Unsteady work after starting engine

I've bought direct autogas injection equipment for better economy. After installation I noticed, that engine works unsteady just after starting it, although it allways starts on petrol. Sometime engine even stops running. But this unsteady work lasts only for ~1 min just after starting. Later it works steady. It seems, after starting, engine doesn't know what to do - it starts with 800 rpm, then 1100-1000-500-1200-500 etc. After ~1 min it stabilizes on 800. My car is Audi V8 91' 4,2L. May be mechanics in autoservice forgot something while asembling motor? Some ideas?

Thanks