1978 ford f250 4x4 with carb problems

ford
f250

#1

I have a 4x4 ford with a 400 c.i.d that has a 750 holley 4 barrel double pumper. what jet sizes do I need?


#2

More info please. Are you just now installing a 750 Double Dumper on that 400 or has it been on there for some time and you’re just now having problems with it?


#3

Whatever’s in it now should work just fine.


#4

I’ve played around with jetting on a couple cars, and I’ve found the only real way to figure out what’s happening is to use a “rolling road” and test the motor at various RPM’s. That way, you set all the jets for the various throttle openings and speed ranges they operate at. Not exactly the answer you’re after, but there you go.

If it’s running now, you can do a couple basic tests, but they won’t be very accurate. Accelerate under power until you hit the mid-range RPM (red-line is at 5K, stop at about 3500), and turn the key off at that speed. Coast/bring the car to a stop, and pull the plugs. Depending on the state of the plugs, you can jet up or down.

It’s been decades since I’ve done this, though. My memory may not be so great.

I’d go to a shop with a rolling road, that knows how to handle this stuff. They’ll also have the right jets, in stock.

Good luck…


#5

I think you are over carburated. Those engines are stump pullers and 500 cfm would be more than adequate unless you have changed the cam.


#6

I second Rod’s opinion. More carb isn’t always better, and it’s much easier to install too much than not enough. A 750 double pumper would be at home on a street/strip big block or a hot race small block, not on this torque monster. A 500 cfm carb with vacuum secondaries would work nicely on this engine.


#7

My vote is with Rod Knox and mark9207.


#8

It had a 2 barrel previously and the carb is off of a pontiac 440


#9

No cam but it has rebuilt heads on it that haven’t been driven down the road yet


#10

It is my friends truck I think that carb would be right at home on my 351 windsor bored 30 over


#11

Unless that 351 is hot cammed and at least moderately modified that carburetor is still a bit much even for that application.
Boring alone doesn’t accomplish much unless it’s done in conjunction with a bunch of other stuff.


#12

The sooner you put the original carburetor back on it, the sooner you will be happy…


#13

Oh no. No no no no no. It’s an essentially stock 400? That 750 is way too much, no matter what you do with the jets. Did you put a 4-barrel intake on it? What kind? If you insist on a 4bbl, I’d go with an Edelbrock–the one that’s an AFB clone.

BTW, Pontiac never made a 440.


#14

has an edelbrock 4bbl intake. never have liked the edelbrock carbs intakes are good though


#15

its my friends truck and I will let him learn on his own I think he would get better mileage out of it with the 2bbl.


#16

I second that motion


#17

At 4000 RPM the engine will draw less than 400 cfm and the engine’s horsepower peaks at 3,800. The engine cannot produce enough manifold vacuum to pull fuel from the secondaries.


#18

Stick with the Edelbrock carb. Using that 750 will be like trying to get a drink of water from a fire hose.