Just like a vet can examine an animal's poop to get an idea of its health, you can pay attention to the quality of your bikes exhaust to get an idea of what's going on inside the engine. There are 3 types of smokes you might find coming out of a sick engine:
1. White Smoke
2. Blue Smoke
3. Black Smoke
White Smoke:
Piston Rings are usually the culprit. They must have worn out completely and your engine lube is entering the combustion chamber.
Blue Smoke:
Usually the most difficult to detect of all, blue smoke is usually cause by malfunctioning valves, like fried Valve seals, damaged Valve guides or sometimes even slightly worn piston rings.
Black Smoke:
If you see large amounts of Black Smoke when you start the bike and less amount when its warmed up it means either your engine is running rough or misfiring. It could happen because of several reasons like your Carburetor Choke has malfunctioned, or the Carb Jets needs cleaning. Another culprit could be your Ignition system, which includes CDI unit, HT Lead, Magnet Coil, etc etc.
BOTTOM LINE:
Bottom line, black smoke is not caused when engine oil enters your combustion chamber because when that happens the smoke is white and very obvious. Engine emits Black Smoke when you have problem with your carb or engine is misfiring. So GET YOUR CARB AND MISFIRING FIXED.
The Googly:
Acha Black smoke ka eik aur issue bhi hai. When the smoke is black, you cannot detect blue smoke…..so chances are maybe your engine is producing blue smoke too. Counting in the age of your engine and mechanics advise of head rebuilt may suggest you also have blue smoke, matlab Head ka kaam hai. Mechanics can tell by listening to the sound of your engine running.
I don’t think you need a full engine overhaul. Agar dil chah raha hai aur pocket ijazat de rahi hai tow go for it werna first get your Carb and Ignition system checked, if the issue still prevails go for engine Head rebuilt.