I'm sure this has been asked dozens of times by now, but I couldn't find it.
I have a 78 f150 2wd 351m/c6 with 2.75 gears in the back. The only real modification to the motor is a comp 268 cam and a cloyes timing set, set straight up.
The carb is a stock 2150 with #63 jets in it, and currently I average about 14 mpg highway. I don't know if that is good or not but it seems pretty decent to me, given what it is.

The truck seems to run ok power wise, but is pretty dead after about half throttle. Out of sheer frustration with my pinging issue (colder plugs seemed to resolve that-don't know why I never thought to try that, duh!) I decided to toss in a set of jets out of some odd spreadbore motorcraft carb that used metering rods. Those things were huge, mileage dropped to about 9 if that. However the engine ran very well, with no signs of running over rich. The 'dead' zone in my powerband disappeared with a significant increase in power all around.

I don't know what the powervalve in the carb is calibrated for, but it definitely does come in when I punch the throttle.


Am I going to need to go to a 4 barrel to keep the mileage and the power?
Or is there a way to modify/ adjust the enrichment system so it delivers more fuel when needed? It seems to me that it needs more fuel, not necessarily more air. I really like the simplicity of the 2150, but I do have an autolite 4100 I could replace it with if need be.

 

Hey 46
Welcome to FTE.
Sounds more like a timing problem to me.
If it were mine I would put the old jets back in and verify your timing.
Starting around 12 initial then 34-36 ish at/by 2800 rpm.

 

I took the big jets out pretty quickly, I can't drive very far with that kind of mileage....

The base timing is currently set to 10 with the vac advance unhooked/capped.
As soon as I hook it up, it jumps to around 20.

As far as mechanical advance goes, I think it's all in at 30-something by about 3k.
I don't recall exactly though, it's been a while since I played with the distributor curve. I'll have to verify that tomorrow.

 

Good deal.
Yeah make sure your timing is right first.
That'll give you ping fits if it's not and also fall on it's face.
Good luck.
Let us know how it goes.

Edit
Im a moron I just went back and saw 351m so forget what I said about target timing.
I was thinking FE
Still sounds like timing.

 

What sort of numbers should I be looking for then? I had spent a lot of time early on with the timing yet oddly can't remember the exact figures. If it wasn't getting enough advance under load, would the extra fuel from the previous jets really help all that much?
The increase in power was substantial.

 

I believe Ms should be about where you are 10 to 14 initial and I'm gonna guess 30-34 advanced.
I don't know for sure though because I've never run an M.
Easy to look up though with Google as your friend.
All I can tell you is the symptoms you were describing sound more like timing issues than carb.
Having said that, it's a whole system where everything relates to everything.
It sounds like you have been in your carb and pretty much know what's going on, so it should be close.
If it were mine I would start with the timing and get that squared away, making sure it's advancing. On my latest truck I had to free up the advance mechanism inside the dizzy as it was stuck. Gotta check all that stuff then move on to the next thing it could be (maybe back to the carb, who knows).
Let us know how you do tomorrow and maybe an M bud will chime in soon.

 

Checked the timing out this morning, my memory wasn't too bad.
With vac advance capped off, 10* at idle with about 30* by 2500 rpm.
I gave it about 2-3* more base timing, since the colder plugs seemed to have resolved the ping issue. Of course there was a slight improvement, but nothing really major.

 

Ok timing verified.
And it sounds like your carb is adjusted properly?
Back to your original question.
Do I need a four barrel for more power. Yes.
Mileage will probably suffer.
You have a cam in there that can use more fuel.
I am assuming its a 268H which will idle nice.
So basically you seem to be at the point where you have to ask yourself what you want.
Does it run nice now?

 

It runs decently, it just seems a little restricted to me. Especially if I have to get on it to pass someone or tackle a steep hill.

 

Ok, here is what I know about carbs from lots of reading. A spread ore 4 barrel will give you better gas mileage and more power. The reason is, the primaries are actually smaller than a 2 barrel, there for when under normal driving conditions, uses less fuel for better mpg. But, when you need that extra fuel, the secondaries kick in and give you all you need. So, as long as you keep your foot out of it unless you need it, you will get better gas mileage.

Don't know for sure with the 351, but the optimal carb for a 400 is around a 600 to 650 cfm 4 barrel. Not sure where a 4150 comes in there, but I believe it is lower.

 

I get you, but we can't feel what you are.
Sounds like a 4bbl will be what you're after.
But there is some expense in that equation.
Sounds like the ***** in your court.

You know I've read/heard that for many years but I haven't been able to confirm that in the real world.
Prolly 'cause if I have it I'm gonna use it.
Just for reference though.
A Holley 600 has 66 jets.
46 says his jets are 63s.
Understanding there is a whole different style of circuit in those two carbs I just don't know if I believe that.
I am certainly not the last word on this though.

 

The last 79 F150 4x4 I had had a pretty decently built 400 w/ 4 bbl headers and a good amount of head work. I managed to get 12 mpg with it. Didnt make alot of difference whether it was town driving or highway driving. The Bronco I currently own, I have managed to coax 10.1 mpg out of with a 351m ,small erson cam made for torque/towing, running headers 600 vacuum secondary holley and no porting on heads. Why the difference? I really dont have a clue other than the porting. Efficiency makes power but also makes mpg gains as long as you dont go crazy with the right foot.

 

I think you are right.
Efficiency is the key.
You have to mash on the pedal harder to get things moving with an inefficient engine.
For instance, I'm getting almost the same mileage now with a 441 4bbl as I did with a 360 2bbl. And I haven't even optimized the mileage yet.
Why?
Don't have to mash the pedal as much.
Efficient engine (more torque).
But I put a lot of money ( total rebuild )into the motor to make it efficient.
I think the point of 4 bbl vs 2 bbl still equals less mileage on an engine with no other mods.

 

I drive with a pretty light foot most of the time but could use the extra power under heavier loads- something that little 2150 has trouble keeping up with.
If I were to use the autolite 4100 I have, shouldn't I get similar highway mileage in theory? Since the 2150 is half of a 4100 for all intensive purposes and I would likely stay out of the secondaries.

 

Hey experimenting is half the fun of having these things.
You got a set of headers laying around?