Erratic Timing

[ScamperTom]

My car has been completely out of tune lately, dropping 300 rpm and stalling in gear. In the course of chasing down the problem I found that the timing was completely out of wack. I educated myself on the ins and outs of timing and went at it. Here's what I found:

The timing is erratic, losing and gaining almost a full point with the rpm steady. The dist. rotor had some play side to side. I tore apart the distributor and found that the weights looked like this:

It appears that those pins are worn which is causing the rotor to have extra play. My question is would this cause the timing to behave erratically as it does, or is this normal? The cam/distributor drive gears have no play.

Further, it appears that the springs have already been replaced with light/heavy ones. But checking the timing it looks that I'm getting 25*+ at 4500 rpm. The cam assembly is marked 15r, which I assume means will give me 30* at full advance, which would match what I saw. Shouldn't I be looking for full advance at about 3k, and shouldn't I wan't less than 30* mechanical so I can run more initial timing? My vacuum canister (brand new, the old one was shot) is marked 9.5, so that should give me 19*. Can I run initial timing at 5* and hook up the canister to full manifold vacuum to give me ~20* at idle or do I need to replace all of this to get a good timing curve?

318 not sure what year
19 hg at 1100 rpm

Thanks for y'all's input! From what I've read looks like there are some real tuning gurus on this forum.

 

[Alaskan_TA]

Erratic timing in a small block is often due to a worn timing chain & gears.

 

[67Dart273]

Well there are several things, and factory Mopar distributors are not known for "rock steady." What are you calling "a full point?" One degree? That's NOTHING

So far as full advance, I'd say AT LEAST 3K with the big spring shown. You should be able to look that all up in the manual, if it's a stock dist and correct year. Bear in mind that the newer these dist's get the "worse" they get for slow, long "smog" advance specs

So far as spark scatter, instability, Yeh, timing chain wear, distributor intermediate shaft and bushing wear, general wear in the dist itself, bushings, advance "hat" and weights, and even the slop up and down in the shaft and the slop/ wear in the drive tang/ slot.

 

[AJ/FormS]

Yeah what he said, Check that by sawing the crank back and forth, engine off of course, against the valve springs. Try not to move the cam. Any play at all is not good. But if you rotate the engine to get the balancer lined up with the index, you will be able to read the cam chain stretch in degrees. Two degrees is kindof normal . By 4 or 5 it's getting to be risky business with factory 318LA chain. Risky in that when it jumps it sometimes spells the end of the engine with at least 8 bent valves.
As to the D if the flyweight are not sticking,then she is fine.... except your thinking is correct; 30* in the D is correct for an Idle timing of 5*.
The engine depending on what it is, you didn't say, will be much happier with double to triple that 5*. But then you have to shorten the slots to take out the equivalent amount of timing for to set the power-timing, which,generally, must not exceed 36 degrees.
That one long loop spring is a tricky bugger. It is usually worth about 4* but may not give it all until 4000 rpm or higher. Because only one spring is now handling everything else, it is sometimes too fast after the initial-reset, to keep your engine out of detonation. So you gotta be careful in how much you can do on a stock engine.
Oh wait, you did say; I see it's a 318; which is what I assumed,lol.
As to the worn pins, I have brazed some hard brass on there which lasts I guess forever cuz no one has ever come back yet,lol.

 

[ScamperTom]

I’ll have to go double check for play in the timing chain. It’s only one degree error +/- but it’s still frustrating. The engine was supposedly rebuilt about ten years ago with higher compression and cam, but you never know. The oil definitely stays too clean for the engine to be original, tho.

Good to know it’s not a huge deal. I’ll probably just put a tack weld of the pins and file them down. There doesn’t seem to be any play anywhere else in the distributor except maybe in the tang/slot. The intermediate shaft and bushing doesn’t have any noticeable play.

So to get the timing to advance earlier, should I ditch the big spring? I don’t really know what it’s doing past 4k...

 

[AJ/FormS]

So to get the timing to advance earlier, should I ditch the big spring? I don’t really know what it’s doing past 4k...

That long looper does nothing to probably well past 3000. If you map out your timing on a graph at every 400 rpm intervals you will see exactly when it starts/quits, and how much it contribute which is seldom more than 4*. Fast curves are not always the answer. Getting your your WOT power-timing sorted is job #1 . Then you do whatever you can for other conditions.

1 degree on the lite means nothing, less than nothing to the engine, unless it is the one degree the initiates detonation. But that is easy to resolve,lol.

 

[ScamperTom]

That long looper does nothing to probably well past 3000. If you map out your timing on a graph at every 400 rpm intervals you will see exactly when it starts/quits, and how much it contribute which is seldom more than 4*. Fast curves are not always the answer. Getting your your WOT power-timing sorted is job #1 . Then you do whatever you can for other conditions.

Thanks! That seems pretty clear. I'll have to look more into what curve is right for my engine, but I'm not spending much time past 4k anyway, so it doesn't make much sense to keep a spring that won't really do anything.

1 degree on the lite means nothing, less than nothing to the engine, unless it is the one degree the initiates detonation. But that is easy to resolve,lol.

Haha that's what I'm worried about! I guess I'll just set it at 33* and feel safe.

 

[AJ/FormS]

After you shorten the slots to say 22 degrees, and run an initial of say 12* for a total of 34*, then that long looper will no longer top out at the same Rpm as before.
I run one of those on my HO367; But In mine, it begins operation at 2800 and is done by 3400, and it only brings in 4*. This allows me to run a medium fast curve to 2800 for sharp throttle response yet by slowing it down after 2800, I can run 87E10 @185psi cylinder pressure with aluminum heads, at full timing, without detonation. So, that little tiny spring has , over 125,000 miles, saved me untold thousands of dollars in not having to run better fuel.
BTW 2800 for me is 20mph in first,33mph in second,45 in third, and 64 in fourth...... so in normal driving, I never miss those missing 4 degrees. And at hiway speed in 4th the timing is locked to 28*. To that, my Vcan adds 22* for a cruise total of 50*, very conservative.
I think I stretched that long looper just a hair. That was over 15 years ago so please forgive me if I cannot recall. You have to be real careful cuz there's no going back; once it's too long it is no good to you.

 

[Mattax]

Thanks! That seems pretty clear. I'll have to look more into what curve is right for my engine, but I'm not spending much time past 4k anyway, so it doesn't make much sense to keep a spring that won't really do anything.

Like AJ and 273 said, you have to measure and plot out what is in there before making changes.
A stock 318 is relatively efficient at low rpm, and doesn't need as much advance at idle and low rpm as something more radical.
However, at least the early years of the 318, the timing should increase with rpm until at least 4000 rpm. That's a 2 bbl stock cam and compression - not hot rod.
As anti-smog requirements got tighter late 60s early 70s, the initial timing was reduced and longer quicker initial advances were used to get the timing back were it needed to be off-idle.
Getting above 3000 rpm with an electronic ignition, the heavy spring and longer advance was sometimes used to offset the time lost by electronic switching.

I'd set the initial timing at whatever it needs to be for your car. Then go from there. Setting it at 2600 to 3000 is for hot rod and racing setups.
If its not stable at the idle speed, try increasing the primary spring's initial tension. That's done by rotating the spring perch on the cam plate.
If you need weights in better condition, ask halifaxhops.

 

[Mattax]

For example, acceptable timing range for '68 using 5* initial (per Plymouth FSM)

CAP and CAS 318's at least through '69 have the initial timing spec'd at either TDC or 5* ATC.
From 1400 rpm to 4600 rpm the timing very similar to the non-smogged 318 shown above.

 

[67Dart273]

Also from long ago, I think Crackedback posted this. Here's a chart showing approximate advance re: slot length. Also, are you aware of the aftermarket FBO advance limiter plate? Some guys have used these and say they work OK
How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races
=======================================================
Dimensions for Mopar distributor slots, in DISTRIBUTOR degrees

Modifying advance slots
degrees / slot size
6.............. .340
7................355
8................375
9................390
10...............405
11.5 ...........420
12...............435
13...............445
14...............460
15...............475
16...............490
17...............505
18...............520

 

[JoeSBP]

I don't think I have ever owned a carbed engine that would hold timing rock solid at idle, always bumps +/- a degree or two

 

[Mattax]

FWIW.
Even though the '68 distributor used roughly the same advance as your plate has (30 crank degrees), the '68 vacuum advance provided more than the replacement pod you bought. The range spec'd in the book suggests something around 24 - 26*.

What mechanical curve you can use for your engine will depend on the details.
If its got EGR and/or smog cam (where the exhaust closing was extended) that's going to change things a little.
And if its later 4 barrel equipped 318 (police or truck) different again. Probably a longer initial advance, and slower secondary advance.
Then based on that, its not too hard to match an appropriate vacuum advance.
You can get clues on what the 318 is from using the paint color, intake, date info cast on the block, and of course engine number of you can see it.

 

[ScamperTom]

After you shorten the slots to say 22 degrees, and run an initial of say 12* for a total of 34*, then that long looper will no longer top out at the same Rpm as before.

I'd set the initial timing at whatever it needs to be for your car. Then go from there. Setting it at 2600 to 3000 is for hot rod and racing setups.
If its not stable at the idle speed, try increasing the primary spring's initial tension. That's done by rotating the spring perch on the cam plate.
If you need weights in better condition, ask halifaxhops.

OK so sounds like I shouldn't run the advance as is. I need to set initial at idle, then (if necessary) shorten the slots to make the mechanical advance match. Only once that's done can I fiddle with the springs to get the right curve. I think I've been getting ahead of myself!

For example, acceptable timing range for '68 using 5* initial (per Plymouth FSM)
View attachment 1715306919

CAP and CAS 318's at least through '69 have the initial timing spec'd at either TDC or 5* ATC.
From 1400 rpm to 4600 rpm the timing very similar to the non-smogged 318 shown above.

This seems similar to what's in my 1972 service manual. I think my engine is 77 (the number just might be 71?) with electronic ignition, not lean burn.

My advance is 15, right in the middle, so those degrees would be 3*, 9.5*, and 15*, if I'm guessing right. Since I can't idle below 1100, and my timing light shows 1* at 1100, then these numbers would match what I'm seeing, meaning that my initial timing is 5* retarded. Yikes.

Also from long ago, I think Crackedback posted this. Here's a chart showing approximate advance re: slot length. Also, are you aware of the aftermarket FBO advance limiter plate? Some guys have used these and say they work OK
How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races
=======================================================
Dimensions for Mopar distributor slots, in DISTRIBUTOR degrees

Modifying advance slots
degrees / slot size
6.............. .340
7................355
8................375
9................390
10...............405
11.5 ...........420
12...............435
13...............445
14...............460
15...............475
16...............490
17...............505
18...............520

Thanks, this is just what I was looking for! I am debating getting that plate, but it might just be too easy for me... lol

I don't think I have ever owned a carbed engine that would hold timing rock solid at idle, always bumps +/- a degree or two

Good to know this is normal. Thanks!

 

[ScamperTom]

FWIW.
Even though the '68 distributor used roughly the same advance as your plate has (30 crank degrees), the '68 vacuum advance provided more than the replacement pod you bought. The range spec'd in the book suggests something around 24 - 26*.
View attachment 1715307081

What mechanical curve you can use for your engine will depend on the details.
If its got EGR and/or smog cam (where the exhaust closing was extended) that's going to change things a little.
And if its later 4 barrel equipped 318 (police or truck) different again. Probably a longer initial advance, and slower secondary advance.
Then based on that, its not too hard to match an appropriate vacuum advance.
You can get clues on what the 318 is from using the paint color, intake, date info cast on the block, and of course engine number of you can see it.

I was wondering if 19* was too much. I've read that the total advance shouldn't exceed 50*, but I guess you really can't dial in the vacuum advance until you get the mechanical right. Just a question: The adjustment in the can only changes the amount of vacuum required for advance, not the actual amount of advance, right?

My engine was built by the PO, but he provided no build sheet, so I have no clue what spec. I should go look at the codes, tho, it will at least tell me what it started as.

 

[Mattax]

Thanks, this is just what I was looking for! I am debating getting that plate, but it might just be too easy for me... lol

If you can weld a tack , your are almost always better off not using that plate.

I was wondering if 19* was too much. I've read that the total advance shouldn't exceed 50*

That's about right, but clearer to describe it as timing at high speed cruise (60 - 65 mph).
Lets say the engine is turning around 2800 rpm at 60 mph, and you observe the vacuum is around 17"Hg.
Then the mechanical timing at 2800 plus the vacuum advance should equal 50 * BTC.
example:
The '68 318 graphed above shows the distributor with 5* initial. At 2800 rpm it could have a timing at 24*.
At 17" Hg it would have no more than 27* more, probably closer to 26*. 24 + 26 = 50*

but I guess you really can't dial in the vacuum advance until you get the mechanical right. Just a question: The adjustment in the can only changes the amount of vacuum required for advance, not the actual amount of advance, right?

You got it! It shifts the line on the graph left or right.

My engine was built by the PO, but he provided no build sheet, so I have no clue what spec. I should go look at the codes, tho, it will at least tell me what it started as.

LOL. OK. Do you think its a basically stock rebuild?

 

[Mattax]

My advance is 15, right in the middle, so those degrees would be 3*, 9.5*, and 15*, if I'm guessing right. Since I can't idle below 1100, and my timing light shows 1* at 1100, then these numbers would match what I'm seeing, meaning that my initial timing is 5* retarded. Yikes.

On a early smog engine, you're actually not that far off. Plymouth spec'd the '68 & '69 smogged manual transmission cars with 5 *ATD initial.

That said, those early smog setups are a PIA and since you're not doing a points resto, there's no reason to try to that.
Bet if you give it some more initial, you can then turn the idle speed screw (aka idle stop screw) down and get a slower idle speed.
Then fiddle with the idle mix screws. Turn them in until the speed drops a little, then back out roughly 1/4 turn richer.

Do this in a small steps. In another words repeat the process a couple times. As the engine warms up, you should be able to get idle speed down to 600 - 650 rpm.

 

[ScamperTom]

That's about right, but clearer to describe it as timing at high speed cruise (60 - 65 mph).
Lets say the engine is turning around 2800 rpm at 60 mph, and you observe the vacuum is around 17"Hg.
Then the mechanical timing at 2800 plus the vacuum advance should equal 50 * BTC.
example:
The '68 318 graphed above shows the distributor with 5* initial. At 2800 rpm it could have a timing at 24*.
At 17" Hg it would have no more than 27* more, probably closer to 26*. 24 + 26 = 50*

OK now that makes sense. And of course that would explain why people drive their cars to get the vacuum advance dialed in, and how adjusting the amount of vacuum needed to begin the advance would also affect how much advance you actually get at a certain speed.

LOL. OK. Do you think its a basically stock rebuild?

Well he SAID high compression and mild cam, but I'm getting 18 hg at 1100, so it must be pretty mild. Fine by me, this is purely a streeter. He threw on edelbrock intake and headers though, and got rid of the egr system.

On a early smog engine, you're actually not that far off. Plymouth spec'd the '68 & '69 smogged manual transmission cars with 5 *ATD initial.

That said, those early smog setups are a PIA and since you're not doing a points resto, there's no reason to try to that.
Bet if you give it some more initial, you can then turn the idle speed screw (aka idle stop screw) down and get a slower idle speed.
Then fiddle with the idle mix screws. Turn them in until the speed drops a little, then back out roughly 1/4 turn richer.

Do this in a small steps. In another words repeat the process a couple times. As the engine warms up, you should be able to get idle speed down to 600 - 650 rpm.

That's next on the to-do list. Hopefully the timing will actually make the difference this time.

 

[Mattax]

Well he SAID high compression and mild cam, but I'm getting 18 hg at 1100, so it must be pretty mild. Fine by me, this is purely a streeter. He threw on edelbrock intake and headers though, and got rid of the egr system.

That does suggest its not too radical. It still might like a curve with a little more advance in the begining. Funny thing is that electronic ignition distributor's curve may already do that for you!

Sketched the '72 maximum timing onto the '68, and also marked your 1* at 1100 rpm.
Based on what you've posted, if you can get 5* at 800, that would be pretty good. Then see how it drives.

If it has some cam in it, it may not like 0* or even 5* at 600 rpm.
If it is some cam, probably will like an initial more like a pre-smog 273 4bbl (or maybe a police pursuit 318 4bbl using a 340ish cam, I've not seen those timing curves)

 

[Mattax]

Here is a comparison of the '72 max specs. with the 273 High-performance specs.

 

[ScamperTom]

That does suggest its not too radical. It still might like a curve with a little more advance in the begining. Funny thing is that electronic ignition distributor's curve may already do that for you!

Sketched the '72 maximum timing onto the '68, and also marked your 1* at 1100 rpm.
Based on what you've posted, if you can get 5* at 800, that would be pretty good. Then see how it drives.
View attachment 1715307121

If it has some cam in it, it may not like 0* or even 5* at 600 rpm.
If it is some cam, probably will like an initial more like a pre-smog 273 4bbl (or maybe a police pursuit 318 4bbl using a 340ish cam, I've not seen those timing curves)

I've been looking at the figures I wrote down, and realized that I can't remember if I set the initial timing at 10* or 0* to record them... so they're pretty much useless. Either way they seem fairly close to the stock 318 specs you posted. Quick short advance at first then a long slow curve.

I worked on the idle tuning a bit this afternoon and was pretty successful I think. I realized I had previously cracked the secondaries open too far, so I closed them and was able to drop the idle down to 600 in gear, 750 in park with 10* of initial timing. Any more and it would kick back on the starter (is that technically detonation?) That should allow me to approximate the 273 curve you posted. If I aim for 32* total (how close should I really push it for a mild engine?) then my vacuum can should come in just about perfect, theoretically.

 

[Mattax]

I worked on the idle tuning a bit this afternoon and was pretty successful I think.

drop the idle down to 600 in gear, 750 in park with 10* of initial timing.

Great progress!
Based on what you're experiencing, should be able to get the difference between in gear and out a lot closer.

Any more and it would kick back on the starter (is that technically detonation?)

I don't think its technically detonation since there is no flame burning.

Kickback is a good description - everyone gets the picture.
However kickback at over 10* is unusual with a hot cam. May be a fair amount of compression in the engine, and the cam although not stock, may not have much overlap.
You may be able to get better idle quality (ie. power) with 8* than 10*. The more power it has at idle, the less the rpms will drop when put in gear.
Also try an 1/8 and 1/4 turn richer on each of mixture screws.

If I aim for 32* total (how close should I really push it for a mild engine?) then my vacuum can should come in just about perfect, theoretically.

Lets come back to that.

But so as not to leave you totally hanging - In my experience, the critical area to get mechanical timing correct to work with vacuum advance is 1800 to 2800 rpm. That's when the car is cruising along for 45 minutes and you give it a little more throttle for a slight uphill and it starts to ping.

 

[Mattax]

Some more points on the timing curve will be really helpful. hint hint

Does timing stay at 10* BTC when the engine slows to 600 rpm?

Big difference if it looks like this.

versus.

If the timing is advancing between 600 and 750 rpm, that would help explain the reason the engine rpm dropped so much when you put it in gear.
That's easily correctable, but we need to know before making corrective actions.
If the timing is not advancing until 750 rpm, that's fine, but if so, it may too much above 1600 rpm as illustrated in the last graph. Again, once we know this, we can choose the appropriate adjustment.

 

[Mattax]

If you need to delay the start of the advance, put more initial tension on the primary spring.
One way to do this is rotate the spring perch on its eccentric.

When you have the distributor out, you can file a screwdriver into a tool to do this. Then in the future you don't have to disassemble the distributor to adjust the perches.

The other way to do this is welding a bead to the inside of the slot. Without a little more info, I'd be hesitant to do that just yet.
But if you're thinking around 32* is probably the maximum timing, and initial will be around 8* or 10*, that does seem to be the way this is heading. In other words 22 to 24 in the slots...

 

[ScamperTom]

Wow so much awesome info! Thanks! Like an idiot I dropped one of the carb sight plugs while checking the fuel level....Can't find it anywhere. Anyway as soon as I can get another (I might be able to borrow one) I'll fire it up, try 8*, and note where the timing curve actually is now. I don't think it's advancing between 600 and 750, but I'll definitely have to double check.

I did take if on a quick drive after I set the initial at 10, and it was pinging under load, so I'll have to adjust something.
Thanks for the help! I'll keep y'all posted.