Vacuum Advance Adjustment Distributor

[Neuner]

I want to fine tune the vacuum advance on my distributor. It's a motorcraft and every search states I should be able to insert an allen wrench and make adjustment but it doesn't appear so.

The tip of the distributor port appears to have been mangled by one of the PO and so it limits what i can try. I've been able to insert a 1/8" allen but it goes in far without making any type of screw contact.

Am I able to replace just the vacuum advance part of the dizzy without swapping the whole thing?

 

[Torxhead]

I have used an 1/8" allen wrench on mine to stop stop pinging. As I recall, it was about two turns clockwise which did the trick. I would have to suggest purchasing one of those remanufactured distributors as they are made to pre-computer advance limits plus getting a new pick up coil, gears, vacumn advance, and bearings installed.

 

[RatDog8o8]

Good luck finding a replacement vac advance unit. They're out there, but I ended up doing like Torxhead said - getting a complete unit. Came with everything.......cap, rotor, the entire package. (Can't find the receipt, but I know it was less than $50....)

- (edit) $42.37 on eBay from this seller:

http://stores.ebay.com/Auto-Parts-Titan

 

[Neuner]

Think I'm getting confused. The distributor is not original. It looks fairly new except for the tip of the vacuum advance being chewed up for some weird reason. I can't find anything about it except for the Motorcraft stamp near the advance port. I'm finding remanufactured ones at local stores for cheap.

I was just wanting to fine tune the vacuum advance.

 

[Torxhead]

You might find a local automotive electrical or ignition shop that could help you out. I bought mine from one of the big box 4wd outfits, it was made by either Crown or Omix. Maybe even drilling out that closed off nipple to clear for an allen wrench would work.

 

[Neuner]

Maybe even drilling out that closed off nipple to clear for an allen wrench would work.

The hole is still large enough that I was able to get a smaller allen wrench down through the hole but it went pretty deep and I was not able to sense a screw type insert. Are most vacuum advances, especially motorcraft, made to be adjustable?

 

[Torxhead]

Looking online I was able to find this and they say 3/32" allen wrench:How to Adjust Vacuum Advance Timing | eHow
They also have a bit different method than just turning the screw and running with it.

 

[TeamRush]

I want to fine tune the vacuum advance on my distributor. It's a motorcraft and every search states I should be able to insert an allen wrench and make adjustment but it doesn't appear so.

The tip of the distributor port appears to have been mangled by one of the PO and so it limits what i can try. I've been able to insert a 1/8" allen but it goes in far without making any type of screw contact.

Am I able to replace just the vacuum advance part of the dizzy without swapping the whole thing?

SOME BASICS that might help you, and for the guys that find this thread later on 'Search'...

1. INITIAL TIMING,
What you get by cranking the distributor around in the engine,
Is just that, INITIAL TIMING. NOTHING MORE.

Initial is the BASE TIMING your engine gets when it's trying to start.
No vacuum until the engine starts, so no vacuum advance yet,
No Centrifugal timing since the engine cranking will never get up to the RPM that activates the Centrifugal advance.

SO,
Initial Timing is STRICTLY for getting your engine running when it's cranking on the starter.

SOME IDIOTS will tell you to crank in a bunch of Initial, all that does is make the starter work harder to overcome the cylinders firing way too soon when you are cranking the engine.

-----------------------

2. CENTRIFUGAL ADVANCE is strictly engine RPM related.
When the engine RPM speeds up, weights in the distributor overcome spring pressure and advance timing.

Inversely, as engine RPM slows, the springs pull the weights back in, and Centrifugal advance decreases.

*IF* all the mechanical parts are working correctly, properly lubricated, ect.
The advance will be EXACTLY the same at any given RPM.
This makes Centrifugal directly RPM LINKED.

Centrifugal Advance *SHOULD* be your MINIMUM SAFE ADVANCE for the engine.
When the engine is under MAXIMUM LOAD, and ready to stall out, there still should NOT be any engine pinging, knocking, ect.

Under Maximum Load, the engine will have little or no vacuum, so no vacuum advance will be pulled 'In',
And the Centrifugal Advance will be the ONLY added advance, and should be set to SAFE levels to preserve the engine from detonation that would damage it.

------------------------------------

3. VACUUM ADVANCE,
Vacuum advance is where your ECONOMY comes from, and helps keep things like overheating from happening.

The engine builds vacuum when it's NOT loaded hard, and has a fairly steady throttle position.
If you open the throttle, vacuum drops.
If you LOAD the engine heavily, vacuum drops.

The Vacuum Advance is LOAD SENSING, and adds ignition advance when the engine can tolerate it, automatically removing it when the vacuum drops due to loading or throttle opening trying for more power, putting more load on the engine.
(This is the reason that automatic transmissions often have 'Vacuum Modulators' that help the Transmission know when to 'Kick Down' into 'Passing Gear', and when to shift up into highway gear, since it's a direct indicator of engine load)

-----------------------------

4. You can spend a month of Sundays trying to adjust vacuum advance, but the 'Correct' way is to use a vacuum gauge to see what the vacuum signal is doing when you drive.

There are three things you simply can not do without when tuning your vehicle for your particular set up and driving style,
ACCURATE timing light,
Vacuum Gauge you can read from the drivers seat,
Tachometer so you know what RPM you are having 'Issues'.

Not saying if you monkey with it long enough you might stuble into a 'Good' tune, even a blind pig finds an acorn once in a while,
But you won't know what that tune is unless you have the above,
And you GRAPH your ignition advances...

--------

I have to put a side bar in here,
Every ignition graph you have ever seen is probably WRONG.
I was taught wrong, looked at wrong 'Ignition Maps' until I started working on ignitions nearly full time and figured out why they were wrong...

Ignition 'Maps', or Graphs with vacuum advance listed on an RPM based scale is WRONG.
There is no way to tell what the vacuum signal of an engine is going to be based on engine RPM.

The usual graph for ignition timing has RPM across the bottom,
Advance listed up the side.
You can't include vacuum in that RPM graph, it's just going to be wrong no matter how you try to do it...

RPM graph should show your initial at your idle speed, around 600 RPM, and move up with RPM from there.

YOU SHOULD MAKE A SECOND GRAPH,
One with VACUUM SIGNAL across the bottom, with advance up the side.
(get this with vacuum pump on distributor, timing light on your timing scale, see at what vacuum signal reading you get what advance)

Add centrifugal to the vacuum signal you are seeing when you have an 'Issue' and that will tell you EXACTLY which advance is giving you issues so you can change it.

----------------------------------

Now, when you have an 'Issue' with vacuum advance, it will NEVER be at idle, since there isn't any load,
It will NEVER be with acceleration, since the throttle blades are open and you have little vacuum,

Vacuum advance issues will ALWAYS be with PTC (Part Throttle Cruise) at a fairly steady throttle position, and while in high gear at road speeds.
(Since any other conditions the vacuum advance won't be applicable...)

LOAD on the engine is VARIABLE,
Gear Ratio, Tire Size, 1:1 Ratio in high gear, or an Overdrive Transmission , towing a trailer, pulling something heavy, ect. all effect your vacuum signal/vacuum advance.

This is one of the biggest mistakes people make and drag into my shop,
Factory 2.73:1 Rear Gear,
1:1 High Gear or Overdrive in Transmission ,
LARGER THAN STOCK TIRES!

This knocks the final drive ratio WAY DOWN,
The engine can't get up to an effective operating range to build reasonable vacuum,
OR,
The vacuum can is tuned to pull in vacuum advance with all this load on the engine at lower than optimum engine RPM.

In road gear, PTC driving, the complaint is always the same,
Hesitation, Pinging, Bogging when they try to accelerate just a little or going up hills...

Large tires and 'Airplane' gears have the engine turning lower than optimum RPM,
The carb is on main jets, no other fuel enrichment (no power valve helping out or accelerator pump helping out with fuel delivery),
LEAN FUEL MIX, TRYING TO ACCELERATE, ENGINE BELOW OPTIMUM RPM, and they can't figure out why it's not 'Working'...

You have TWO choices here, and it may take both if the guy chooses really large tires...

You can richen up the fuel mix, but that wastes fuel anytime you are off idle,
You can back the vacuum can down a little to take some of the advance out of the timing at PTC.
Sometimes it takes both.

-----------------------------------

*IF*...
You don't have stupid tall tires on airplane gears, an Overdrive Transmission ,
You can usually get away with a PTC vacuum signal reading,
Then adjust the vacuum can to top out advance right at or slightly above the PTC vacuum signal.

*IF*...
You have REASONABLE tires on reasonable gears, you can usually have the vacuum can top out advance slightly BELOW the PTC vacuum signal,
So you get maximum advance/economy at PTC.

The vehcile I just had in there had a TON of armor, winch, ect. all adding weight, 35" tires, and you guessed it, 2.73:1 gears.

To make matters worse, the guy had an aftermarket header installed with a free flowing exhaust, which leaned out the fuel mix even more...

I had to both richen up the main jets AND set the vacuum can to about 4 in.hg. ABOVE his PTC vacuum signal to get the 'Pinging' and 'Hesitation' complaints under control...
(otherwise the detonation 'Pinging' was going to destroy his engine)

This effectively eliminated HALF his vacuum advance since he'll never pull enough vacuum at PTC to bring it into operation,
But his vehicle weight & drive train choices simply wouldn't allow the engine to tolerate any more vacuum advance.

---------------------------------

ASSESS YOUR CHOICES!
Contemplate the consequences...

Make Choices WISELY,
Tune for your particular vehicle weight and configuration.

Keep your advance curve SAFE!
You aren't driving a race car, so you don't need to tune it to the absolute limit,
Get the SAFE performance out of it you can, but be SAFE if you want that engine/drive train to last.

DO NOT LISTEN TO MORONS!
The 'Super Duper' this or that add on never does what the retailer selling you the item says,
And the guy excitedly telling you what it did for HIM doesn't have YOUR VEHICLE or drive the same way you do...

The biggest thing I've learned from over 40 years of racing, and over 15 years of working on folks' Jeeps is,
EVERY SINGLE VEHICLE/DRIVER IS DIFFERENT,
There is NOT a 'One Size Fits All' or 'Super Duper' part of any kind, and the effects of any swap need to be considered.

Carb & Exhaust changes mean tuning changes.
Swapping ignitions (to HEI clone or something not factory) mean tuning changes.
Tire & Gear changes mean tuning changes.
If you don't 'Map' or Graph out your current 'Tune', you have no idea where to go when you need to change 'Tune'.

TOOLS!
Vacuum Gauge, Vacuum Pump, Timing Light & Graphing are the ONLY way to tell what you have now, and to find the TUNE you are looking for!

 

[IOPort51]

Classic Inlines - Ignition System & Timing

I found this to be interesting.

 

[mstar]

The post by "TEAM RUSH" is a MUST READ article for anyone attempting engine tuning. The author is dead on in his description of timing events.
I have debated these principles with other "experts" that where schooled incorrectly and they probably will never figure it out!! One exception I will make is on initial timing. For my V8 engine with a very large flat tappet solid cam it is helpful to run more initial timing for smoother idle and better vacuum for power brake booster.I have a reduction drive starter that handles the extra load fine. The total advance is Limited to safe level. This is on a street driven hot rod. Again thanks for a great article and some vindication on my understanding of timing events. Thanks Again for your effort to educate.

Mike

 

[mstar]

Classic Inlines - Ignition System & Timing

I found this to be interesting.

I did not read the entire article in the link you provided, but it seems to mirror
what Team Rush is say.The only exception may be the fact that there where a few different ways the vacuum advance was used over the years as far as ported or manifold but the principles remain the same. mike

 

[TeamRush]

The post by "TEAM RUSH" is a MUST READ article for anyone attempting engine tuning. The author is dead on in his description of timing events.
I have debated these principles with other "experts" that where schooled incorrectly and they probably will never figure it out!! One exception I will make is on initial timing. For my V8 engine with a very large flat tappet solid cam it is helpful to run more initial timing for smoother idle and better vacuum for power brake booster.I have a reduction drive starter that handles the extra load fine. The total advance is Limited to safe level. This is on a street driven hot rod. Again thanks for a great article and some vindication on my understanding of timing events. Thanks Again for your effort to educate.

Mike

NOW WAIT!
You said you took 'Exception' to something I wrote on Initial timing,
Then turned right around and said you have a gear reduction starter to compensate for the very thing I warned about!

No exception, you just compensated for the results of what I wrote...
Which is the long and short of 'Timing' issues people have, and have to work with and/or compensate for...

INITIAL should be set somewhere the starter NEVER lugs, the engine just whizzes around like it's supposed to.

If you drag down the starter motor with a lot of initial timing, and I mean EXCESSIVE AMOUNTS,
(There are morons out there that will tell you to lock down centrifugal, remove vacuum, set initial to 38° to 45° and lock it down!)...

Then you need to do one of two things,
1. Would be an ignition 'Cut Out' switch.
A switch that allows the starter to get the engine turning WITHOUT the ignition firing way too soon with a lot of initial,
Once the engine is cranking good, you have fuel in the cylinders, you flip the IGNITION switch and the cylinders fire.

This is common practice with racing engines that have a TON of Initial timing and not much else.

2. If the initial is REASONABLE, just a little higher than the starter/engine 'Likes', you use a gear reduction starter to help with the starter load.

Gear reduction starters DO NOT help the starter drive gear to ring gear contact overload, the drive gear and ring gear will still have excessive wear over time, but the engine will start reasonably well.

This is common on force fed engines, like supercharged engines.
Supercharged engines or engines with WAY TOO MUCH duration on the camshaft like a LOT of initial timing to keep from overheating at idle, or from having low vacuum problems.

You simply proved what I was saying, your vehicle isn't an 'Exception', you simply worked around the issue so you can run a camshaft that doesn't exactly agree with the RPM you want and the vacuum signal you need.

And this isn't attacking you at all, it's simply analyzing what you said and explaining why it's happening.

And just for the record, I ran supercharged engines, high lift/long duration cams for YEARS with exactly the same issues you have... That's how I know what's going on.
I've blown the starter right off the engine on more than one occasion, broken more starter drive gears than I can remember, had to run vacuum pumps & tanks to have brakes, ect.
Yup! Been there, Done that, paid a fortune for it!
I was fun, and painful, but I won't do it again... Too expensive.

---------------------

COMMON, MOSTLY FACTORY ENGINES,
Will do fine at around 6° to 8° initial timing.
This lets them start well when the engine is 'Cold', loose from metal parts being contracted and not sealing well,
And when 'Hot' (hot starts are a big complaint when the Initial is jacked up), the metal parts are all expanded, the engine is making MAXIMUM compression,
And the fuel/air mixture is staying MIXED, so it's ready to really burn when it hits the cylinders and the spark plug does it's thing...

Too much Initial on a hot engine will burn MUCH more rapidly than when the engine is cold,
And the initial will try to send the piston back down the way it came up, instead of letting it roll over the top and go down on a proper 'Power' stroke.

If you are just a 'Little' too advanced, the piston/rod gets banged down on the crank in a straight line, over loading pistons, wrist pins, rod and crank bearings.

So keeping the Initial Advance reasonable is a good idea.
--------------------

If you want more 'Seat Of The Pants' take off power,
Then consider using LIGHER WEIGHT SPRINGS in your Centrifugal advance.
This will let the RPM linked advance happen faster in the RPM range, and you WILL FEEL the difference right away.

Again, keep it SAFE!
No sense in destroying your engine over time just so you can get more wheel spin taking off...
You need to be aware of the signs of detonation, hesitation, pinging, knocking, and adjust your advance accordingly to eliminate these problems.

If you have the factory '78 to '86 distributor in your vehicle,
There is another way to get more 'Seat Of Pants' power,
That is swap the advance head inside the distributor side to side.
This involves taking the distributor out of the engine and rotating the drive gear 180° on the shaft to keep rotor phasing correct,
But every Jeep/Motorcraft distributor came with TWO advance slots on the advance head, and Jeep always used the smaller of the two.

Between adding some centrifugal advance total, and bringing that advance in with some lighter weights,
Most people see substantial 'Seat Of The Pants' power increase,
And this costs you exactly $5 for springs and some time to tune the Centrifugal advance.

Just learn the warning signs and back it down when you encounter them...

Keep in mind that if you change the weight/load of the vehicle (Larger tires, towing, ect.) you will have to change the 'Tune' again to compensate for that weight, rpm/load change on the engine.

Jacking in 'Initial' timing to gain that extra 'Seat Of Pants' feeling is the WRONG WAY to go about things, yet the myth persists,
So do the complaints about starters failing, ring gears having teeth missing, hesitation, pinging, knocking, hard starts, ect.

 

[TeamRush]

AND... I know what you mean about tyring to 'Educate' someone that hasn't found out he was taught wrong, either by bad text books, or by someone he trusts...
They just WILL NOT think it through and budge an inch on the subject, no matter how you present the information to them...

We were simply taught WRONG,
Every text book I referenced had the VACUUM, a load based advance,
Right on the RPM based graph for centrifugal, and the two CAN NOT exist on the same graph simply because the LOAD isn't RPM based.

This took me about a month to wrap my head around, identify, quantify, and develop a system in graphing timing curves when I started doing distributors/ignition that the text books were WRONG,

And everyone I talked to was WRONG since they learned from text books and didn't work with timing enough to figure it out,
Or they learned from someone that was the 'Correct' way things worked (The MYTH)...

When working with MSD, I regularly ran into 'Competitors' that spouted the 'Myth', and at first I tried to correct them only to be called an 'Idiot'...
Later, the guy running the Research & Development department explained why the COMPETITORS having things screwed up all the time HELPED us, and that was why we shouldn't attempt to correct them at all...

You can BE the smartest guy in the room, and not need to prove it,
Or you can argue with an idiot, and onlookers can't tell the difference between the two since they don't know which one is right...

--------------------------

The debate between 'Spark Ported' and 'Manifold' vacuum rages on every forum to this day.

The simple fact is, spark ported and manifold vacuum are about the same once you open the throttle blades.
Spark ported signal tends to be a little lower than manifold, but that is easy to adjust for in the vacuum canister on the distributor.

The real advantage to spark ported over manifold comes at idle.
You don't have ALL the vacuum signal pulling in full vacuum advance at idle,
So you often avoid the issues that happen with full vacuum advance at idle, like overheating on a marginally adequate cooling system,
And like less tail pipe emissions since you aren't completely exhausting the oxygen in the cylinder before it hits the exhaust pipe. (lowers oxides)

For me, it's response of the vacuum can it's self,
Spark ported is a little buffered, and responds faster to throttle changes since it's sourced between venturi and throttle blades.

You get IMMEDIATE drop when you close throttle blades, and you get a buffered signal, slowing the advance slightly as it comes 'In' which gives your fuel mix time to adjust before the advance hits at PTC.

All in all, it really doesn't matter to anyone that isn't tuning right to the edge for economy,
Or trying to pass a tail pipe 'Sniffer' emissions check.

 

[mstar]

Well that was a mouth full My situation not extreme at all.Actually did the 18*initial advance because there wasn't enough cent. in the dist to get the total I wanted 36*. The reduction drive starter(powermaster on a F.E. Ford)was part of the original engine package.Getting a little O T for the jeep guys so thanks again for info. mike

 

[gutthans]

Good tech info from 'Rush'...A few thoughts...

You ought to know how much total mechanical your distributor gives. On todays' cheap (low octane) gas, 36* (initial plus mechanical) is about as much running advance as (medium hard) acceleration will tolerate. My unit has 20*- 22*...that makes my initial a 14*-16* max.

Depending upon your on-board equipment, you may get better vacuum results from full manifold rather than ported. Many motors can be tuned to either.

Use a timing tape to find what your motor actually does with advance. Use a driving vacuum gauge to assess what the vacuum is doing.

Again, depending on your motor/equipment the limit for cruise should be between 50*-56* @ PTC. This will be your initial, plus mechanical, plus whatever vacuum advance is available.

Remember that if you run a Holley, richening the mains will not help low speed lean conditions much at all...you will be running on the idle/transition circuit up to about 1200 RPM. If you run around town on big tires use 3rd gear and/or keep RPM's up around 1500 -1800.

Lastly, no matter what you try...drive it for proof and change one item at a time in between runs. That will be the final determination of your effort.

Autolite 985, In-cap HEI, 14* initial, 22* distributor, 16* on manifold vacuum, idls set @ 700 RPM. Curve starts @ 850 RPM/Ends @ 2400 RPM. 4.6 stroker, 19.5 MPG on BFG 33's w/ 4.10 and Lock-Right....and yea, I know the Teams thoughts on the HEI 'upgrade'...I use a mellonized MSD gear on a nitrided Comp Cam gear and have machined and spacered it to ride center/center on the cam gear.

 

[TeamRush]

Well that was a mouth full My situation not extreme at all.

Actually did the 18*initial advance because there wasn't enough cent. in the dist to get the total I wanted 36*.

The reduction drive starter(powermaster on a F.E. Ford)was part of the original engine package.Getting a little O T for the jeep guys so thanks again for info. mike

How much centrifugal do you have? The advance head is usually stamped with the number and an 'R' for right hand rotation.
(IE: R 5.5/7.5 in some of the computer controlled vehicles,
Common in pre emissions I-6 engines was R13/15)

The advance heads can have anything between 5.5 to 21 degrees, it's just a matter of finding the correct advance head,
Or making the slot in your advance head longer with a grinder, but I suggest you SNEAK UP on that particular way of doing things since a grinder CAN get you into trouble really quickly!

 

[TeamRush]

Good tech info from 'Rush'...A few thoughts...

You ought to know how much total mechanical your distributor gives. On todays' cheap (low octane) gas, 36* (initial plus mechanical) is about as much running advance as (medium hard) acceleration will tolerate. My unit has 20*- 22*...that makes my initial a 14*-16* max.

This is purely a technical point,
ALL distributor advance is 'Mechanical'... There is no electrical or digital component in these obsolte '70's/'80s distributor ignitions.

'Centrifugal' is the technically 'Correct' term for the advance weights/springs,
But common usage is 'Mechanical', another one of those 'Myths' that have persisted through the years, mostly from text books that were simply wrong.

As for centrifugal advance, like I said, it's the minimum SAFE advance.
If your engine will tolerate 36 degrees (Initial + Centrifugal) then by all means run it, and you do what works best for you,

Given you have REASONABLE vacuum signal, and that's not always the case,
I would stick with closer to 24 to 28 degrees Initial + Centrifugal, and make up the difference with Vacuum advance when the engine wasn't loaded so hard for better economy.

24 to 28 is PLENTY for reasonable take off 'Power', and the vacuum will take over and add another 8 to 15 degrees when the engine can easily tolerate it.

After all, I'm NOT drag racing with my Jeep, lots of low speed operation,
Taking off in traffic, pulling hills off road, ect.,
So low end driveability is my main concern...

The other thing that come to mind right away is,
15 to 16 degrees INITIAL... that is at idle, and right off idle...
When does your centrifugal start?

By using smaller/weaker springs, you *COULD* get the centrifugal to START SOONER, and reduce that initial for take off power.
Usually, the centrifugal starts somewhere between 1,200 RPM & 1,500 RPM, which is WAY TOO LATE for good down low power.

On a light weight vehicle with REASONABLE drive train ratios I like to see the centrifugal start about 800 or 900 RPM, just about the same time the carb starts to draw from main jets, and while the fuel enrichment is still active.

I like to see the centrifugal 'All In' by 3,000 RPM at the latest, and on light weight, reasonable drive ratio vehicles, it can be 'All In' by 2,500 with little or no issues...

Like I said, it's a dance between fuel management, Centrifugal, Vacuum and driveability/ final drive ratios and your right foot, so it's a little different for everyone... that right foot can throw a money wrench into about anything I put in print, so I'm hesitant to tell anyone to tune right to the ragged edge!

Depending upon your on-board equipment, you may get better vacuum results from full manifold rather than ported. Many motors can be tuned to either.

Use a timing tape to find what your motor actually does with advance. Use a driving vacuum gauge to assess what the vacuum is doing.

That's what I've said all along...
Spark ported sometimes solves some intermittent transition issues from acceleration to PTC,
Some guys simply don't have spark ported vacuum on factory carbs,
Some guys want full hammering vacuum for this or that... It's personal choice.

I'm inclined to have a SMOOTH transition from idle to no vacuum advance/starting centrifugal advance,
From acceleration/no vacuum to PTC on full vacuum, so I use spark ported vacuum probably more than other tuners I know...
It's personal choice, so I'm not going to argue with the guy that says he 'Likes' manifold over spark ported, it's his choice.

Again, depending on your motor/equipment the limit for cruise should be between 50*-56* @ PTC. This will be your initial, plus mechanical, plus whatever vacuum advance is available.

This is WAY TOO HIGH for any factory distributor I know about!
If you get into the mid 40s it will be a miracle, even more so if your mostly stock AMC engine Jeep tolerates that much at any time!

If that's specific to YOUR engine, then state that.
The way it looks is you are RECOMMENDING people jack in up to 50 to 56 at PTC, which a stock AMC engine WILL NOT TOLERATE.

A FACTORY V-8 distributor, PRE EMISSIONS, will give you about 8 degrees Initial for good starting,
About 18R or 21R for centrifugal, the timing of which will be slow, starting about 1.200 RPM and all 'In' by 3,000 RPM,
The vacuum can will give you about 8 or 10 degrees (emissions cans MUCH less),
And you will wind up with about 39-40 degrees total advance at around 3,000 RPM and around 8-12 in.hg. vacuum signal.

Let's face it, with the average AMC engine, the last one rolled off the assembly line in '90 or '91, so it's tried, and most are at least 30 years old,
That's PLENTY of advance at a reasonable time to keep your engine living for years to come...

*IF*...
You were to build an engine yourself, and you know EXACTLY what you are doing, keeping things that can cause pre-ignition out of the engine, keeping the camshaft reasonable and working with the factory heads/reasonable intake & exhaust,
And you DID NOT stick stupid large tires on 2.73:1 gear sets,

The you could potentially tolerate up to about 50 degrees with few or minor detrimental results.

I'm running 3.31:1 gears on 31" tires, Torque Flight automatic in an '86 GW, FULL REBUILD with headers and improved compression/camshaft, and I'm only cranking 47 degrees at full advance,
Knocking back the ignition by 8 degrees when I tow anything...

It cooks tires, runs 70 MPH on the interstate, gets 17 MPG on highway, and pulls anything I put behind it... (Gets about 12 MPG towing anything heavy)

And mine is pretty standard for everyone I've talked to that's running similar...

Remember that if you run a Holley, richening the mains will not help low speed lean conditions much at all...you will be running on the idle/transition circuit up to about 1200 RPM. If you run around town on big tires use 3rd gear and/or keep RPM's up around 1500 -1800.

By 'Mains', do you mean 'Main Jets'?
If so, that is correct, but I disagree with when the main jets start to work.
Holley is fully adjustable via air bleeds to get the main jets working sooner.

Idle speed, usually 600 to 750 RPM, are idle screw territory exclusively.
Power Valve and Accelrator Pump enrichment will get you from 600 to 1,000 RPM or so where your main jets should start to add to enrichment.

Somewhere around 1,200 RPM you are going to overpower the idle mixture circuit capacity, doesn't matter what Holley you have.

Depending on VOLUME of accelerator pump, and the discharge nozzle size you choose, you will run out of accelerator pump about 1,400 RPM on average.
You can make it much sooner, or much longer, but AVERAGE will be about 1,400 RPM on a hard take off.

You should be FULLY into main jets by then,
And the Power Valve will be working to richen up the fuel mix in a big way,
And will continue to work until you build enough vacuum to close that power valve.

Power Valve should be set to shut down about 2 in.hg to 4 in.hg BELOW PTC point, so it opens up right away if you tip into the throttle a little.

At PTC, you SHOULD have full vacuum advance, the vacuum should have closed the power valve, and you *SHOULD* be running on main jets only,
Your leanest fuel mix being on main jets.

Since there are so many fuel enrichment circuits, accelerator pump, power valve, ect. on a Holley, they are VERY tuneable, and you can run the PTC right up to the point of being a little lean since both accelerator pump and power valve (Properly Adjusted) will add fuel if you need a little more throttle to get up a hill or catch up with traffic during PTC.

When adjusted CORRECTLY, this should all be SEAMLESS AND AUTOMATIC,
Timing drops via vacuum advance, power valve opens via vacuum, and as soon as you tip into the throttle the accelerator pump should give you an instant shot of fuel before vacuum even gets a chance to drop and linkages on vacuum devices to move...

Lastly, no matter what you try...drive it for proof and change one item at a time in between runs. That will be the final determination of your effort.

This is the #1 thing I tell people!
ONE CHANGE AT A TIME!

TAKE VACUUM AND RPM READINGS WHEN YOU HAVE AN 'ISSUE' SO YOU CAN TRACK WHAT'S GOING ON!

Autolite 985, In-cap HEI, 14* initial, 22* distributor, 16* on manifold vacuum, idls set @ 700 RPM. Curve starts @ 850 RPM/Ends @ 2400 RPM. 4.6 stroker, 19.5 MPG on BFG 33's w/ 4.10 and Lock-Right....and yea, I know the Teams thoughts on the HEI 'upgrade'...I use a mellonized MSD gear on a nitrided Comp Cam gear and have machined and spacered it to ride center/center on the cam gear.

YOU HAVE DONE YOUR HOMEWORK!
Sounds like a strong combination!

I was wondering about the 22 degrees of centrifugal advance, now I know!
Never seen a Jeep/Motorcraft with FACTORY 22 degree centrifugal advance head in a Jeep...

Autolite plugs are EASILY Fired by a single coil ignition, so that's a good deal, keep the gap reasonable, 0.035" to 0.045" so you don't overload the internal insulation inside the ignition coil,
Those 'Super Duper' plugs, and CRAZY large gaps to produce excessive voltage have ruined more coils than I can count... All bad ideas...
All in all at least as good as 'Stock' and probably better than stock since you are paying attention to details and doing research,

33" tires on 4.10:1 gears gives you a final drive ratio BETTER than stock vehicle with stock tires and 2.73:1 gears, so that's better than stock.

16° + 22° = 38° before the initial. Most vehicles run fine on 38° total,
But STROKERS take a little more ignition timing...
Not sure I'd get it with INITIAL, but a 52° total I'd keep a CLOSE watch on even in a stroker!
I'm sure you are since everything else about this build sounds so close to dead on what I would have done it's not funny!

Starts at 850, ends at 2,400 is VERY close to what I would have done.
When I'm tuning for power out of the hole, I start VERY early and finish up early.
I take a TON of when I say I start centrifugal at 850 or 900 instead of the 'Conventional' 1,200 RPM all the 'Text Books' say you are supposed to,
But if you want to get that vehicle moving EARLY, then you MUST get the timing started EARLY.
If the machining/build will tolerate the timing, there IS no such thing as 'Too Early'... It's what the engine will TOLERATE, not what some desk pogue writer thought sounded good when he wrong the text book 45 years ago...

Manifold vacuum on those stiff HEI cans will give you VERY fast vacuum advance drop when you goose the throttle, and that should be keeping you out of trouble. It's the only good thing about HEI vacuum cans, they have a VERY stiff spring in them,
(Two things, they are easy to limit if you get too much total vacuum advance)

I'm taking it you don't do a lot of HEAVY TOWING WITH THIS?
Sounds like a mud bogger to me, It should REALLY clean the tires when you need it to!
Down shift, blip the throttle and throw CAT SIZE MUD CLODS IN ALL DIRECTIONS! :drool: Sounds like a TON OF FUN!

----------------

As for the HEI 'Upgrade', it's a move sideways, just another '70's factory ignition to me. Purely mechanical/electrical engineering standpoint.

When the guys have a 28+ year old ignition in a Jeep, and they start talking about an 'HEI Upgrade', it goes against common sense.
If the gear ON the distributor is 28+ years old,
Or discount 'Sore' ,
It's time for a 'Tune Up' with some quality parts...

AND,
If you are going to buy QUALITY parts, why not upgrade to the better 'Ford' version of same?
It's ZERO cost increase over a comprehensive 'Tune Up', same price as the factory , but QUALITY factory replacement ,

And that *USUALLY* solves 99.9% of the complaints WITHOUT pulling the distributor and spending $300+ on an HEI Clone and starting the tuning process all over again when the factory unit usually just needs adjusting.
$5 worth of springs, an 1/8" allen wrench and a vacuum gauge usually gets them running just fine...

Just like any other ignition from the 70s, HEI/Clones come with their own set of 'Issues', and you have to spend time/money fixing them,
Gears, install height issues, blown through rotors, good caps/plug wires, ect.

So you are $300 behind in the race, you have to pull/replace the distributor, which is a big deal for some guys (a snap for others),

Since YOU paid attention, got the correct install height, gear mesh, correct gear, ect. you know what I'm talking about here,
Every system has it's 'Quirks' and 'Issues'...
With HEI it's blowing through rotors, with Motorcraft it's leaving the key on and toasting the module...

They are all greasy from the inside out to me, and they all have 'Issues', it's just a matter of what skill level the guy working on it has that is of primary concern to me...
And how deep the pockets are, I don't want to spend someone else's money unnecessarily and overload them on the mechanical/electrical information they should probably need to know...

Cap, Rotor, Wires, Plugs, 'Grounds' are pretty easy, if they still have 'Issues' after that then we go deeper...
It's never the 'Advanced' guys that ask the basic questions...
Like, "I have the factory '1980' distributor cap/wires, should I buy an HEI?"
The answer is always "NO! TUNE IT UP,
GET THAT 34 YEAR OLD OFF THE DISTRIBUTOR AND WHILE YOU HAVE IT OFF, USE THIS UPGRADE STUFF"

*IF*...
It turns out to be a bad distributor, module AND coil,
Then at least they have reasonable plug wires, and they are only out the cost of cap & rotor when they order the HEI/Clone...

I stress plug wires A LOT!
Once you have a good set of plug wires, and they fit CORRECTLY, it doesn't matter what ignition you put under them.
PLUG WIRES CAN WASTE UP TO 80% OF YOUR SPARK ENERGY ON THE WAY TO THE SPARK PLUGS!
So, it makes sense to have a good set of plug wires...
Just like it make sense to spend $3 to $5 extra on a brass terminal distributor cap, doesn't matter what distributor is under it, this has NOTHING to do with the distributor, it's all about that high voltage spark energy getting WHERE it's supposed to go, WHEN it's supposed to be there!

I got one about a month ago that had spark plugs in it that hadn't been made in 12 years!
When I asked the owner, he said the local 'Shop' (now out of busness) had done a 'Tune Up' on it a 'Couple' of years ago, the shop has been out of business for at least 5 years...
And the spark plugs were 12 years old, the spark plug wires were FACTORY 1980, and the distributor cap was changed, but was the bottom of the line local discount 'Sore' version...

No wonder it had a 'Miss'....

Six plugs, one distributor cap, one rotor, 7 high voltage wires later,
We found a TON of vacuum leaks, fixed those,
Found the choke wasn't pulling off and the high idle was stuck in place, fixed those,
Now it starts in the cold weather, warms up, runs reasonably well for something that was 34 years old and had been running rich with a ton of vacuum leaks for who knows how long...

Now that it starts and RUNS, the guy wants to put 36" tires on it!
On 2.73:1 gears!
On a worn 34 year old STOCK engine!

Just another day in Jeeping!

 

[Neuner]

Ugh ~ another thread and question derailed...

I didn't ask about how to set the timing or advance. I asked if my vacuum advance was adjustable before going to purchase one that was. Those were some unnecessary extremely long posts with information that is already found in a general internet search. If someone wants to have all of that included, they can ask that question themselves in another thread.

Thanks for the help by mentioning the 3/32". I haven't tried it yet because I haven't had time.

 

[DHugg]

I want to fine tune the vacuum advance on my distributor. It's a motorcraft and every search states I should be able to insert an allen wrench and make adjustment but it doesn't appear so.

The tip of the distributor port appears to have been mangled by one of the PO and so it limits what i can try. I've been able to insert a 1/8" allen but it goes in far without making any type of screw contact.

Am I able to replace just the vacuum advance part of the dizzy without swapping the whole thing?

Neuner, if a PO mangled the aperture into the vacuum advance input port, you can reverse that distortion with a 1/8 inch or less punch. I get many of my tools at Sears, Craftsman brand, and the punch series I have all have sloping tapers to the final punch diameter. Or think about any steel hammer-struck tool, like a nail set or the tool you use to create a drill bit starting depression.

What's the worst thing that could happen? You could damage your already-ruined vacuum advance port.

 

[Flex BT]

Teamrush, thanks for all the info. I did the teamrush upgrade a year or so ago and read I should try to up my initial advance from 6-8 to 10-12, this is the first time I've hear that's incorrect. I wonder if this might be the reason the Jeep is having a much harder time starting?