Probabbly it is the cold start injector and/or the auxilary air valve.

These are special added controls to the engine during warming up.
A cold engine needs more air and fuel than is going trough the normal iddling valve(hot engine) is goining.

You have to check this.

The cold start inj injects a richer mixture during start.
The aux air vale provides more air to the engine

succes
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