tappets get disturbed very often for some bikes, you may check clearance as per the user manual suggests , the exhaust won't get clogged in 18k kms , but sometimes children insert things into the exhaust opening , you can check that by listening at the exhaust note , it should be clear ..

he already mentioned that piston belongs to carb of your bike . you should also check carb's diaghpragm for any cut or tear .
most used technique is open the spark plug and bring it close to the metallic part of your bike and kick or push self start to check the spark .

Answers in bold.

Induction coil (if thats what kiraxe means by HT coil) is traditionally found under the fuel tank , bolted to the frame . It doesn't get sudden death but weakened and intermittent so it's very difficult to pinpoint . It is damaged easily by moisture and can cause weird problems .. symptoms include engine dying suddenly as if out of petrol ... then starting up again on kick and operating ok for sometime again . Any auto electrician shall be able to test and tell you if it's ok or not .
Vaccum piston is the needle-valve slide mechanism in the carb I think .

Ok, will get the induction coil/ HT coil checked too this sunday..
Needle-Valve Slide in carb also seems ok..
Yesterday I compared my bike's spark with another & guess what, my bike produced more spark then the test bike.
And about tappets, my mechanic told me to come with atleast 2-3 hrs of spare time, so I'll get it checked on sunday.
.
And I'm also trying to figure out WTH bike behaves normal with choke ON (any reason for this??)
So plz tell me how a choke works on stunner, does it reduces the air flow or increases the fuel flow or both.

Ok , even if spark is good .. check induction coil .. it can produce good spark one moment and then short out in operation intermittently ... as I told you already ... that makes it a pesky problem to pinpoint ... i've had it . Occurs mainly in a wet bike during monsoon .
Choke does the same thing if I'd choke you .. i.e strangles the air supply to the carb . This seems to be the clue in your case ... your bike is working better with less/no air to the carb .

okay, will get it checked tomorrow by the same mechanic..
btw, wanted to know the process of checking it, just as a guide.

your problem indicates that there is water in petrol. Water in petrol causes the bike to run as if it is in reserve and also on choke it will run better. Run the bike in reserve for 30-40 km and then take it to a mechanic. Then open the tank and dismantle the tank and drain all the petrol in a bottle. Then clean the inside of tank with a cloth and keep it in hard sunshine for 1-2 hours. Then fill clean petrol and also drain the carb and clean the jets as water will be sticking to it.

@sibun,
carb & tank was cleaned recently (i thought the same water thingy) but problem still persist.

Auto electricians know how to test it , I dunno how they do that .. never seen in my presence . I just know that it's a step-up transformer with coils inside .

Guys, got the tappets and induction coil checked.
Both are ok.
The mechanic wanted to open up the engine too.. Suspecting partial engine seize but i declined him. Because i'm sure its not engine problem, it would have emitted smoke.. Power & fuel efficiency drop should be there.. Right. So at last he tuned the carb and he tuned it super rich..(usual AFR screw position is 4 turns but its now slightly more than 5) Problem has reduced but its still there.
The mechanic says its happening because of inadequate fuel supply.. Where & what's the problem, help me figure out.

i) find a tiny hole somewhere in the fuel-tank cap and clean it . It's a pressure equalizing vent . You can blast it with wd-40 .

If this doesn't do the trick then ,

ii) Drain as much petrol you can by removing the fuel tube & opening the petcock to reserve . Disconnect the fuel-gauge electrical wire plug . Unmount the fuel tank . Open the cap and turn the tank upside down on some vessel to drain the last part of the fuel . Loosen the nut that fixes the petcock valve beneath it and take out the petcock valve as a whole . A fuel filter(strainer) thingy should also come out of the tank with some wiggling .Carefully clean off this whole system as far as possible with a bowl of warm water and tide powder in it and old toothbrush , wash and dry well . Check for rust blockages etc . Buy a new piece of fuel tube . Reassemble and refit in reverse order . If there is no water or gumminess in the drained fuel , you can reuse it after filtering .(I use a wad of cotton stuffed tight into a plastic funnel as filter for this ) .

If this doesn't do the trick either , next is the carburetor & float system . Btw .. how did you clean the carb ?

I didnt cleaned the fuel filter.. everything else was done (silly me) will do it asap. Can you precisely locate where the fuel filter is? I mean inside the tank or with the petcock? Few pics of the procedure would be very very helpful.
And for cleaning carb I followed this:How to Clean a Motorcycle Carburetor, the RIGHT way. Evan Fell Motorcycle Works

Older honda bikes have a tube like plastic protrusion with sieved walls extending inside the tank as fuel filter , some bikes have similar sieves inside the petcock mechanish . Stunner being a newer model I don't exactly know .. but surely your service center engineer will be able to tell you exactly where it is in a sec . Whatever it is , it'll be just a simple sieve like thing . Clean it .
What did you use as carb cleaner ? And your process was thorough ? I mean .. you can easily get the problem you have from some mistake in carb cleaning or re-assembly . Considering that it cures somewhat on choke application .

Will talk to the svc guys soon, about the fuel filter.
And for carb cleaning i used petrol, old tooth brush & a thin wire for jets. And yes the process was complete, without any mistakes (I've done it earlier too)