And does it actually stop?
If so, it is most probably the lack of 2T oil and prolonged riding like this may result in a siezure. (Though smokers can endure such torture for like ages before they actually sieze) Try adding a lil more 2Toil with the petrol.

Try above and check the carb may be running lean..

Thanks guys.
It does not stop as I get the speed down to about 40KMPH and it goes ok for a km or so, I have not gone beyond a km at 40KMPH as I'd have reached my destination (my office )
I'll increase the 2T oil flow, get the carb fine tuned and let you'll know if that helps.

Finally I got to go to the mech, but what a rip-off! This is what happened...

I went to a mech and told him about my bike pretending to stop, he removes the spark-plug and tells me that this is the wrong one, Yamaha uses W5BC and what was in there was a TVS's spark plug. And had to spend Rs70 for it, after which he tuned the carb...

I would like to know if the change in spark-plug was the problem, as the other mech told me it does not really matter.

Yamaha provides BPR7HS spark plug for all the bikes, use either Mico or NGK both are equally good

After which I went n got a little stickering done. I was still not happy, there was something missing! So, I went to a pretty "famous" mech in RT Nagar, Bangalore and asked him to do something to get my bike to go a little faster as well as to get the bike roaring when I go beyond 50KMPH. He took about 4hours, modified the muffler and did some tuning for which he charged me 600bucks! This morning when I took the bike out, it was only the roar which sounded awesome, but the performance was still the same

Hee hee what to say man, he might have just pulled off the three muffler caps and given you away and charged you 600 bucks. if you had taken it to a gas welding shop nearby 60 to 100 matter over, just modifying your muffler doesn't increase performance all you get is only sound

He had also mentioned that if I wanted it to reach about 130-140 KMPH, I had to get the piston changed and this would take 2days n 2.5K.

If you have his number call him up and ask what piston ? what size ? what make ?

My suggestion keep the stock piston and get it ported.

So, on the whole I spent 1K just for the roar, spark-plug n little stickering

Mistakes will always make a man learn better, it's good if you realize it.

Guys, do you know any mech who is really good n cheap to get my bike where I want?

Which part of bangalore is nearer to you ? i mean near to your residence !