hey folks, not even really sure where i should begin. I bought a 625 cfm carburetor for the 400 in my 79 f150. hooked everything up and truck starts right up. but it wont stay idleing, i have to keep tapping the gas or it just idles down and dies out. Eventually she'll get warm after doin this and is still kinda high idle. Played with the idle screw a bit and I notice the flap on the top of the carb, under the air cleaner stays all the way open. So i actuated it closed with the linkages on it, and the truck will idle down and stay pretty good as long as i hold it. i let go, flap opens, then truck idles up, then back down and dies

please point me in the right direction cause im not sure where to begin. not to in tune with carbs.

also if u need more info let me know Thank You

 

If closing the flap makes it run better you might have a vauum leak which throws off the mixture. You should be able to hear a high pitched hissing noise from the spot of the leak. Plug that and adjust your idles and you should be good to go.

 

yehp, that was it. i left one of my unused ports unplugged. she idles pretty smooth now.
thank you Harley