I have bajaj discover 135 KS of 2008 model. I had run of 50 kms per day having 40% city ride and 60% highway ride and the bike was giving a superb mileage of 65-70 kmpl. after 4 year now i have 16 km run per day with 50% city and 50% highway ride and now bike's mileage has suddenly dropped to 35-40 kmpl. the total run of bike is 36,000 kms lifetime. I have changed the spark-plugs suggested by the authorised bajaj workshop and its just 3000 kms old. The authorised bajaj workshop are not able to solve the problem as they tested the bike on highway which gave them 50kmpl.
i think the problem is in the clutch plates as the rpm of bike goes high(i have kick start model which doesn't have the rpm meter but i can sense it through the engein noise ) on normal speed of 40 kmph. Other thing is now i have to release the 90% to 100% of clutch in order to move the bike on fully pressed clutch.earlier it was around 75%- 100% release on which the bike moves. after i myself adjusted the clutch,the release of bike occur in 75%-85% but rest of the release of clutch has no effect whether i release it or not that is after 85% it has no effect.
Workshop guy says there is nothing wrong with the clutch plate but my question is why the release of clutch is that much changed along with the pick-up of bike also dropped.

suggest me what factors are affecting the mileage as its has suddenly dropped.