Engine smoke burning eyes a bit after changing the assesmbly

[danishhayat4567]

I’ve recently changed my Car(Vitz)assembly from 1000cc to 1300cc, after installing the assembly they welded a piece of pipe to my exhaust as it wasn’t reaching the assembly. Everything is running good except the burning eyes smoke.
The mechanic told me its because of the filter in exhaust that has been removed and by installing a filter the eye burning smoke will go away.
Plus the fuel consumption is also very high. 13km/liter on highway.

[hash4u]

O2 sensor installed properly?
proper Cat?
check light working and off?
exhaust leak checked thoroughly ?

[danishhayat4567]

Yes my mechanic said that the sensor is okay and there’s no leak in exhaust he said the filter in exhaust has been removed by it previous owner that’s why it’s burning eyes. But the thing is that before changing the assembly it never happened.