sir i am a 12 cbse passout student. i got last year 65% and in mains i got 40/360i jhave dropped and planned to give clat but now i again think of choosing engineringsince i have 4 months left.is it possible to crack iit this year with such a short interval of time ...plzzz suggest what to do in detail,.

You will have to really work very hard. Focus on 30-40 chapters out of 80 chapter in JEE syllabus. Plan you weekly schedule and time management should be there.
You know last 105 was the cutoff for which 30-40 questions is necessary. So read complete NCERt for chemisrty and do few topics in Maths and Physics . You can check wieghtage of different topics on given link.
http://www.askiitians.com/iit-jee/main/preparation-tips/