Chevrolet Repair: carb problem, cfm carb, fuel economy

Question
I just traded my Edelbrock comp. series carb for a quadrojet. I didn't like the fuel economy that I was getting,5-6 mpg. I believe it was about an 800 cfm carb. My problem is that now I cant seem to get it to run right. Is it possible that I need that big of a carb to make it run right? It is in a 1979 GMC 1 ton dually with a wrecker bed and weighs in at 8000 lbs. but I don't think thats my problem because when I open it up, everything smoothes out. It just sounds bad at low rpm's and almost dies at idle.

Answer
Hi Mike,

How big did you put in?  What size motor?  

If you have a 350 motor, a 650 should be minimal.  If you have a 400 motor or bigger, you need 750 or more.  Your problem is at low speed, you may need to adjust the float, or the front jets delivery.  I suspect the carb is not big enough.

I think VAN may be more help.  He is another allexperts
Chevy helper.    He deals with trucks this large and older vehicles.  

C J S