HI I changed the head gasket on my nephews Ibiza 1998 1.2ltr single point injection. it accelerated before the head gasket change

His head gasket blew and his engine was kicking out white smoke. so i assumed it was head gasket so i changed it with no problems. The problems started after starting the engine The engine starts and run really ruff and although the timing is good it will not accelerate infact i have to feather the accelerator to keep it idleing. I see that the accelerator only adjust the air flow so im a bit stuck.

can anyone assist with this problem of no acceleration when pressing the accelerator?