My 1988 2wd 2bbl carb truck does not run well after starting up and runs rough for at least 15-20 minutes. On first start up it puts out alot of black soot. Also on the colder days it sputters a few seconds after I turn it off. Overall the exhaust seems to have a bad polluted smell to it.

If I turn the motor off after 20 minutes after its first start of the day and then turn it on it runs great.

I have been advised this might be the AAP Diaphram and not a choke issue?

If possible advise what you think it is and what is a good web site to get lower cost Carb parts for my truck instead of paying alot of money at the dealer for parts??