You can not expect, and will not get, the fuel mileage of a modern fuel injected vehicle.
You are trying to push a vehicle with the aerodynamics of a concrete block though the air at highway speeds, and you have a lot of air turbulence under the vehicle that even a concrete block doesn't have to deal with.
You are dealing with a 3 speed or 4 speed MANUAL transmission in most cases, which is often in the wrong gear for maximum fuel efficiency, where the modern high mileage cars often start with 5 or 6 speed automatic transmissions to keep the engine in exactly the correct RPM range,
You are running Carbuerators or simple speed/density throttle body fuel injection,
And an engine that was designed in the 1920s, and has about a 550 RPM 'Economical Power Band', so it's VERY hard to squeeze fuel economy out of that engine,
And most times, we are dealing with tire size, gear ratio, and other modifications that are NEGATIVE to fuel mileage.
Having said all that, there is no reason you should not be able to get 15 MPG or even closer to 20 MPG...
(which is about roughly half what the modern passenger cars get that all look like electric shavers.)
Fuel economy is a constant dance between fuel delivery, ignition timing, engine RPM and vehicle load.
The wrong FINAL DRIVE ratio will kill fuel economy very quickly,
And it's the #1 problem I see.
FINAL DRIVE is the gear ratio from the output of the crank shaft to the rubber patch on the road surface.
The first thing people do with Jeeps is bolt on 'OTHER THAN STOCK' size tires, and any increase in tire height changes the final drive in EVERY GEAR.
The 'HIGHER' the drive ratio, taller tires, 'Taller' gears all slow the engine down, and keep it from getting to the 'Power Band' in the RPM range where it gets the best fuel mileage,
So the engine is always 'Lugging' the vehicle around, that uses more fuel.
Think of it this way,
You have a 12" breaker bar at the differential end of the axle,
And you have standard size tires, the breaker bar will be Easier to turn the axle/tire than if you have taller tires.
The vehicle will move FURTHER with every turn of the breaker bar on taller tires,
But it will be more difficult to turn the breaker bar, so you will have to put more muscle into the process of moving the vehicle.
At any given highway speed, the engine is turning SLOWER, so you are NOT reaching the 'Sweet Spot' for fuel economy,
And you are lugging the engine to keep up with the speed you want to go, which is dumping fuel to 'Lug' the vehicle along at the speed you choose.
The next factor is worn out parts, mismatched parts, ect. at the engine.
A prime example are engines that have seen better days,
As engines wear, compression drops, and they become less efficient,
And as parts wear, fuel metering, ignition timing, ignition energy to the plugs are effected, and you aren't even close to getting the correct fuel mixture or the spark energy delivered in sufficient currents, or at the correct time anymore.
The next factor is MISMATCHED PARTS,
With all the interchangeability, people are sticking (factory) parts on their engines that have no place being there,
And then there is the aftermarket that 'Guarantees' you big gains if you just use their product.
Nothing like having WAY too large a carb, way too large a camshaft, way too large an intake and/or way too large an exhaust on your engine,
Then expecting it to work miracles.
Carb or fuel injection fuel/air flow MUST be matched to the intake, camshaft, head and exhaust sizing/type if you want to make any power or fuel economy.
And you MUST have an ignition that works with the fuel mixer you are running... PERIOD.
I'm going to TRY to explain how some of this works, and what you can do to make your engine TUNE match your vehicle with it's tires, gears, transmission, and other parts that may or may not be working correctly, and may or may not be 'Optimum' for your particular application.
A custom tune WILL wring some extra power and economy out of your engine,
But no one can work miracles, and unless you are willing to drive around in a 'Micro Electric Shaver', don't expect 40 MPG...
One bad component in the ignition, doesn't matter what you do to anything else, it's simply not going to run CORRECTLY.
The engine is mechanically sound,
(no 'Flat' camshaft lobes, no rockers that are worn out, no lifters punched through, no low compression cylinders, ect.)
*IF* the engine DOES NOT have a bunch of vacuum leaks and has a reasonably working carb,
Start with the ignition.
The issues with your current Ignition are solvable, and for a reasonable price/minimal effort.
Your '78 factory distributor is fine.
The purpose of the distributor is,
1. To produce a 'Trigger' signal. (low voltage 'Primary' side of the ignition.
2. To adjust to produce that trigger signal at the correct Engine RPM (Centrifugal Advance) and Engine Load (Vacuum Advance).
Theses are the MECHANICAL functions, has nothing to do with electrical.
3. To deliver the HIGH VOLTAGE spark energy pulses to the correct cylinder (Cap/Rotor)
Your factory distributor does the first two pretty well,
And if you don't like what/when it's doing things, it's VERY adjustable.
#3 is a different story...
Jeep choked when the electronic ignitions came long with double the voltage of breaker points.
That's the reason for the 'Tall Rotor/Wide Cap' upgrade.
Double the voltage means you need roughly DOUBLE THE SPACE between that high voltage and a 'Ground'.
Tall rotor does that, gets the spark energy up and away from the electrical 'Ground' of the distributor shaft and housing.
Since you can't insulate the high voltage coming through the cap, you have to put physical distance between the high voltage and a 'Ground'.
Tall rotor gets the spark energy up and away from the 'Grounds' in the distributor, shaft, housing, ect.
And the wide caps spaces the terminals out so the spark energy can't jump to the wrong terminal (when things go correctly...)
Step 1 of the 'TeamRush' ignition upgrade for AMC V-8 engines, '78 to '90 AMC/Chrysler 'Jeep' vehicles...
Check your distributor for a cap alignment 'Tab'.
Some distributors from the factory have them, some don't.
Right smack in the middle of the word 'Motorcraft' cast into the distributor housing...
YOUR CURRENT DISTRIBUTOR CAP FROM 'JEEP' USES THE 'ALIGNMENT NOTCH' TO LOCATE THE DISTRIBUTOR CAP CORRECTLY.
THE UPGRADED CAP WILL USE THE 'ALIGNMENT TAB' SHOWN IN THE PICTURE.
The screw hole is there, and threaded,
You will need to find a SHORT screw that threads into that hole,
And you will need to fabricate the tab if you don't have one on the distributor already.
(this is NOT a stress part, so it can be made of plastic if you want to, but metal holds the bends better)
Razor blade, scissors, tin snips or something to cut a piece of metal from the can,
A hole punch, drill, awl, something to make the hole,
Some needle nose pliers to shape the tab.
The width of the cap adapter notch will show you the width, and he shape is shown above.
END OF FABRICATION, and a lot of the factory distributors already have the tab,
Reman distributors from the parts store rarely have the tab already in place.
DISTRIBUTOR CAP UPGRADE.
"Off The Shelf" parts,
Cap Adapter, Cap, Rotor all from '79 Ford F-150 Pickup with V-8 engine.
Doesn't matter what engine size, all V-8s use the same distributor cap/parts.
This would be my recommendation for a 'Correct' distributor cap with the other components included,
You can't do any better than MSD for a distributor cap for this application.
What this does NOT show is your cap will come with a plug wire retainer that keeps the plug wires in place on the cap, a VERY good idea.
This is a TOP QUALITY distributor cap, replaceable center electrode, high quality brass terminals,
And a cap material made of the highest electrical insulating material,
And the under side of the cap is designed to break up any ionization between terminals so you don't get cross fire from one terminal to the other.
The cap is also vented to remove ionized air from the cap, that reduces the potential for ground fire and cross fire.
The rotor is pretty basic, nothing you can't buy off the shelf at any parts store, same with cap adapter, so you aren't paying for the full on race versions that are 'Proprietary' and you have to order from MSD.
The cap adapter is NOT a wear part, it should last you for years to come,
The rotor will need replaced about once a year.
Your crappy factory plug wires will no longer fit because of the new style of cap terminals,
Much better than the 'Socket' terminals 'Jeep' used for many years and should have change when they used a high voltage electronic ignition...
You have two choices for plug wires,
You can find an 'Off The Shelf' application that 'Will Work' but aren't high quality and won't fit the engine well,
Or you can get some 'Cut To Fit' wires and have high quality wires that will last years to come, fit correctly, and work very well.
My recommendation is for 'Cut To Fit'. It's more work,
But the results of using high quality wires will make your ignition work so much better for years to come, and you only do this ONCE every 10 to 20 Years with MSD wires...
Cost is VERY reasonable since they last at least 10 years...
These plug wires come with EXCELLENT silicone boots and insuation that won't split, crack, or rot like 'Rubber' or vinyl coated wires will.
They are Low Resistance, Low Impedance wires and deliver the spark energy to the plugs with out loosing most of it on the way.
The plug end terminals/boots will bend up to 90° to help you get a perfect fit.
They have stainless steel 'Snap Lock' terminals on both ends, no more having a plug wire back off a terminal and you get a positive tactile 'Snap' to let you know they are locked in place.
These sets come with instructions, cutting and stripping gauge, crimping tool and do REALLY well for any street or trail vehicle.
IF you go this way, let me know, there are some tips that will make assembling these wires MUCH easier...
Some tips and common sense for install of wires...
DO NOT run the wires in a bundle!
Use NON METAL wire looms, keeping the wires 1/2" to 1" apart from each other.
DO NOT drape them over metal. Metal will suck your spark energy out like crazy.
Don't LOOP a wire around the cap/distributor,
Start it going the correct direction from the distributor cap.
Looping adds wire, and that adds resistance.
DO NOT allow the wires to rest directly on an exhaust manifold or header tube! (common sense thing!)
Don't 'Pinch' a wire between other things mounted in the engine bay... (Common sense again!)
Now, for better ignition, makes sure BOTH engine heads have a DEDICATED GROUND WIRE,
Use copper based 'Never-Seize' on the spark plug threads.
That will help complete the 'Ground Circuit' and the spark plug becomes the most attractive "Ground" in your ignition system, and you will start getting spark to the CORRECT plug where it belongs...
Once you start getting the spark energy to the plugs where it's supposed to be, and you start getting the plug GROUNDED properly so it can fire like it's supposed to,
Consider some of the other upgrades listed in my signature line...
Like tuning the distributor/carb to work with your engine, tire size, gear ratio and in the RPM range you normally drive the vehicle...
Timing, or TIMING ADVANCE, comes from three places,
INITIAL, what you dial in by twisting the distributor in the engine,
Is used to START THE ENGINE, and that's all.
Stupid large amounts of Initial don't do anything but make the engine hard to start,
And that's VERY hard on the engine...
When you are cranking the engine to start it,
There is NO VACUUM, so no Vacuum Advance,
And there is not enough engine RPM for the Centrifugal Advance to be working,
So no Centrifugal Advance.
INITIAL IS STRICTLY FOR GETTING THE ENGINE STARTED EASILY,
Even when it's hot and has full compression,
and when it's cold and the combustion process is slow.
Too much INITIAL when the engine is warm and has high compression, but no oil pressure,
And you just hammer the pistons down on the wrist pins, hammer the rod down on the DRY rod bearings, hammer the rods down on the crank and it's DRY bearings...
REASONABLE INITIAL ADVANCE is in order here, 6° to 8° Initial is usually plenty without doing any real damage.
CENTRIFUGAL ADVANCE IS YOUR SAFE BASE LINE TIMING ADVANCE.
CENTRIFUGAL ADVANCE WORKS LOCK STEP WITH ENGINE RPM.
When you take off from a stop, there is no Vacuum to speak of, so the Vacuum Advance is NOT helping you at all.
When you load or 'Lug' the engine, there is no Vacuum, so no Vacuum Advance to help you.
The CENTRIFUGAL is there ot start the ignition process SOONER as engine RPM increases.
The crank turns faster, so you need to start the ignition process faster,
And that is what CENTRIFUGAL Advance does.
CENTRIFUGAL ADVANCE WORKS LOCK STEP WITH ENGINE RPM.
Wight held back by spring pressure are what activates the Centrifugal Advance.
As engine RPM increases, the weight overcome the spring pressure, and swing outward,
And that advances the ignition timing.
The springs/weight are found in the bottom of your distributor, under the vacuum advance plate and the reluctor.
It's the COMBINED spring pressure that counts, the springs DO NOT have to be the same size,
You can 'Mix & Match' springs to get the advance you are looking for at any given RPM, up to the maximum amount of advance allowed by the limiter stop and slots.
This shows two different slots with two different amounts of total CENTRIFUGAL advance,
In this case,
One is 13° Centrifugal Advance, shown with the limiter arm in the slot,
One is 18° Centrifugal Advance, show not used, no limiter arm in the slot.
The distributor 2 pictures above is 18° & 21° centrifugal advance slots on that particular advance head...
When you go to tune, you will chart your engine RPM, then chart the advance CENTRIFUGAL adds with the Vacuum disconnected, as you increase RPM.
(This usually means you will need a timing tape on the balance since the Initial plus Centrifugal Advance will exceed the timing scale on the front cover)
Then you will know EXACTLY how much timing advance the Centrifugal is adding at any given engine RPM when this is done, so you can compare and see what any changes do to your timing curve.
If you want to add a little more 'Zip' in your low gear take offs, then you can use lighter springs in the distributor, and you will get advance SOONER in the RPM range, and that will give you more 'Power' when you take off.
Don't get carried away, and listen/feel for signs of detonation so you don't damage your engine.
Since this is your fall back 'SAFE' ignition advance,
IF you pull a trailer or boat, or lug around a bunch of crap,
This timing adjustment MUST be kept 'Safe' for when you do that stuff.
If you run around without trying to pull the 'Titanic', then you can usually increase this part of the timing curve without a lot of issues.
VACUUM ADVANCE IS THERE TO MAXIMIZE YOUR FUEL AND EMISSIONS EFFICIENCY.
Vacuum can ONLY BE CHARTED BY VACUUM SIGNAL!
You will see timing tables with vacuum charted on them, and that IS NOT the way to do things.
Your vacuum signal can and will be all over the place depending on the engine loading and throttle position,
Every time you open the throttle blades for more speed/power, the vacuum drops and the vacuum advance goes away.
When the engine pulls hard or 'Lugs' down, the vacuum advance goes away to stop detonation with that increased load,
And adds timing advance when the engine IS NOT loaded hard, and vacuum is high.
Your distributor came from the factory with adjustable vacuum advance,
Allen wrench through the vacuum nipple,
So you can 'Fine Tune' the ignition advance to work SPECIFICALLY with your tire size/gear ration (Final drive ratio), your engine RPM, the load the engine sees (Expressed in 'Vacuum Signal')
If you want to mess with the vacuum advance, you will need a hand vacuum pump, an ACCURATE vacuum gauge, a timing tape on the harmonic balancer,
And you will need to chart the vacuum signal you show while driving around, at idle, under SLIGHT load, under part throttle cruise.
With a accurate vacuum signal to advance map, you will know the EXACT amount of vacuum advance you have at any given vacuum signal the gauge shows.
Once you have seen to getting some spark energy to the plugs,
And you have seen to a reasonable base timing that is 'Working',
Then turn your attention to the carb.
AMC V-8 ENGINES CAME WITH MOTORCRAFT 2100 OR 2150 CARBS.
Like all carbs,
*IF*... You have vacuum leaks everywhere, bad gaskets, leaking fuel, worn out throttle shafts, blown power valves and a carb full of crud, you are NOT going to get results.
So if you shake the throttle shafts and they move up and down or forward and backwards,
You have fuel leaks,
You aren't running a good fuel filter,
You can turn the idle mixture screws 'IN' and the engine continues to run,
You have intake or carb base gaskets leaking,
Are running a little bitty air cleaner,
Your Choke isn't working,
You are wasting time and money tying to tune this engine.
These carbs have some tuning capabilities that some other carbs don't have.
The MC 2100/2150 was designed by Holley, and many of them were built by Holley for Ford and AMC, so they share a lot of the same tuning traits as a Holley.
To start with, there is a needle/screw type fuel flow adjustment for IDLE MIXTURE.
These screws are to be turned IN and OUT EVENLY at all times!
The only exception to that rule is if you have an exhaust gas 'Sniffer' and have access to EVERY CYLINDER'S EXHAUST so you can determine which idle mixture screw needs a little adjustment.
If you don't have that $50,000 machine and exhaust manifolds were you can sample each cylinder,
Then keep the screws EVEN AT ALL TIMES.
(and that goes for every other common carb on the market)
Turn your idle mixture screws all the way in, count the number of turns it takes you to screw each in, once they bottom out LIGHTLY (DO NOT crank down on these screws!),
Then decide a 'Medium' between screws if they were both about the same, and back them out,
Write down how many turns out they are....
When tuning your idle mixture,
You wan to back the screws out EVENLY (no more than about 4 turns out normally) and then start/warm up the engine.
Turn the screws IN EVENLY until your RPM drops about 50 RPM,
And/or you are showing the highest STEADY vacuum reading (between 18 In.Hg. and 22 In.Hg).
MAKE SURE YOU KEEP ACCOUNT OF WHERE THEY LAND!
(You will need to duplicate the adjustment later when you test the power valve!)
Next comes the 'Accelerator Pump' which will activate and put extra raw fuel into the carb when the throttle blades move.
If the accelerator pump doesn't throw fuel shots IMMEDIATELY when the throttle linkage is moved,
Then it's time for a carb rebuild.
If you see bubbles or weak streams, or one stream stops before the other, it's time for a rebuild.
Linkage adjustments will allow for VOLUME and DURATION of accelerator pump shot if you need to tune it later during moving tests.
The next test is the 'Power Valve'.
The power valve is a NORMALLY OPEN (takes engine vacuum to close it) valve that lets extra fuel into the carb/engine during low vacuum/high load situations.
With the engine running,
If you turn the idle mixture screws all the way IN, and the engine continues to run,
It's getting fuel from someplace, usually the power valve is blown out and the engine is drawing fuel from there.
That means you are WAY over rich under all conditions with the exception of wide open throttle,
And it's a HUGE fuel dump that just kills any economy.
If you find the engine runs with the idle mixture screws all the way in, back the screws out to where you had them adjusted and rebuild the carb, or at least replace the power valve.
This is your primary fuel metering any time you are above about 800 RPM.
The size of the main jets is stamped onto them on the thick rims, and you can change jets pretty easily in an MC 2100/2150.
Main Jets supply fuel during NORMAL PART THROTTLE CRUISE conditons, when throttle is steady and vacuum is reasonably high, you should be running on Main Jets Only.
When you 'LUG' the engine, or accelerate, both the power valve and accelerator pump add fuel for that enrichment.
Man Jets are VERY finely tuned to deliver fuel at a constant mixture rate for the amount of AIR moving through the carb, so they are variable at any given speed and very consistent.
Enrichment is needed if you load the engine or try to increase RPM, and that is where the enrichment circuits come into play,
These all have to be balanced if you expect any real fuel mileage and smooth operation when accelerating, part throttle cruising or idling.
Now that you have fixed the vacuum leaks, fuel leaks, voltage leaks and have the engine idling fairly well, it's time to tune Carb and Ignition together...
Without this, your vehicle is never going to run the way it was intended to.
This DYNAMIC Testing/Tuning will make your carb fuel metering and your Ignition work TOGETHER on your specific vehicle, and will take into account all the 'Issues' you might have with tire size, gear ratio, mismatched parts, ect.
It won't be 'Perfect', but it will give you a reasonably smooth running engine for the limitations you have...
The first test is a HOOT!
Find someplace you won't get arrested,
And try to do FAST standing starts!
Just as hard as you can jump on the throttle through 1st gear, high range!
This is the hardest you are ever going to load that engine from a standing start...
You are looking for 'Hesitation' or 'Stumble' off the line.
If the engine hesitates and doesn't want to gain RPM quickly,
Noses over Immediately,
Then your accelerator pump isn't' delivering the fuel shot FAST enough.
There is slop in the linkage somewhere...
If you have a start of a take off, then the engine noses over,
Your accelerator pump isn't delivering a long enough DURATION fuel shot.
The linkage isn't adjusted for a LONG DURATION fuel shot, and you are running out of fuel before the main jets/power valve take over...
Now, once you get the accelerator pump stumble out of the way,
Then you can add LIGHTER SPRINGS to the distributor, and do the same thing.
With the ignition timing coming in SOONER in the RPM range, you should have a 'Seat Of Pants' feeling of increased power.
GO EASY ON THE SPRINGS, THIS WILL EFFECT YOUR 'SAFE' TIMING CURVE ALL THE WAY UP THE RPM RANGE!
What's good for you in first gear my not do so well in 3rd or 4th gear under lean fuel conditions!
So, if you have two 'Heavy' springs, go with a heavy and a 'Medium' weight spring,
If you have a heavy and a medium, go with a heavy and a light spring,
ONE STEP AT A TIME,
YOU SHOULD NOT make big jumps here, you will have to test the ENTIRE fuel/ignition tune before you come back here and try lighter than the springs recommended...
Don't forget to check the balancer with a timing light and timing tape on the balancer,
Adjust your timing map so you know EXACTLY when that extra advance is coming 'IN' in the RPM range...
Once you can get into traffic without being DANGEROUSLY SLOW,
Try a little around town driving,
Listen/feel for the signs of detonation, hesitation or pinging/knocking.
If you don't have any,
Then do a highway test...
Vacuum gauge installed so you can get some vacuum readings...
Part throttle cruise, 55 MPH, flat land, steady speed.
Note down your vacuum signal, this will determine the power valve size you need in the carb...
Now, slightly accelerate, like you are hitting a head wind or trying to go up a hill, see if you can make the engine hesitate or knock, loose power.
If so, this is a problem with vacuum advance.
You will probably need to enrich fuel (bad idea, covering up the spark advance issue),
Or a adjust your vacuum advance so you don't get that hesitation.
In some extreme cases, you might have to limit the amount of centrifugal advance or vacuum advance to keep that hesitation/knocking/pinging at bay.
I'll go into that when I feel better and have more time.
Now that you have a 'Part Throttle Cruise' (PTC) vacuum signal reading,
You can change the power valve to something that works with your vehicle set up.
Power valves typically are set to open about 2 In.Hg. BELOW that PTC vacuum signal.
If you are mostly street driving, then 2 In.Hg. will give you fuel a little sooner when you are trying to add a little speed in traffic,
Vacuum advance will pull out some timing as you give the engine a little throttle, and the power valve will open adding fuel when you need it during mild acceleration or when the engine 'Lugs' bucking a head wind, or when you go up a hill...
If you off road a lot, you can set the opening point a little lower so the vehicle doesn't 'Jump' when you are trying to 'Finesse' an obstacle, say 2.5 or 3.0 In.Hg.
If your PTC is showing a 8.5 In.Hg. vacuum signal, then you install a 6.5 closing power valve.
Any time your vacuum drops below 6.5 In.Hg., the power valve opens and adds fuel.
If the same vehicle did a lot of off roading, then you might want to take that same 8.5 PTC vacuum signal and use a 6.0 or 5.5 In.Hg. power valve.
I'll cover one more part about Power Valves after main jet sizing information...
TUNING MAIN JETS.
Main Jets are intentionally left a little rich in most vehicles.
When you start getting the fuel mixture a little on the lean side, emissions go up,
And as discussed before, the factory didn't know if you were driving around with an empty vehicle and no top, or a vehicle full of junk and trying to tow around a loaded car trailer through the mountains...
The 'Best' (also the most expensive) way to set your main jet fuel mixture is to use a 'Rich/Lean' indicator.
This is a Oxygen Sensor screwed into the exhaust and samples for Oxygen in the exhaust.
Too much oxygen, and you are lean, Too little oxygen, you are rich.
If you have long tube headers, it will be a little more expensive, since you need a HEATED O2 sensor. As the exhaust gasses cool in the tube, they don't heat the O2 sensor sufficiently for an accurate reading.
Those of you with 'Shorties' and factory manifolds should be OK with the regular O2 sensors.
An O2 sensor based 'Rich/Lean' indicator will have a leg up on the fuel situation, you can watch your fuel mixture displayed in the cab of the vehicle, and make adjustments to the jets during part throttle cruise when the power valve is CLOSED.
As a side note, if you use a multi-meter on a 'Junk Yard' O2 sensor, you can have a 'Poor Mans' version of a 'Rich/Lean' indicator, search for instructions on how to do this, and how to read what is displayed.
IF you don't buy a 'Rich/Lean' indicator, this is much more difficult these days of Unleaded fuel.
Unleaded fuel doesn't make anywhere near the plug reading that old leaded fuel did...
You Can PLUG the power valve, and go out, make some full throttle passes on NEW plugs, and see where your fuel enrichment is.
DO NOT use the engine for a 'Brake', when you get to the end of the hard pull, push in the clutch and shut the engine down to preserve plug readings.
You are looking for a very light 'Tan' color,
Wheat straw 'Light Brown' is too rich, and White is too lean for normal operation.
When you are done, remember to put the power valve back in the carb!
Without it, you will have to jet up to compensate, and that defeats the entire deal of tying to lean out main jets a little.
Once the carb is tuned,
You go back to the Ignition,
This time you are adjusting the VACUUM ADVANCE to work with the leaner fuel mixture.
You should do the 'Slight' acceleration test again from PTC, and see if the engine hesitates or not, this is an early sign of detonation and shouldn't be allowed to happen.
You can add vacuum advance until you have that hesitation, then back the vacuum advance down until you are just UNDER that hesitation.
When you are just under that hesitation, you are running as much vacuum advance as your engine/fuel mixture will allow for the final drive ratio and load you are running, so this will be your maximum safe advance at PTC.
*IF*... Your try and pull a trailer down the highway when tuned this way, you WILL HAVE DETONATION...
You are set up for MAXIMUM advance with a LEAN fuel mixture, and adding weight WILL cause problems.
For 'Short' heavy loads, pull the vacuum advance line off the distributor and plug the line,
This will take ALL vacuum advance out of the ignition curve, and you should be fairly safe for moderate loading of the engine/vehicle...
If it's a REALLY heavy load, or you plan to go very far with that load,
It's time to RE-TUNE the engine to pull that load safely.
Remember, you have gained some economy by taking the 'Safety Margins' out of the factory system,
And if you intend to lug around a heavy load, you WILL have to tune for that extra load on the engine!
Once your engine is tuned, LET THE VACUUM GAUGE RE-TRAIN YOU HOW TO DRIVE FOR FUEL ECONOMY!
That vacuum gauge will SERIOUSLY open your eyes to what you can do to increase fuel economy!
The higher the vacuum signal reading, the better your fuel economy...
You will learn where to place your shift points to keep the highest AVERAGE vacuum reading,
You will learn what happens on hard acceleration and rapid throttle setting changes,
And it will teach you where the 'Sweet Spot' gear/vehicle speed gives you the best economy.