Failing emission test big time

[dushanf]

Dear all, 
I have a 1997 Mitsubishi Lancer bought brand new. The car struggled to pass the vehicle emission test in the recent years. In the last year, after installing new spark plugs, new air filter, the CO level was still above the standard 3. Somehow I managed to get it through the test last year, but this year the CO level is above 9. It initially even rejected from the test as it is above the high peak values. This is after installing a brand new carburetor and tuning, cleaning the newly installed and almost unused spark plugs, with a still brand new like air filter which is very clean. 
However let me give you some background info, the car has been sitting for months close to an year. I used to start it and let it idle without running just to charge the batteries. I top up oil from time to time as there is a slight oil burning issue which was there for a while (3 - 4 years roughly) and the colour of the engine oil is still good, but I didn't change the oil. Some of the current engine oil is nearly an year old however, I think I top up like half of it or so six months ago with fresh oil and the color of the oil is not bad. 
I noticed this problem when the car was stationary and when I used to drive it daily, it used to pass the test. 
Now even after reducing the fuel intake from the carburetor, with a brand new carburetor and almost new air filter, almost new spark plugs, even the mechanics are finding it difficult to understand the problem. Can clearly see the excessive black exhaust. 
        RPM    HC    CO
Standard        1000    3
Idle        1070    1384    9.39
2500 RPM    2450    812    9.99    

Any thoughts? 
Thank you in advance!

[Hyaenidae]

Is it equipped with a catalytic converter? and if it is, is it still intact?

[dushanf]

Yeah it does. I'm not sure the condition though, I too suspected whether the catalytic convertor be the culprit. Although I fixed a muffler, the sound of the exhaust at some point increased heavily to the extent to suspect that there is a problem with the exhaust. 
Any place I can check and may be replace or clean it? How much would it cost to replace it?
Thanks so much for the prompt response

Edited February 7 by dushanf

[iRage]

Okay...you should take your car to a garage and get everything sorted out. Your car can have issues ranging from cat converter going back, O2 sensors etc are going bad, gaskets going bad and fluids mixing up, carbon and sludge deposits, etc...your fuel can be contaminated, your oil can be contaminated (as you just simply top up and not do an actual change).

You said your oil is not a good color...what color is it? Do you know if coolant is leaking into it? 
You said there is black smoke, which is a typical indication of an incomplete burn. This itself can be because there is insufficient air going into the combustion, the fuel is bad (moisture could have built up in the tank; if leaving idle for a long time, you should leave the tank full), to the spark plugs going bad (even though you changed it a year or so ago the car left idle could have caused them to go bad). 

Leaving a car idle and just starting it to charge the battery is not the best thing to do. There is no guarantee that the engine comes to the proper operating temperature (what you see n the dash is NOT the operating temperature of the engine and the oil !). There are other components that need to get lubricated which doesn't happen when you don't run the car, without this lubrication, gaskets, etc..can go bad if the engine doesn't reach operating temperature. Also if the car doesn't run there is no proper pressure running through air valves (intake and exhaust) then moisture can build up and even carbon deposits can build up.

So it is really hard to answer your question because your car seems to suffer from lot rot (car illness that occurs when the car is just left parked for a long time).