I just drilled a hole for a grommet and hose in the firewall and wound the hose around the steering column a couple times and it sits on top of the column out of the way. Been that way for many years.

You may also want to check your timing advance at the rpms where you get good mileage and bad. Especially the vac advance, faster should be a bit lower vacuum and the unit may retard for less mileage.

 

i would love to have a inline 600 carbbed engine for my superduty!

 

Anything new on this topic, AbandonedBronco?

 

Not too much. I don't think I'll be able to look much further into it until I get a Wideband O2 sensor, which I'd like to do after Christmas sometime.
Just no real way of knowing what's going on without one.

 

I'm supposed to be on break right now from FTE but couldn't resist putting in my $0.02 since nobody here has mentioned it yet.

You dropped from around 4500 to 5000 feet to 2000 feet in elevation. That's a bit denser air, and, when coupled with the same amount of gas (in other words, not adjusting the carburetor for the drop in altitude) this means your fuel mixture is leaning out a bit.

Then, you also dropped the size of the power valve in the carburetor and stuck in a set of leaner jets.

I have a feeling that you had a pretty lean condition going on when you were driving on that stretch of interstate. Too lean can hurt mpg too, and make the motor work too hard to move the vehicle.

Just my thoughts.

But, what's weird is that before you changed the parts out in the carb, you were getting better gas mileage at a higher elevation than you were at a lower elevation. This means that with a richer fuel mixture you were getting better mileage. Then, on the trip back, you leaned that carburetor out with different jets and a different P/V and it started sucking up the gas.

I'd try a richer jetting, readjust your floats, resetting the mixture with a vacuum gauge, and following Kev's advice about using a P/V that's 1/3 of the manifold vacuum.

Then, back off your speed to about 55 mph and see what you get. I'll bet it'll be a noticeable improvement.

 

Good points. If you are running lean, you are probably using a wider throttle position to get the same speed.

 

The wideband O2 sensor will tell the tale and keep you from guessing whether to richen or lean the fuel mixture.

 

AbandonedBronco,

The following is just for comparison of a modified 4.9L [yours] vs. a more closely stock 4.9L [mine]. I just completed a weekend trip of approximately 215 miles, each way. I hooked up my vacuum gauge and monitored the vacuum closely. Keep in mind that mine has a Carter YF and factory exhaust.

The following was with the transmission in O.D. [manual];

45 mph = 15" Hg
50 mph = 13/14" Hg
55 mph = 12" Hg
60-62 mph = 10" Hg

I did not exceed 62 mph, so I do not know what it would have been @ 75 mph, but that would be interesting.

I wonder why my truck's numbers are lower than yours and yet it returns 20-22 MPG @ 55-62 mph? By the way, @ WOT and going uphill, the vacuum drops to 2" Hg.

 

That's actually one of the reasons you got good mileage.
The lower the vacuum, the better the gas mileage, AS LONG AS your fuel system doesn't go into a rich mode (like a 4bbl getting into the power valve).

A lower intake vacuum means the throttle is open more, which reduces pumping losses. The more closed the plate is, the harder the engine has to work and pull against it to get fuel in. It's kind of along the same lines as a restrictive exhaust.

The 1bbl doesn't have a power valve in it that dumps gas in when the vacuum gets low, so as it opens up, it just makes it easier on the engine.


That's why leaning the fuel ratio out nets better gas mileage. You have to open the throttle more to get the same amount of fuel to power the vehicle. If you go too far, though, the vacuum drops into the power valve range. Or, in the case of a carb like the Carter 1bbl, it gets too lean and damages the engine.

My best mileage on my other Bronco, when I have the vacuum gauge on it, is usually around 8hg to 10hg. Just as long as I can keep it from dipping below 6.5, where the power valve opens.


At least, all of this has been my understanding.


And who knows, without the vacuum gauge on there, maybe that's exactly what I did? Leaned it out too much that it went into power valve territory to cruise at those speeds. A smaller (4.5) power valve, or a little richer might fix the issue.

 

In the book, Ford shows a Duraspark with two pigtails,and an altitude compensator,(theroretically that varies timing based on atmospheric pressure or altitude).I live in Denver, and have a 1981 E150, 4.9 with factory 4 speed,(stick),and want that setup really bad,to see if it would help out my mileage.
Timing makes a big difference.
IMO,if the carb needs more air,(leaner at altitude), you have to put the pedal down harder, to get the same speed.A carb does not have a brain, so more pedal,more air,and if there is more air going through a brainless carb,it is going to suck more fuel too.
Just thinking out loud.
Anybody got a high altitude setup? Can I buy it!!

 

I'll add this in. My 86 F150 and 89 Bronco at one time had essentially the same drivetrain. 5.0 EFI, AOD, 3.55 gears and 31" tires. The only difference being what Ford changed from 86 to 89. The Bronco weighed IIRC about 500lbs more.

The F150 always averaged about 1-1.5mpg better than the Bronco. This was overall averaged but similar enough with city vs highway driving. I don't have direct, back to back testing though. Just that feel that you get after you've put 30-40k+ miles on a truck. You just know what it's going to get for MPG. I mostly did the same things with them, went the same places, etc. The F150 was always better on gas...

There also seems to be variations in the exact same trucks that are likely just differences in build tolerances, maintenance, etc, so maybe I just got lucky with the F150 and/or unlucky with the Bronco...

Or there's something else.

 

Even without the infinite number of variables that can impact MPG, no two engines are necessarily identical in performance (including MPG) even when identical in every other way from the factory. And that is why tuning is an individual engine specific matter. A vacuum gauge, AFR meter, etc., are great tools for finding that sweet spot for an individual engine and that spot is not of necessity the same for all engines.