I have a 1977 f250 460 with c6.
As far as I now everything is stock on the motor. The carb is a motorcraft but I cant see what model it is, there is a ID tag that I need to get the numbers off of so I can see what I have.
I don't know much about carbs so I'm trying to learn as I go, but what has me wondering about the secondaries is going up inclines. We have a canyon that is a decent grade and long, I was towing a empty flatbed trailer and with my foot all the way down could never get it above 40 mph it just felt like that was it. I have been out playing with no trailer and it feels like I can get them to open up when it down shifts, but cruising along at 55 and stomp it nothing.
I am worried about pulling our camp trailer with it and that is the main reason I got the truck with a 460.
Right now money is tight and the truck runs good on flat ground but any ideas of what to check?

 

Did your pickup shift down while pulling the trailer up the grade? your kickdown linkage may be out of adjustment if it did not. Vacuum secondaries will not open until there is sufficient need for them to from the air traveling thru the carb and that takes higher rpms usually. try shifting it down manually next time and see what that does. If not that then your trailer is probably heavier than you might think. or it could be several things but thats what I gathered from what you said. Hope it helps..

 

I agree, my 460 would gain speed up a very steep incline with a 2400lb trailer loaded with a 8N tractor, bush-hog and grater box on board!!!!
At 40mph in high gear, if will be hard to gain speed on a steep incline loaded.

 

i beleive the stock motorcraft is a 605cfm mechanical sec. carb but has an air dampner in the secondaries to block them until engine load reaches a point where the demand for more air opens them. i hate those carbs so i switched to a 750 vac holley (#3310) with the #1932 mr gasket adapter and felt like it gained 50 hp! no joke the gain was incredible! i would check the secondary linkage and take the air filter off and see if the rear barrels move freely if you plan on keeping that carb. i agree with above, my truck would pull a hill with ease whether i had a trailer loaded or nothing at all. they have incredible torque.

 

Thanks for your replies, and that makes me feel better how your trucks tow. I've been out looking at the carb, checking vacuum lines and cleaned the linkage on the carb. It down shifts a lot easier and feels more responsive when I step on the pedal.
But I haven't towed with it since the above mentioned.
My next plan is get a better carb.

 

Many car 460's had the spreadbore Motorcraft carb. 4350 series. Ok when right, but usually, they are not right!!

He has a 600cfm OEM Holley on his truck.

 

the post said motorcraft so i figured his was the same as mine was originally (spreadbore) i think its the cars that had the squarebore holley style from the factory.

 

I did a lot of work on them on my 76 460 van. I dont believe they flowed more that ~450 cfm---read that figure somewhere. Pull the air cleaner and see if your carb has a square open hole between the secondary air butterflies (thats the upper butterflies.) It is not the same spreadbore as the quadrajets, its a unique Ford pattern. The primaries are even smaller than the Q-jets and secondaries are way smaller than the Q-jets. The 4350 is an evolution of the 625-700 cfm 4300 carbs which were also fords spreadbore. My 70 mach I has a 4300 on it and they were on some 429 autos.

I had the similar problem pulling boats and 31' travel trailers. It just does not supply enough air to support 460 cubes under load, suckin lots of air. The primaries are way too small to support much load, so you are constantly into the secondaries, and the mix is not as good. That carb was intended to attempt mileage figures from a 460 (My opinion). I eventually went to a square bore holly (had to change intakes as well rather than use an adapter) ~600 cfm. That gave appx 300 cfm from the primaries, with a stiff spring in the secondaries. When possible, you need to run on the primaries for the best mix and vacuum.

My .02 anyway, hope it helps some