controlling temperature on a charcoal grill when smoking meat

Hi,

I am smoking a chicken for the first time.  I haven't done much smoking so I don't know the tricks of the trade.  I am not using a smoker because the one I have isn't working.  Instead, I am using my weber grill.  I've put the coals on one side and smoking chips inside tin foil on top of the coals.  On the other side is the chicken.  

I am having a lot of trouble regulating the temperature.  At first, it was too hot, about 480 so I took half the charcoal out.  Doing this brought the temp down to about 390.  I partially closed the vent and this brought it down to 350, then almost closed the vent completely, this brought it down to a little under 300, but after about 15 mins, the wood stopped smoking.  I was essentially extinguishing the fire.

It seems that to keep the fire going, the vent needs to be more or less completely open, but this results in too hot of a fire.  Does anyone have any tips on how to keep the heat in the 225-250 range, and keep the fire going and smoking?

Thanks,

btw, I'm using a Weber Performer and lump charcoal