REPLACE SPARK PLUGS ON MY 99 MIRAGE AND IT RAN PERFECT FOR A WHILE THEN IT STARTED PUTTIN AND SPITTING BLACK SMOKE FROM THE EXHAUST...WHAT COULD THAT BE?

I wouldn't guess. I would check the mixture if carbourated or maybe it has an issue with the injector timing, if fitted.

Hi there.
well when you throttle up to about 2000rpm do you get blue smoke coming out the exhaust.When driving on flat road will it power up as normal with out missing or getting hot.
what colour was the old spark plugs were they black or light brown.
When you start engine and run for 3 minutes then let it idle and remove oil filler cap do you get smoke coming out.
Number 1 if yes then engine is lacking power due to piston rings worn answer rebuild engine.
Number2 spark plugs brown [good] if black soot as num1.
Number 3 if you get black smoke or blue ,then the engine is lacking compression could be your problem as num1
OTHER WISE GET YOUR TIMING CHECKED AND SET TO BOOK.

The spark plugs may have become oil fouled. Nothing to worry about. Just replace the spark plugs, start the engine and let it idle for 10 minutes.

On petrol engines, the presence of black smoke from exhaust indicates that the air / fuel is not correct. If we check and find the spark plugs on them a layer of black smoke, so no air is uncertain.
The fix is very simple: replace the filter element from the air filter, clean or replace the spark plugs and, if necessary, adjust the air intake flap from. Black smoke usually disappears after the first fixes (replacing the air filter and clean or replace the spark plugs can be done even by the driver.

If you have not replaced plugs start there A bad map sensor will also cause black smoke and poor running

These things are notorious for having crank sensor failures. Not too expensive and not too dificult to replace. I would try that first.

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check firing order. if okay possible timing belt jump timing..

check the fuel pressure regulator.

Do you get black exhaust smoke when first started? If so, then there is too much fuel at first which probably fouls several spark plugs, but as the engine warms up, the plugs start firing again. If no black smoke--is the check engine light ON? Read the code(s) if so and follow the clues.

If you have spark, try changing the fuel filter. If you don't have spark, get back to me.