Is it fair to say that a new muslim always have effect of kufr in him instead becoming muslim.
First of all, it's a huge favor of Allah(SWT)upon us when He guides us to the path of Islam. Do not let it go by any means.
The evil thoughts you experience is from the Shytan, and he will try his best to make you fall in Kufr.
This type of whisper of the shytan is experienced by practicing muslims as well, but we have to fight it, life is a test.
And remember always fight against this thoughts, do not give up.... Islam is the only true religion.... so do not loose hope.
Sahih International Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful." [Sûrah Az-Zumar: 53]
So do not loose hope in the mercy of Allah(SWT), He is always with us. I don't know if you are facing this trial, but whoever is facing, convey it to him/her.
May Allah(SWT) forgive us and guide us to the straight path. Ameen.
Of course this is because the new Muslim will be changing his ways (not drinking alcohol, no girlfriends, no gambling etc..) and the Satan will tempt him by all means necessary to cast doubt in him as he has left all worldly desires for Allah and His Rasool (pbuh). For example, if a man leaves alcohol, he will want to go back to it, but he must fight it!
The revert to Muslim will have trials upon him by Allah to test his faith. Just saying "we believe" is,not enough. The following verses will be of help as in Surah Ankubut:
And following concluding Hadith: