Carburetor Re-Jetting

[martincom]

Well, I've been procrastinating re-jetting my sweptline's carburetor for nearly two years. The doubling of the price of gasoline finally prompted me to get it done, even though I only drove my sweptline approximately 2000 miles annually. My mileage was 13MPG city. I'm hoping for 18 MPG when I've completed the task.

Reading Sparkplugs

Back before air/fuel meters and when dinosaurs roamed the earth (according to my daughters), we determined whether a fuel mixture was rich, lean, or about correct by "reading" sparkplugs. This typically consisted of operating the vehicle at a steady highway speed for a couple of miles and simultaneously shutting off the engine and shifting into neutral. In this manner, the "burn" coloration of the electrode insulator could be captured when the carburetor was operating from the main metering circuit. Of course, this meant removing sparkplugs on a hot engine and inspecting them on the shoulder of the highway. This usually resulted in Chevrolet and Ford drivers honking their horns on the way by and shouting something laced with expletives you couldn't understand. Not to mention, burnt fingertips and wrists when doing so on a big block Mopar. You could easily consume an entire weekend re-jetting a carburetor.

Cost Justifying an Air/Fuel Meter

With age comes wisdom, for at least some of us, and the reluctance to experience pain, both physically and to our egos. Not to mention our time became far more valuable. So I really did not desire to re-jet a carburetor by reading sparkplugs. The painless method to re-jet a carburetor is with an air/fuel meter, which eliminates sparkplug reading. The only problem is cost. They're expensive. I have about $700 into mine. Thus the big reason for the procrastination.

However, with the price of gasoline what it has become, I determined it would pay for itself in 5 years via increased mileage. Assuming I would realize a 5MPG gain and even with my low annual mileage of 2000 miles, I would save $154.00 annually. So the air/fuel meter would pay for itself in 5 years. That's not a great return on an investment, but it is not a bad return, either. Besides, all my muscle cars could stand a jetting update as gasoline formulations have changed dramatically since I had last done so.

The Air/Fuel Meter

I chose the FAST brand of air/fuel meters, Model 170402:

https://www.fuelairspark.com/gasoline-a ... ensor.html

This was the dual sensor unit, which would be overkill for a sweptline. However, I'll utilize it on high performance engines and being able to analyze air/fuel ratios of each cylinder bank has value. This package came with weld-in bungs, for the sensors, that are welded into each exhaust pipe a foot or so from the exhaust manifold and with corresponding intercabling lengths. The exhaust system of my sweptline will need to be replaced in the not to distant future and once the carburetor re-jetting is complete, I would have no need to do so again in the foreseeable future. So I opted to also purchase the optional tailpipe sniffer and longer intercabling. I could have fabricated the intercabling, but by the time I purchased the minimum order of required connectors and cable, I would be saving little, if any. So I just purchased the longer cable.

https://www.fuelairspark.com/replacemen ... -lead.html

https://www.fuelairspark.com/ez-clamp-o ... t-kit.html

Additional Equipment Required

In addition to the air/fuel meter, a tachometer and vacuum gauge will also be required.

Prerequisites

As ignition time will impact air/fuel ratios, the ignition timing must be optimized prior to re-jetting. See this thread for details on how to perform ignition timing optimization:

viewtopic.php?f=19&t=44352

Carburetor CFM

If utilizing a non-OEM carburetor, be sure its CFM rating is appropriately sized for the engine. Bigger is often not better with CFM. There are many online CFM calculators that simplify the formulas utilized to determine CFM:

https://www.summitracing.com/newsandeve ... calculator

Carburetor Specifications

Knowing the specifications of the installed carburetor will provide a baseline to begin from and save disassembling the carburetor to determine such. Factory service manuals typically include such specifications and aftermarket carburetor manufacturers typically publish them on their web page. In my example, I'm re-jetting a Holley 0-8007 carburetor, that is replacing a much too large (cfm) Carter AVS a previous owner had installed.

Equipment Setup

1. Install a towel on the top of the dash to protect the finish.

2. Connect a vacuum gauge to manifold vacuum and place on the dash.

3. Connect a tachometer and place on the dash.

4. Install air/fuel meter and place on the dash.

Position tailpipe adapter so sensor is not at the 6 O-clock position, to prevent moisture from condensing within the sensor. Be sure to secure cables so they cannot blow loose and be drug on the road surface.

Initial Measurements

Proceed to a fairly level roadway and accelerate to 55 - 65MPH. Observe displayed air/fuel ratio and record. Observe air/fuel meter and vacuum gauge while gently accelerating to the point manifold vacuum drops below power enrichment level, which is the size of the power valve on Holley carburetors. Note how much the air/fuel increased in richness. Aggressively accelerate to the point of the secondaries coming in on four barrel carburetors, note the air/fuel ratio. Note the manifold vacuum level when at idle, in neutral with a manual transmission or when in "Drive" with an automatic. Compare and analyze measurements to the air/fuel ratio chart, below, and determine where adjustments are necessary.

With my '71 D100, the air/fuel ratio was 16.0 at highway speeds, indicating a lean condition. When gently accelerating, the mixture did richen to 14.0, indicating the power valve was opening and adding the proper amount of fuel, once the main jet is corrected. However, the power valve was opening late, at 6.5 inches of manifold vacuum, when accelerating. This corresponded with the installed power valve, a 6.5, per the specifications. Holley's recommended power valve selection criteria is a value that opens at half of manifold vacuum when idling in neutral with a manual transmission or in "Drive" with an automatic. My '71 D100 has 16" of manifold vacuum when idling in "Drive". This would equate to a power valve rated to open at 8" of manifold vacuum, which would be the optimum opening point based on the analysis of air/fuel meter readings and manifold vacuum. The air/fuel ratio under hard acceleration, secondaries opened, was lean. However, once the main jet were corrected, I concluded the air/fuel ratio would fall into the desired range.

As this was my first experience with the FAST Air/Fuel Meter, I did not have any experience as to how to gauge the impact of each step of a jet change. So I ordered jet pairs for each of the next three steps richer. Which would be jet pairs 52, 53, & 54 as the specification on the installed Holley indicated it was built with 51 jets. The specs also indicated the Holley was built with a 6.5 power valve. I ordered a 8.0 power valve and appropriate gaskets.

Upon replacing the power valve and the main jets with a 52 pair, one step richer, I performed another test run. The air/fuel ratio was 14.5, which is near the perfect 14.7. The power valve came in at the rated 8" of manifold vacuum with an air/fuel ratio of 13.0. Hard acceleration, secondaries open, yielded an air/fuel ratio of 12.5. I was very satisfied with the results.

Idle was a bit rough. Rather than adjusting idle mixture in neutral, I did so in "Drive" and adjusted for smoothest idle by "feel" as the manifold vacuum reading was fluctuating and not offering much resolution. I also increased idle speed to 900 RPM. These two adjustments yielded a smooth idle when in "Drive".

After both optimizing ignition timing and re-jetting the carburetor, the improvement in throttle response was definitely discernible. Throttle pressure was also noticeably less for customary traveling speeds, signalling a significant improvement in performance and efficiency. I checked the gas mileage after 70 miles, which is probably too small of a test range, but I was anxious to see how it improved. I was a bit disappointed, it had only improved from 13 to 16MPG. I was hoping for 18MPG. However, much of this was idling while I experimented with different adjustments. So we'll re-calculate utilizing a much broader range of travel.

I did not have to make any changes to the secondary circuits of the Holley. This particular model was not equipped with a secondary metering block, but rather a metering plate. So there are no changeable mixture determining components. The metering plate can be drilled out to richen the mixture, which requires a set of wire gauge drill bits. The problem with drilling is there is no going back, without replacing the metering plate. Alternatively, Holley does a secondary metering block kit to convert from the metering plate.

Part Sources

While Holley jetting components are widely available, I source Holley carburetors and parts from All State Carburetor. They are very knowledgeable and have a comprehensive inventory.

https://allcarbs.com/?v=920f83e594a1

Carter Ball & Ball, AVS, AFB and Edelbrock carburetors all utilize the same jets. Rather than a power valve, Carter & Edelbrock utilize calibrated springs to determine at what vacuum level the power enrichment circuit opens. There are tuning kits available, that consist of an assortment of three springs for the AFB, AVS & Edelbrock carburetors. I'm not aware of any source for tuning springs for a Ball & Ball. I source Carter parts from Mike's Carburetor.

https://www.carburetor-parts.com/

[712edf]

I would consider any carbureted V8 getting better than 15 mpg dang near a miracle, car or truck, unless it was driven 50 mph downhill all the time. Even my modern V6 & V8 gas powered rigs with EFI don't get but about 17 mpg. Others claim 18, 20 maybe even more &I'm not calling them liars, but that has never been my experience.

But with your testing equipment & a bunch of tuning I have no doubt you can pull the feat off. Keep us posted.

Bucky

[nutz]

wow not bad 3mpg is a 20% improvement
my 12 hemi doesn't get that ,but it rarely sees the highway
funny how adding fuel can give better fuel mileage

[nutz]

wow not bad 3mpg is a 20% improvement
my 12 hemi doesn't get that ,but it rarely sees the highway
funny how adding fuel can give better fuel mileage
the other varible is spark plug heat range
from what i remember the blueing on the threads will tell if they are at optuim combustion temp
great write-up thanks

[martincom]

my 12 hemi doesn't get that ,but it rarely sees the highway
funny how adding fuel can give better fuel mileage
the other varible is spark plug heat range
from what i remember the blueing on the threads will tell if they are at optuim combustion temp
great write-up thanks

The increase in fuel economy, MPG, was a combination of both optimizing the ignition timing and the re-jet. I went from one to the other, almost in succession, and never took the time to track the change with just the optimization of the ignition timing. Based on my "feel", I would say most of the improvement was realized from the ignition timing optimization.

We have a '15 Grand Cherokee w/5.7 Hemi. We average 19 city and 21 MPG hwy with it. The bottom falls out when I pull my car hauler or the 18' enclosed to 8 MPG (80MPH). If I slow down to around 70 MPH, it improves to 9 MPG. I had an SRT Magnum, until someone ran a red light, with the then 6.1 Hemi. I'd average 19.5 city and 22 MPG hwy with it. Both these MPG ratings are from the instrument cluster menu, which is accurate.

With the lean condition, the combustion pressure would not be at optimum and the efficiency of the engine suffers. With that, fuel economy.

I utilized the Champions specified in the service manual specifications.

The truck has the four blade radiator fan. I'm going to swap it out for a clutch fan. According to a number of magazine articles where they ran dyno tests to compare the horsepower loss of the various cooling fans, I should have a net savings of 20 hp with the change. We'll see how that translates into fuel mileage.

[nutz]

nice , its cool to be able to see the progression in your process
i would think making those tweaks it would be easy enough to optimize it more
to help compensate for normal ware and different fuels maybe adding some timing or retarding or using a different heat range
might be possable to sneak another mpg
the best thing a new engine can do is change timing/fuel on demand

[martincom]

I thought about changing from ethanol to non-ethanol premium. However, I couldn't find any information on how they are increasing the octane to 92. I don't know if they were utilizing an additive or the more complex (higher cost) refining method. Straight gasoline has 30% more power than ethanol, but that doesn't translate into 30% more mileage. The EPA states it will yield approximately a 3% increase in mileage, but considering the typical EPA employee, that is biased. I came across a few articles published on the internet where they reported 6 - 9%, depending on the vehicle. Even at 9%, the cost difference of non-ethanol is quite a bit more, close to 20%, the last time I checked.

I'm only expecting a tenth or two from the clutch fan. At the price of fuel, that will pay it for itself in a number of years, perhaps quicker once we move to AL, where I can drive the truck year around, verses 6 months.

Gas mileage is like racing. We had an auto shop instructor, in high school, that was a successful stock car racer. So he'd somewhat mentor us in our efforts to make our street racers go faster. One of the things he told us has stuck with me over the years and I applied it to more than just racing: "Finding a single item, on a car, where you can reduce the weight by 100 pounds is difficult to find and expensive you do. However, finding 100 places, on a car, where you can remove a pound is easy and affordable."

[nutz]

nice sounds like the same old timers that i learn from
the problem is the most saving cost the most $$$
is the cost of the higher octane fuel going to off set the mpg
i daily drive and mostly work on audi ..and the price of running premium will almost equal the mpg
but the car will self tune to the fuel
so if you run a higher grade and you tune to it would the saving justify the gains
the biggest savings would be to add a overdrive...but does the cost justify the savings