I have a 85 f-100 that has a 302 fi... with a c-6 and a 3.08 locker.... it will not , will not , get but 14 mpg.... I have a edelbrock intake and a holley 600 cfm single 4 with vac secondaries...
Do you suppose that I could pick up some mileage by changing over to a carb and what will I have to change out to do that....????

 

You'll probably lose economy.

Make sure that you have all the tune-up stuff covered: good spark plugs, good wires, good coil, good rotor, clean air filter, tires aired up.

You'd be better off finding a way to get the freeway Rs lower. That'd probably get you up to 16-18mpg after a nice freeway haul. At the end of the day, though, you've got an OOOOOOOOLD truck that's not too far off what a modern truck gets. I had a 4.8L Silverado that would do about 19mpg with no load and driving like a saint.

 

Timing and emissions. You need to remove all emissions equipment, esmissions junk hurts MPG terribly. Then advance your timing as much as it will take without detonation. 12*-16* initial is where you'll likely be, though youwill probably have to limit your total mechanical advance. Also run an adjustable vacuum advance and run it on ported vacuum, with as much advance as it'll take at highway cruise. Adds more timing with light throttle and won't advance at heavy throttle and heavy load. Once you do that lean out the primary jets as much as you can while avoiding a lean surge/bog at steady highway cruise. Keep secondary side jetted for best acceleration, may have to up it a couple sizes to compensate for a lean primary. Headers will help slightly. Long tubes into a 2.25" exhaust would be good. Porting the heads will also help. Clean up the exhaust port bowl and short turn.

You should be knocking on 20 MPG. My '86 Supercab got 17 MPG highway with 351w/C6/NP208/3.55:1 and it wasn't tuned for MPG. I now get 12.5:1 highway with 460/ZF5/NP208/3.55:1 but again, not tuned for MPG.

 

...wow.

Don't do that. Just leave the EFI in. It's programmed to get way better fuel economy than a carb could hope to do. If you're really desperate to do something, rebuild the motor, add some compression into it and a nice set of heads, then put on some decent aftermarket fuel injection on top.

Or, just put a minimal amount of money into tune-up parts and leave the stock injection alone.

 

Keep the FI, do a good tune up including good low ohm wires, a high power coil, bigger gap on the plugs, and cleaning or replacing the injectors,

Then advance the timing as far as you can without it knocking.

 

Apart from driving style, I think the C6 is the main factor, especially with a 302.

My two trucks are both 351w; the F150 is injected with a C6 & the F250 is carbed with an AOD ( I don't use OD) - same axle ratios & overall condition but the F150 is on 15" wheels vs 16".

Despite being heavier & carbed, the F250 gives 16.5mpg compared to the F150 giving 15mpg.

I like C6's (as much as I can like any automatic) but they do cost in fuel.

 

The suggestions to make sure the state of tune is good on your truck are a good
starting point. The rear end ratio you have is good for mpg but the C6 trans is not
very good because of the 1 to 1 ratio in high gear. Something that worked for me
on my 1981 F100 with a modified 351W was to change the carb from a 4160 600CFM
Holley to a 600CFM Motorcraft/Holley 4180 carb. Lot of folks don't like the 4180 but
it can be a very good carb if rebuilt properly. The major advantage it has are annular
type primary boosters that do a better job of atomization of the fuel than the standard
Holley boosters do. Another good carb choice would be an older Ford 4100 4bbl
in either 1.08 or 1.12 size. The 4100 also has annular type boosters but because they
are 1960s vintage they may not have the necessary vaccum hookups your 85 needs.

 

The Autolite 4100 is a great carb for MPG, much better than fuel injection. Perhaps fuel injection without all the emissions BS would be better for MPG but that doesn't exist without spending a fortune, even then a set of Webers would match it (though not make sense on a truck). A carb is much cheaper and more reliable. That extra money spent on a *good* EFI system will buy a lot of gas. Personally I refuse to rely on EFI to get me to work every day, I only use carbs on my daily drivers. I don't like EFI much but on a toy I'd tolerate it as if it breaks no big deal.

 

I know I may be jumping into a hornets nest here but the 4100/2100 carbs are junk and there are few carbs I dislike more. Sure they run fine but they all have one huge flaw that I can't just get past, they cause flooding. The way the power valve is located and plumbed causes the bowl to empty into the engine when you shut it off, so if you come back 5-15 min later it's flooded.

And FWIW Weber side drafts are awsome, best carbs I've ever worked with and they can make great since on a smaller truck V-8, they just don't make them big enough for bigger motors. And they are major $$$$ to get and tune, having to buy 8 jets or emulsion tubes to make just about all adjustments really adds up.

 

The money spent on a carburetor, which would in turn make the car get worse fuel economy and more temperamental, would be an even bigger waste.

I don't know why you're so afraid of EFI. It can adjust to varying environments SIGNIFICANTLY better than a carburetor, and is mechanically much, much simpler.

Fact of the matter is, there are few, if any, off-the-shelf carbs that will run well in a truck. Most are designed for ~3000-3500 lb cars running minimal load and expecting consistent performance in mostly similar conditions.

The self-adaptive nature of just about any EFI set-up will ensure that your car will run pretty well at 7000 feet even if it was set up for sea level. (Nothing can compensate for the general lack of air going through the motor, but at least your car won't be running pig rich)

The most mechanically complex parts of an EFI system are the fuel pump and the throttle body. Nowhere near the amount of stuff that could wear down / gum up and fail on a carb.

For me, my truck is not my only form of transportation, so the 4180 stays until I've got more significant expenses paid off.

DocM - are you driving this truck every day? If not, I'd just do a tune-up and forget about it.

 

Carb does nto mean worse MPG, should be at least the same, probably better once rid of the emissions nonsense. There is nothing temperamental about a good Holley, they just plain work. On my truck I ran a 600 CFM 4160 for about three years. Two on the original 351w and never adjusted it once after installing it. Ran great and got me over 17 MPG highway at 75 MPH with a C6 and 3.55:1 gears. ran that carb for a year on a ~400 HP 460 and never had to fiddle with it. This is daily driving. Including two trips of 1500 miles in a week on vacation. I now upgraded to a 4150 750 CFM double pumper which after a jetting change is working great for me. Nothing temperamental about it.

No clue waht you're talking aobut with regards to no off the shelf carbs that work well in a truck. How about nearly any Holley? Even Edelbrock is fine though not my preference.

I've daily driven cars with Autolite 2100 and Autolite 4100 carbs with no trouble. Even in below freezing, just set the choke, turn the key and immediately can drive away like normal. No nonsense, just works. I used to drive 165 miles/day to/from work in my '68 Galaxie with a 302 and a 4160 Holley. Never any trouble.

I've taken my trucks in the woods in water and mud and never had any trouble when all my fuel injected friends stall out and run rough, if not stall out altogether. I'll stick to my inefficient and temperamental carbs, as they just plain work through anything and never give me trouble.

EFI has all sorts of funky things. Every EFi vehicle I've driven raises the RPM after you let off the gas to shift. That's retarded. WEars syncros makes shifts not as smooth, harder to double clutch, just all around dumb. Sensors and all sorts of nonsense to fail. I've seen so many eFI vehicles get towed or not start because some sensor died even though everything is mechanically sound. That' not a quick simple roadside fix. I'll stick to my simple technology that just plain works and is reliable.

 

There's a difference between working and working well. The difference between a "perfect" tune and a "good" tune is not gonna be noticed from the seat of the pants.

The booster demands, vacuum signals, etc, are gonna be very different in a truck vs a car, even WITH the exact same engine setup. (down to the bolts and nuts)

MAP sensors, MAF sensors, etc, can all compensate for this. Carbs have different size/shape ports to jimmy-rig it pretty close to what's needed.

Yeah, a sensor can keep an EFI car from running, but more times than not there will be tell-tale signals before they just "pop". (Significant loss in fuel economy, car running like crap, etc).

 

Call it whatever you want, but nothing beats the easy tunability, reliability and consistency of a good carb.

 

I'll chime in on this EFI vs carb debate. A well tuned EFI will almost always make more power and get more MPG the a well tuned carb. A carb just can't adjust to every running situation like a good EFI system like these Fords have. Nor can they adjust to differing outside/weather/altitude conditions like a good EFI system can. Also the EFI system includes ignition timing in the constantly self adjusting tune, something of course a carb system can't do.

Sure an EFI system has more to go wrong, so if reilabilty and easy repairs are more important then MPG and power, a carb is the way to go. But a little knowledge and tools/parts kept handy will fix most EFI systems rather easy when they do break down.

These Ford EEC-IV systems are good also, only complicated enough to work well, pretty simple to fix, and only a small idle follower function(that's what keeps the RPMs up). My BB Chevy would spin the tires with the idle follower function alone while I was standing on the breaks, I HATED that.

To the original poster, keep the EFI and sell the carb stuff to tune/upgrade the EFI.

 

Let's not forget that driving style has a LOT to do with gas mileage. If you drive it easy and don't drive it like you stole it, you'd be surprised how much better your mileage is. Aside from saving a ton of wear and tear on your engine, transmission, etc.

I'm only getting around 9mpg with a 2bbl 302/C6/2.75 setup, and I drive like a grandma, lol...14mpg is great compared to that!