could be one or more of many things, try...

checking plugs on back of engine are correct, thermo timeswitch, coldstart valve, aux air device etc.

Check that your aux air device on back of engine is working properly ie opening and closing.

Check idel control screw is set correctly.

check air pipes to aux air device and the throttle body are clean and S*** free.


these are just some basic things to try, there are moany more possible reasons lke fueling etc.