I have a factory motorcraft 4b carb sitting atop my 460 ported, polished and gasket matched stock dove heads, running around 9.5 cr. stock cast iron exhaust manifold.

I am driving long distances these days and have disconnected the secondaries from opening up(--just for the time being)

With just the primaries operating the engine accelerates very smoothly but when it gets around 50 or 60 mph (2600 to 2900 rpm, running 4.10 gears) with the pedal to the floor the truck seems to have somewhat uneven surging-- like its being held back and can't really let go all the way.

Is this a carb adjustment or maybe a mechanical problem? Nothing in the wheel bearings or trans seem hot?

Secondly, -- and this seems really strange to me, going up a long steep grade with the throttle only partially open I get significant pinging, but if I put my foot to the floor the pinging goes away: -huh

Any help thanks much

 

One inlet needle is probably not enough fuel to feed this motor. The pinging is probably due to the excessive heat from the lean condition due to inadequate fuel delivery. If your determined to run a two barrel I would get a larger needle and seat assembly for your carb and it might even need a little more fuel pressure to keep up. That 460 needs significant pounds of fuel per hour to make the horsepower you’re asking by flooring it at that speed. If it could flow fuel from the secondary you would have twice the fuel delivery available and you starvation problems will go away. That is if you haven' t burnt the pistons out of it already. If you polished the intake manifold or runners smooth that will lean your mix further as the fuel will fall out of the air stream. The intake tract needs to be a little rough to keep the fuel suspended. It sounds like a combination of poor fuel delivery compounded by the smoothed intake tract. Hook your secondary back up and if it’s still pinging you may have to put in a bigger set of main jets. Anything you can do to get the intake air in the manifolds and head to swirl and tumble more will help keep the fuel atomized and build more power at less throttle position.

 

Well partly I was running too lean. I richin up the two front mixture screws and running muck better now.

Thx HG

 

Those front mixture screws mainly affect the idle mixture. They have almost no affect after about 1/8th throttle position. The only thing you can do to change how it runs down the highway is to change the jets. On my Holley 750 I only run a #68 main jet in the front and on the highway I'm getting right around 12 mpg with 4.10 gears and 30" tires. The reason your pinging was going away when you floored it is I believe because your power valve wasa opening and that supplies addition fuel to the mixture. I really doubt disabeling your four barrel had much effect on your milage. The drivers driving style has the biggest influence. Just don't accelerate hard and anticipate stops so you don't use much brakes and your milage will improve. Then if you get in a jamb or need to pass or pull a load you will have full power that you warmed over big block can give you. I doesn't make much sense to polishg the runners and then restrict the carb does it? I love my big blocks power and I like it so much I don't mind paying at the pumps for it. I suffered with that POS 7.3 for so long. Next summer I'm doing up a 545 and I half expect it to get better milage than my 460 as I will be able to raise my rear gear up to a 3.55 and just loaf that big motor around.

 

Yep,

Riching up the idle screws only seemed to help tone down the poping/back firing when I was deceling-- which is at or near idle. I was getting close to 15mpg until my recent rebuild-- now I get 11 to 13. Your right it did not seem to significantly help my mpg to disable my secondaries-- although with just the primaries it still moves along real good. ( pulled a 5500# load up step grades this last weekend and it just ate it up-- and with no secondaries!)

 

My 357" sand dragster only loses .2 seconds running on the front half of a 950. 3.55 down to 3.75 seconds E.T. for 100 yards on sand. Still stalls at 7200 even with the two barrel. Only 200 rpm less than the full four barrel.

My 460 towing 16,000 pounds GCVW still gets 8-10 mpg even pulling some mean grades at 65 mph that my diesel would only do 27 mph on. There really is no shortage of gas. Just money to pay for it. You really be bummed if in your effort to save $5 at the pump burns the pistons out of your new rebuild. The rods will trash the whole mess in short order if a piston goes. I found I got better mileage with my Holley 750 loafing around compared to the lowly 600 that was on it that I had to lean on to get it to go. Less throttle position equals better mpg. Smaller carb means more throttle position which opens power valve which richens the mixture for more power but at the expense of consuming quite a bit more gas.

 

Hired Guns,
I've been told that smaller CFM carbs get better gas mileage because they have a better "vacuum signal". Meaning, there is more suction pulling the fuel out and helping it atomize. Big carbs dont have the efficency at lower rpm's to break the gas down. At least that what ive been told.

 

Your information is correct but with 460 at highway speed and a load your horsepower will be reduced significantly with a small carb and it will need to be near wide open under heavy loads. With a bigger carb I find myself letting up on it all the time as if I have a strong tailwind pushing me everywhere I go. The result is I am not mashing on the pedal all the time to get it going or to maintain cruise speed on a grade or into the wind. My power valve is rarely open and my accelerator pump rarely gets squirted once I'm underway. I have a C-6 and 4.10’s. For my usage I picked up 2-3 miles per gallon with a 750 cfm. It made my big block act big. With a 600 it was like it was trying to prove itself all the time where now I have to hold it back to keep from getting speeding tickets.

 

Thx for the info HGun,

Yeah, I just have not shelled out the $250? for a new edelbrok 750 or 850 yet.