I will never change my mind about carbs, I can't stand vacuum secondary carbs. Most folks have zero business using one, they just don't understand tuning with them. Where as the double pumpers take much of the tuning out of the equation. I also believe that heavy vehicles generally need a dp carb. My personal experience with carbs is that vacuum secondary carbs are best suited on stock, low performance vehicles.

Well, fair enough.

I think I have a handle on my 3310, seems to run OK. No bogs off idle or at speed, plugs are a brown color, idles at 850... what else do I need it to do? It's probably not razor sharp but close enough for horseshoes and hand grenades.

I agree though, there is a probably a lot of misconception out there about how vacuum secondary carbs work. How many guys are gonna try to tune the secondary opening with the spring kit? And frankly, there's probably more guys that get the kit, convince themselves that the secondaries need to open quicker than necessary and put a light spring in the pod and the car bogs. Then they curse the vacuum secondary because "it don't run right" and toss it. Out comes their 'trusty old double pumper' that went 12.20s with a .509 cam back in 1982.

I guess they are happy with covering up poor transitions between the circuits by dumping fuel. Maybe it works OK. I'd rather not line the oil companies pockets though.

I'm gonna put one of my Thermoquads on it anyway.

I had a 73 or 74 W100 PowerWagon 318/727 LB that I purchased brand new....it had plenty of power, gas mileage was no different then any other trucks of that era....was a great truck

Rickster

Made the 110 mile round trip to work today. Mostly highway driving, about 1/3 in heavy traffic, got 9.3 MPG. Filled up on 87 swill in the AM at my local Sunoco, rolled back in on fumes to the same exact pump went a total 115 miles.

My dream right now is to get two round trips out of a tank of gas.

As far as mileage, a well built and tuned BB should get more, somewhere in the low double digits... maybe 11 or 12 which would be 20%+ better.

Still, if you are only getting 115 miles per tank @ 9 MPG usable tank capacity must be pretty small. From memory a 30+ gallon tank was available for those. Sounds like a bigger tank is in order.

Huh. Maybe my gas gauge is way off. Not sure how big my tank is, maybe I am thinking its more empty than it is.

It would be nice to go farther than 115 miles per tank but it still only gets 9 mpg.

I got 14 one time from a big block truck, one time! Most fill ups netted 8-9 mpg.

12.7 gal should have been about a 1/2 of tank. Do you have the big plastic saddle tank on the driver side of the frame, or are you running off a rear tank? The frame under my old 4x4 had dual tanks at one time. Someone before me removed the big saddle tank and left the smaller rear tank. It was only 16 gal. 120 miles was the limit on that one (and it was a small block).

I had one big block Dodge truck with want was called a 55 gal gas tank out of a school bus mounted in the box. That 50 or so usable gal netted between 425 and 450 miles before it ran out and I would switch over the the trucks regular tank. That was a TNT 440, with a 727, and 4:10 rear gear, 3/4 ton camper special, 73 club cab, with a wood topper, pulling an open car trailer. Truck and trailer fully loaded ready to go weighed 7650 lbs. It sucked gas, but man it pulled! Gene

I got the tank on the driver side. I did some reading on truck tanks this evening, I guess it's 20 gallons?

However, if it is in fact a 20 gallon my gauge is way off because it's reading just about empty when I could actually have almost 1/2 tank left. Maybe I'll siphon out the remainder when it gets low to see how much is left.

In addition to a cam change, I have to think an OD trans in this situation will really help my highway mileage situation. A 20% drop in highway rpm over 50 miles has to make a pretty good dent.

I can't justify driving this thing on a semi-regular basis, I'll spend more on gas in 6 months than I paid for the truck. I've driven it about 1,200 miles so far @ 9.3 mpg...

Looking into getting an A/F gauge before I make any major changes. I got one in my Duster, it's a nice tool to have.

FWIW, I've known nobody who's had one of these trucks and claimed the fuel mileage was any good. Most are in the double digits, barely. BB or SB.

But doing some quick math, I'll base it on your claim of 9.3 mpg average, and a 110 mile round trip, with 1/3 of it in heavy traffic. I'll assume that means a mixture of stop and go, and varying speeds, etc. That would be about 37 miles heavier driving, 73 miles straight highway. If we assume 6-8mpg during heavy traffic, typical for any cammed BB car or truck, that translates to 10-13 mpg highway. Not atypical for a BB pickup. Many members with BB cars struggle to beat that mpg.

I think if you were to step down to a stock or near stock cam, you could see a 2mpg increase across the board, likely more noticeable during your heavy traffic portion of your commute. An overdrive transmission may gain you a mpg or two on the highway, nothing during the heavy traffic portion.

IMO even if you changed a SB, efi and overdrive, you won't see much more mpg that what you have. If it helps at all to put things into perspective, we took a 97 ram 1500 5.2 4x4, no lift, stock tires, on a 3000 mile round trip. On flat level ground at near sea level elevation we got 18mpg if we kept it at around 60mph or less. Upping to 75 dropped us to 13. Once the terrain started to get hilly even in the slightest, we couldn't break the 15 mark anymore, regardless of speed. That's with EFI, overdrive and the improved aerodynamics of a 90's ram, perfect weather, we drove straight through so no cold starts or warm-ups.

If you're making that 110 mile round trip either way, this is simple the wrong vehicle for this application. Based on your driving habits I can't see you being able to improve your average by more than 2mpg. Is that enough to make the difference between this vehicle being viable and not? I would imagine either way you will have to get used to buying gas and think about putting in a slip tank to extend your range.

yeah but that 97 weighs twice as much as his truck

Alright, think I got a plan sort of nailed down.

Spoke at length with Tim @ Hughes today to discuss cams. I told him I didn't really want a Whiplash and said I was looking at some of the low duration hydraulic cams. This is the one he suggested:

HUG SEH1620BL-12

BB HYD FLAT TAPPET CAM 216/220 -112ºLSA

LOBE LIFT .330"/.335"

It gets installed @ 108. Smooth idle, high vacuum, up to 3.55 gear. He didn't think the ones under this would be worthwhile and I tend to agree. I don't want a milquetoast truck.

I asked about making it a single pattern, he didn't think it was worth the minimal expense with my combo. He said the little extra exhaust duration would help with clearing the cylinder.

We talked about heads at a few points, I told him I was considering a set of Stealth heads. He did not seem to think they would add much save for having them in place for future upgrades. Of course the smaller chambers will help but I will likely still be under 9.0 C.R. with the stock pistons.

My reasons for changing them out are more related to not wanting to deal with 40 year old, lo-po iron heads. I would need to replace the springs with the new cam, which I suppose I could do with the heads on but why? I still have a valve tick - no idea what it is but I will assume the worst which means to fix the problem the head will most likely have to come off. If I am taking them off, they are not going back on, especially if I have to spend money on them. So assuming they need some amount of work, it's kind of a no-brainer to go with the Stealths. The springs on the 440 Source heads are compatible with the cam I am looking at so I even 'save' a few bucks not having to buy springs with the cam.

We also talked a little about the intake. I would like to change out the Performer RPM for a regular Performer. He concurred that theoretically the shorter runner will help with velocity to fill the cylinder quicker but I wouldn't really notice any difference over the RPM. If I can find one relatively cheap I will grab it. I actually want it more for the spread bore opening and the choke well so I can run one of my Thermoquads.

I think I am going to go through with the plan as is and wait on the trans for now. IMO it would be prudent to wait on it and see what the other changes do. Swapping in a whole top end is a decent job, should satisfy the need to tinker for a while. Plus, there's always the potential for something to go wrong. Taking the trans out multiplies that tenfold.

Besides, a trans swap is really expensive no matter how you slice it. It is something I want to do though but perhaps just not at this very instant.

Will update when things start coming together.

Just a quick update for those following along at home;

Ordered my cam that I mentioned above from Hughes. Not in stock, should be soon though. Grabbed a Performer intake off Ebay. probably paid too much for it. Whadyagonnado.

Also got an A/F gauge to see where that all is at. I will put that in before any changes to get a good baseline. I'm betting it's pretty close but it will be fun to mess with it.

Decided against buying heads, saved me a bunch of coin! Sometimes you gotta control yourself.

Pretty warm here this winter, might get to this before long.

The a/f gauge is a fun tuning tool. You will learn a lot! Hope you got a wideband not a narrowband.....

The Air/Fuel ratio gauge will be a big help finding out where you are.

Since you desire better fuel economy
you should shoot for a high air/ fuel,
at least 18 at highway cruise part throttle.

If you switch to new iridium tip spark plugs and renew the ignition parts you might be able to successfully ignite 20 to 24 air to fuel.

You should consider swapping to a modern low rolling resistance street tire like the Bridgestone Dueller HL or similar in a tall thin profile. Run the highest air pressure as it improves emergency braking, reduces hydro planing, even though it will wear out the center most tread bar slightly earlier.
This tire change could improve your fuel economy by as much as 2 MPG.

The picture of your truck shows no front air dam below the front bumper.
A large air dam,
coming within 4 inches of the pavement
could improve 70 mph steady MPG by 1.0.
An air dam can be made cheaply from black plastic "lawn edging" with the round pipe like moulding down near pavement.

For further aero improvement consider covering the back part of your cargo box top from the tailgate forward about 4 foot, leaving the 2.5 feet just behind the rear window open to the air.
I know that sounds a bit odd,
but that gives the best aero improvement,
slightly more than a full length tonneau cover.

If the original EGR valve is still on the 400 V8
take it off if you try adjusting the air/ratio up to a high number like 18 to 24.
EGR and lean burn do similar things by reducing engine pumping losses on the intake side but you do not want to do both at the same time.

In an earlier post you mention a vacuum gauge.
If you try high air/fuel ratios
a vacuum gauge is valuable.
At highway cruise in top gear you want low vacuum: 8 to as little as 4 inches Hg
If the vacuum is higher than this,
lean out the A/F more
until the leanest cylinder begins to misfire.

My guess is that your truck with the
leaned out 400 V8,
3.23 diff,
low RR tires,
aero improvements
should be able to get 16 MPG at a steady 70 mph
when the wind is below 5 mph
and the air temperature is 50 to 70 degrees F.

Good luck.

I have an an LM1 with an A/F gauge in my Duster so I am familiar with them. I got a wide band, not sure if anyone even makes a narrow band these days? It's an Auto Meter Cobalt, a digital stepper motor type.

When I was looking, I found that Summit sells a clamp/band thing that you wrap around your exhaust pipe which has a bung built into it. You just drill a hole and put the band on where the hole is. No more muffler shop!

Going to make a mount panel for the gauge in my dash where the radio was.

Great info and tips, thanks!

Bringing this back up. Finally installed my A/F gauge and as expected, the engine is on the rich side.

I am impressed with myself for getting it in the ballpark just tuning by ear and feel. The truck runs pretty well and has no hesitation. It is not "pig rich" by any means but based on the reply above, I have some work to do.

The main thing I was looking for with the gauge was cruise A/F ratio. Currently, it's under 14. 70 mph seems to be the sweet spot for best A/F ratio at around 14.2 but the RPM required to keep it at 70 likely negates any benefit of a leaner mix.

Most of the time the gauge was between 14-13 with dips into the 11-10 range when accelerating and 16-17 when you mat the gas from a roll. After the initial opening, it recovers and goes back to 13-14 depending on how long the throttle is needed to get up to speed. Closing the throttle creates a momentary rich condition into the 12-11s but then it recovers.

Idle mix was on the rich side just under 14. I may have a linkage/cable issue because there are times when the throttle blades don't seem to close all the way without pulling the throttle back with my toe. My throttle return bracket is from a small block so it might not be entirely correct. Will have to look into that one. That's been there since I got the truck so yes, I need to get on that. It also takes a while to idle down after a stop but it's a stick with a heavy flywheel.

Besides that, I have not played with anything on the carb yet, not even the idle mixture screws. Reading up on tuning techniques, I think I may be getting in deep with air bleeds which unfortunately are not real adjustable on a garden-variety 3310. My quaint hope is that one of my Thermoquads will be more tunable in this regard, meaning no drilling of fixed orifices, changing metering blocks etc. and will potentially do a better job with cruise A/F.

I also need to drive with a vacuum gauge to see if the power valve is coming on at the right time. Not sure yet but based on the gauge, I don't see a big rich spike when accelerating so it must be fairly close. It's a 6.5 now.

So the main goal right now is to get the A/F ratio as good as it can be with this combo of parts. I believe the Holley might be limited in it's tunability but again, I have not gotten too deep into this yet so we will see. When I get to the high point I'll start thinking about swapping the cam and intake.

Sounds like you could lean out the main jets just a hair (perhaps 2 numbers) and get just a little more fuel when you mash it by playing with the accelerator pump, not gonna see a whole bunch of improvement based on those numbers. High swirl and tight quench will run better at leaner numbers but those open chamber heads bite a big one and will not like going really lean.

I had the thought that the rich condition that I am seeing when the throttle is closed is unburned fuel due to the cam and low compression. Might need to open the throttle blades a little more but 2 points on the gauge is a lot of hydrocarbon.

I also spoke too soon on the idle numbers. Once it warmed up outside (mid to low 60s, really nice today) and the motor finally got down to it's normal idle speed, it was under 12!

Guess I might need to go back and look at the timing again too, probably could use more?

When I say 2 numbers I am talking jet sizes not AF ratio numbers, probably going to be more of a dog if you get a that much leaner.

Carbs just run rich on decell, I don't know much you can do about it, perhaps you can get an air valve like a reverse blow off valve to open when vacuum gets high and let some air in. This is why the NASCAR cars (before EFI) would always be dumping fire out the exhaust when they let off to go into a corner. Now they can shut off the fuel completely on decell and help MPG... not something easily accomplished with a carb.