I just installed a factory rebuilt 2150 carb on my 77 f-150 351m Its probably not adjusted right or something, but when i give it gas and let off it doesnt idle down very quick, it stays at the higher idle for 3 seconds or so. Im pretty sure the spring is pulling the the throttle all the way closed so i have no idea what it could be??? I wish i knew how to adjust a carb. Any ideas????

 

Does the carb have a throttle positioner device (a dashpot, solenoid, or vacuum motor in front of the throttle lever on the left side)?

Does the idle hold steady and then drop suddenly, or does it drop gradually down to the lower speed? It shouldn't drop suddenly. Sometimes a slightly prolonged, gradual idle speed drop is caused by new throttle positioner components.

 

It does have the dashpot solenoid, and when it does drop its a slow gradual decline in rpm's

 

The dashpot is the old one that was used with the old carb though, and its really not even touching the throttle bracket or whatever you want to call it, so i dont think its the culprit

 

One more thing, i have a few vacuum ports on the carb that i have no idea what to hook them up to, if at all. One is the Fuel bowl vent valve port, i just have it capped right now, where should that hook up to, the other is on the lower driver side of the carb, right by the mixture screw, it too is capped right now, my old carb didnt even have that lol, sorry if i am confusing in my posts, im a carburetor novice who just wants to do this himself

 

F150s were considered HD vehicles in MY1977. On a '77 HD engine , the only carb ports you need to use are the spark port, PCV port, choke pull-down port, and the power valve port (if the carb uses an external vacuum for the power valve).

The spark port is located just below and in front of the choke housing on the right side of the carb. It should be connected to the distributor's vacuum advance canister.

The PCV port is a larger port, usually on the rear of the carb base or on a spacer below the carb. It should be connected to the PCV valve on one of the valve covers.

The choke pull-down port is located on the right rear of the carb at the bottom, and it is usually a long tube with a 45-degree bend pointing up. It should be connected to the choke pull-down diaphragm (just above it).

If you have an external power valve port, it is on the bottom front of the carb, toward the right side. It should be connected to a manifold vacuum port. The stock intake manifold has a vacuum port located in front and to the right of the carb for this purpose.

Any other ports on the carb should be blocked off with plugs.

Check the throttle shaft for looseness in the carb body, and check for binding when the plates close. If the throttle shaft is too loose in the carb body, the throttle plates might be rubbing against the throttle bores when they approach idle position (closed).