconds to get to 60kmh.I am not happy about this because my car has always been a fast speeding car and I want it accelerating as fast as it was before. Please what do I do? The mechanic said there's no fault with the gear, the electrician said all the fuse is ok, so what do I do? Please help
January 31, 2013.

Check ignition timing.
Ensure throttle cable is adjusted correctly and opens the throttle fully when pedal is fuly depressed.

Jan 31, 2013.
Sir, I am so so happy about your response.I would do that tomorrow.
Sir, but is it possible for the ignition timing to be wrong and the car starts and moves?
Sir, let me add an info which may also help you understand how my car worked before I changed the engine and gear.
When I fully depress the old throttle and it doesn't moves rapidly and the tyres spin very fast, the fuse for the reverse light would have cut and when I replace the fuse, the car starts accelerating with fast speed again, but while the fuse is not changed, the car moves normally but wount accelerate fast but gradually.
For this new engine, when I discovered the acceleration was not fast, I checked the fuse and found out it was ok, I now decided to remove the fuse (assuming it has cut), when I put the gear in "Drive" and press the throttle, the car doesn't move an inch but just stays on the spot and keeps making noise while I press the throttle deeper, while in the former engine and gear, the car would still move but wount accelerate fast. When I returned the fuse, the car now moves when I put the gear to "Drive"
This is making me further think some electrical faults thats not balanced with the gear system.
Please let me know if you need more explanations as i'm tired now, the mechanic said the gear and engine are fine, the electrician said he has checked every fuse and they are all ok, now no one know what to do to make the car accelerate very fast as it used to be before I bought this new engine and gear.
I sincerely expect your prompt response.

Jan 31, 2013.
If the timing belt has not been installed correctly, it would affect the ignition timing and performance. I would suggest starting with that if the throttle cable is adjusted correctly.
Did you check for trouble codes?

Feb 1, 2013.