Bajaj Auto — platina bike purchased on aug 2018 having many problems

I have purchased Platina bike in Aug 2018, within 4 months black smoke (carbon) came from silencer. I went to service centre on 20th Dec and they did the service. Again i faced the same problem on Jan 2019. I went to SC on 5th Jan 2019 and they did something and the problem was not cleared completely. On 21st Jan while driving suddenly the bike went from right to left. After that i started facing the same problem whenever we travelled double seat. So i went to SC on 28th Jan 2019 for both the problems. After maintanance they charged Rs 2212 ( including 1200 for AMC). Every service they charged Rs 50 for Battery Terminal (what is the baterry terminal?). I met the sales person in showroom they are not ready to take the responsibilty and forcing me to meet service center I am totally upset with the product and want to do a legal step. Can any one help me for this?