Nissan Micra K11 monopunto problem of 95

Hello everyone, I am new in this forum, and I wanted to consult with you a problem that I have with my micra K11 of 95.
I had a problem, the car hit me many pulls and very sudden, in principle the mechanic told me it was a problem of the CoCo, which I changed for a new one, but continued to fail, it is true that not so serious, but I kept the fault. I told the mechanic again, and he told me that the problem was the monopunto. I changed it for a 2nd hand, a carburation laboratory checked and the mechanic mounted it.
The issue is that he no longer pulls or stops, but he is very accelerated and the mechanic does not know how to tell me what happens to him.
Does anyone have an idea of ​​what can happen to you?
I would appreciate very much that someone could help me.

Surely the step by step engine should not work must check that I walk to rule out since that is the one that accelerates in idle greetings

Disconnect the flow ... It is located in the butterfly body

It is very common for a welding inside the butterfly body.
It happens to almost all these microns.
all the best

Ok, I'll try very thank you