Re-setting the idle mixture

Lately I found that the engine was holding back a bit when giving a lot of throttle. I thought maybe the idle mixture would make a difference. Because I had never done this before I thought I'd try and set the idle mixture on my carb myself by using a vacuum meter. The idea is to set the idle mixture so that the highest possible vacuum is achieved. So I hooked up and vacuum meter and started turning the 2 idle mixture screws. I did get a better idle mixture but it did not improve the holding back of the engine. I guess it has to do with the engine being cold coz with a warmed up engine it picks right up when pushing the pedal down...