Stock Distributor Re-Curve

[Nicholas Gromak]

Hello all,

If you're not into short stories, you can skip to "Sorry for the short story...."

I've been having some drivability issues this summer with my bone stock 1967 Chrysler Newport. I would like to modify the stock ported advance setup for a manifold vacuum setup and tune for fuel economy as a fun little project. This has been my daily driver for 2 summers now and I would like to improve the little everyday hassles that I incur. Here's a story:

In the spring of 2018, I brought the car in for a tire and high zinc oil change where my mechanic introduced me to something called detonation and pinging. He set the initial timing to 7* BTDC and hooked back up the ported vacuum advance as seen in 1967. The car ran fine but as the days got longer and the temperature higher, I started to get pinging again at part throttle. Curiously, cruising at 35-40 mph was fine until I encountered a hill or highway on-ramp. If I held the speed constant, as soon as I started up the hill, it would ping all the way to the top. Highway cruise and overtaking was not a problem.

Anyways, I called him back some months later and he told me to rotate the distributor 1/8" to retard the timing. The car ran like crap, had no power and still detonated in the same exact way. For a time I was driving with no vac advance until someone from a car show I was attending told me to put high octane fuel in it all the time. With that in mind, I reset the timing (to a mark before the retard), hooked up the vac can and drove on 91 octane for 2 months. This didn't help. After that, I went to 94 octane racing grade gas (at $1.65/L or $6.25/gal) for the rest of the summer. This did not solve the problem either.

Fast forward to late August. I did some research about Manifold vs Ported vacuum and I decided to switch. What a difference it made! I got all my performance back, it starts easier and I've been driving on 87 octane without problem...until now. (quite a sight for sore eyes to see $1.30/L or $4.92/gal for gas again)

The complaint now is off idle stumble/stall. In the cold October mornings here, the car will stall every time the accelerator is pressed. It's kind of scary for the engine to die on the freeway in today's morning traffic. Once the engine is warm, the problem almost goes away. Only a slight drop in engine speed until it climbs out of its hole; not a complete stall. I can't advance the timing any more. It's already at 30* at idle with the vac can hooked. And its starting to ping again!

Sorry for the short story. Here's the question:

I want to re-curve the stock distributor to work with manifold vacuum and crappy 87 octane gas. I think I have my numbers figured out but I would like someone's input to solidify what I've got. After playing with a vacuum gauge, I found that my engine likes 27* of initial advance which draws 20"Hg at 800 rpm. This is not possible to use to drive the car but it shows where it's happiest. Through tests, I discovered my vac canister to be 12 crank degrees at 20"Hg vacuum. I would like to set the initial timing at a number such that with vacuum advance added, idle = 27*.

So I want 15* initial + 12* vacuum = 27* at idle with manifold vacuum. The problem I'm trying to figure out now is the mechanical advance. I read a lot of posts of people with hot cams in their cars and HEI distributors shooting for 36-38* total all in at 2500rpm-3000rpm. But my engine is stock. Where are they getting these numbers from. I'm guessing I don't want to go over 45*-50* at highway speed on 87 octane in 38*C, 100*F weather. I did some more tests and determined that my vacuum is stable at 15"Hg maintaining cruise (75% of what it was at idle in neutral). Doing some simple math to fill the gap, I have 15* initial + ??? mech + (12 x .75 vac) = 45*/50* total. My mechanical is therefore between 21* and 26*.

I also don't want it to come in too fast because of the hill climbing detonation explained above so I'll shoot to be all in by 3000rpm. I may never reach 3000rpm on a regular day but this should stretch the curve so it's not too high at part throttle at the expense of usable horsepower at lower speeds. Engine only turns 2350rpm at 60mph by the way.

Anyones' input or advice is greatly appreciated! I just want to know if I'm headed in the right direction after doing all this research.

Thank you!

Nick

 

[Turboomni]

Where do you get these numbers?

 

[Turboomni]

So I want 15* initial + 12* vacuum = 27* at idle with manifold vacuum.

Where do you get these numbers?? Why manifold vacuum? What is the purpose of vacuum advance may I ask? When should vacuum advance come into play? And why?

 

[Nicholas Gromak]

They're just what I'm planning to get the distributor to do. I got the vacuum advance from plugging and unplugging the vacuum canister line while the car was running and reading off the timing tape. Turned out to be about 12 degrees.

The 27 degrees I got from unplugging everything, hooking a gauge to manifold vacuum and rotating the distributor until maximum vacuum was achieved. I then tuned the idle mixture screws and got another inch of vacuum. I went back to the distributor and tried to see if the mixture adjustment affected anything which it did. Increased it a little more until the needle started to jump around a bit, then turned it back to 20" vacuum (stable) and richened both idle screws by 1/2 a turn. All the while keeping the idle below 800 hoping that the mechanical wasn't kicking in that early and skewing the results. I had to dial it back a bit because it would kick back a bit when starting. I got it to what I believe is a reliable setting. At least for starting when hot.

The 15 is what I believe I want as initial timing, with a manifold advance setup, with a vac can that is not adjustable, at 12 degrees.

 

[Turboomni]

What is the purpose of vacuum advance may I ask again??

 

[Nicholas Gromak]

From what I understand it's to increase advance at light load conditions. They say you get more fuel economy at cruise with manifold vacuum advance.

 

[Turboomni]

So why do you want this at idle with manifold vacuum? And why do you want more than 12 degrees at idle with vacuum advance? Vacuum advance is not needed at idle.

 

[Nicholas Gromak]

Hmm... Good point. I would like to use the manifold vacuum more at cruise than idle. I think about it like this: to go down the highway or road, it maybe only takes 50 or so horsepower to maintain speed. Now with no vacuum canister, either ported or manifold, you are limited to the total mechanical advance only. By having your advance come in at a low load, high manifold vacuum condition like highway cruise, (on top of your mechanical advance) you can get a couple more horsepower at the same speed.

I'm no expert but when you adjust the distributor when tuning you notice that if you advance it some, the idle speed increases. What is happening is you're making the engine more efficient for the given air fuel ratio and rpm. The same goes for cruise. You advance the timing at speed, you're more efficient and you don't have to use so much throttle to maintain that speed.

There is only one benefit shrouded in mystery about having this much advance at idle that I won't get into. People say that your engine runs cooler with more advance because the lean mixtures found at idle take longer to burn. This doesn't really make sense to me. I thought that if something burned longer it heated up more but that's besides the point.

The point is that the drawback with ported vacuum is that vacuum advance and mechanical advance both increase together making a steep curve. Lots of punch of the line and horsepower but easy target for pinging on today's fuels. The drawback with manifold is that along with benefits at cruise you have to have high advance at all other high manifold vacuum scenarios including idle.

What's your take on that. Are you a fan of ported, manifold or no vacuum advance connected?

 

[Wildbunch02]

I had a similar off idle stumble with my 75 Nyb which was driving me crazy. I did many of the similar adjustments that you have done as far as timing and idle air mixtures. I bought a rebuilt carb which solved the problem, but the problem returned. After a close inspection I noticed a small fuel leak on the top of the carb were the stem from the acclerator pump hooks to the pump linkage. I swapped carbs since I was still under warranty and that solved my problem.

 

[Turboomni]

Good night,,,

 

[halifaxhops]

OK here is what I need to figure this all out. Carb 2 or 4bbl, engine size and year and any cam etc, and the distributor number.. Here it is in a nutshell. Your ported vacuum port on the carb limits the vacuum advance from going all in at idle and basically turns to manifold vacuum at WOT. The mechanical advance has a max total in degrees that the weight of the springs tells it when to start to advance and also when to be in full, lets just say 20 at the crank to keep the numbers simple. Say the Vac advance is 20 max at the crank also. Ok at idle it should be close to 0 or so at a idle then you hit say 1500 crank rpm the mechanical goes to say 10 and the vacuum starts to kick in. It all increases form engine rpn and vac " as the throttle opens. also as the Vacuum drops at crusing speed the manifold vacuum should also drop and retard the vac advance back a bit. That's the way it should work. Ever change out the distributor with a rebuilt one? Why? They are usually just set in the middle of say a ten year spread to work on a lot of different applications. Hope this helps.

 

[Nicholas Gromak]

I had a similar off idle stumble with my 75 Nyb which was driving me crazy. I did many of the similar adjustments that you have done as far as timing and idle air mixtures. I bought a rebuilt carb which solved the problem, but the problem returned. After a close inspection I noticed a small fuel leak on the top of the carb were the stem from the acclerator pump hooks to the pump linkage. I swapped carbs since I was still under warranty and that solved my problem.

I'm sure my carb is compounding the problem but I've only got 3 weeks left before I put the car in winter storage. I'll check again for any obvious cracks in the exterior but I'm afraid the full rebuild and clean will have to wait until next spring. I did however check that all the adjustments on the carb are as per factory and they were. Accelerator pump and all. I've got the full service manual.

Thanks for the response!

 

[Wildbunch02]

It was not a crack in the carb top but a leaky accelerator pump leaking internally and seaping out of the area where the stem of the pump pokes through the top of the carb. It was pretty easy to spot.

 

[mr. fix it]

Hi Nicholas.
From your avatar, it looks like you have a 67 Newport with a BB and likely OEM distributor?

I don't know what your experience is with engines but it sounds like you have done your homework.

If I'm correct then I hope my advice can help you here.

You should have a checklist of items to make sure you have met the minimum needs to make this engine have a chance at running right.

1) Tight timing chain - you make no mention of the timing chain condition. Without that, your distributor will never operate correctly...
2) Good verified compression - not just it was good 3 years ago when I last l checked a few cylinders approach...
3) Nice clean operational carb with a basic bench setup - float set correctly, choke pull off set correctly good air & fuel filter...
4) Stock distributor with good bushings, cam lobes & points setup if you have points. Dwell setting is more important than gap although both are important... If there is excessive play it will never setup correctly regardless how many hours or parts you throw at the engine.

***
Worn Timing chain will give you a dancing timing marker and the ripple affect in timing will be experienced throughout all rpm ranges.
Sometimes a decent running at idle or under load but never consistent.
***

***
Low or uneven compression will cause the fall on the face issue too but likely low power since the block is tired and can't give you the guts to go. The fall on the face thing can go away as it warms up as the compression will likely increase enough to mask the bigger issue of low compression.
Likely not your problem here but thought I would mention it since it is all a combination of all things here to make an engine run well or as expected.

***
Carburation issues should be sorted out is a huge factor in this equation as well as mention by Wildbunch02
Bad choke setup, if it comes off too early you will get the fall on your face scenario as noted
Dirt in the carb clogging up different circuits in the delivery when warming up
Internal leakage will cause the fall on your face drivability issues when between cold to warm or running in general.

***
Worn distributor will cause you all sorts of grief as well.
Dwell, point gap, misfires, falling on the face, lack of power, etc

I've personally not played with weights and springs like in the old school days for quite some time.
I have used different spring tension springs, different butterfly weights, & vacuum advances that allowed for setting the internal bellows so the vacuum advance came in at different rpm's.
It was all trial & error.
Once I found the sweet setup I would transfer it to different engines as needed.

Finally if all above checks out and you get it running with the stock setup that makes you happy, to save aggravation, if I were you, I would go with the new electronic systems that allow you to dial in your own setting via computer using the tach signal not any vacuum advance.
I'm not sure who offers these but I'm sure it is out there with all the EFI conversion systems.
there is bound to be a distributor that has these features.

I personally would stay as stock as possible, get the engine running as good as possible then go forward playing with timing as you have calculated so precisely after making many notes on how it got to run good then if things go horribly wrong go back to the stock setup that works.

Good luck in your quest for the perfect running engine

Keep us posted on how things go.

 

[Nicholas Gromak]

OK here is what I need to figure this all out. Carb 2 or 4bbl, engine size and year and any cam etc, and the distributor number.. Here it is in a nutshell. Your ported vacuum port on the cab limits the vacuum advance from going all in at idle and basically turns to manifold vacuum at WOT. The mechanical advance has a may total in degrees that the weight of the springs tells it when to start to advance and also when to be in full, lets just say 20 at the crank to keep the numbers simple. Say the Vac advance is 20 max at the crank also. Ok at idle it should be close to 0 or so at a idle then you hit say 1500 crank rpm the mechanical goes to say 10 and the vacuum starts to kick in. It all increases form engine rpn and vac " as the throttle opens. also as the Vacuum drops at crusing speed the manifold vacuum should also drop and retard the vac advance back a bit. That's the way it should work. Ever change out the distributor with a rebuilt one? Why? They are usually just set in the middle of say a ten year spread to work on a lot of different applications. Hope this helps.

Thanks for the reply halifaxhops!

Here's what I got: 383 big block 9.2/1 Compression. 2bbl Stromberg WWC3 carb. Stock iron heads. Stock points distributor (see picture for timing and part number). Stock cam, Stock cast (single dual plane???) intake, Stock cast exhaust manifolds, Single exhaust pipe 1.5" diameter. Also this spring, I had my mechanic replace spark plugs, plug wires, new set of contacts, new coil, new condenser, set the dwell and all that.

From the manual it looks like I was into the mechanical while taking my measurements, oops. Just throwing some numbers here lets say we have 2 engines both set at 12.5* initial like the manual. Both are lightly accelerating up a hill in 3rd. I've got traffic behind me so I've got to speed up. For simplicity's sake lets say the 2 engines start at the same rpm say 2150. According to the manual, 2150rpm would bring mechanical to about 12.5* ontop of initial = 25*. From driving around with the gauge hooked to manifold I know that light acceleration, the vacuum is 10". For ported it might be higher say 15" due to the high load. According to the manual at 10" the vacuum should be around 8* and at 15" it's maxed at 13.5*

The manifold vacuum engine starts at 12.5+12.5+8= 33* but he still has to speed up to get away from traffic. The accelerator is pressed enriching the air fuel mix, manifold vacuum drops to below 5" (from tests), and the timing is retarded from 33 to 12.5+12.5+0= 25* and away he goes preventing detonation as he does.

The ported vacuum engine also starts at 33* but speeding up does nothing except go higher. All the settings are maxed out. "The rich air fuel mix burns faster" or so people say, and coupled with the high degree of timing, ping ping ping all the way home.

My problem with the ported vacuum setup is not only does it ping on hills but it also pings after every gear shift when the engine is suddenly loaded. Also from stop signs.

 

[Nicholas Gromak]

Hi Nicholas.
From your avatar, it looks like you have a 67 Newport with a BB and likely OEM distributor?

I don't know what your experience is with engines but it sounds like you have done your homework.

If I'm correct then I hope my advice can help you here.

You should have a checklist of items to make sure you have met the minimum needs to make this engine have a chance at running right.

1) Tight timing chain - you make no mention of the timing chain condition. Without that, your distributor will never operate correctly...
2) Good verified compression - not just it was good 3 years ago when I last l checked a few cylinders approach...
3) Nice clean operational carb with a basic bench setup - float set correctly, choke pull off set correctly good air & fuel filter...
4) Stock distributor with good bushings, cam lobes & points setup if you have points. Dwell setting is more important than gap although both are important... If there is excessive play it will never setup correctly regardless how many hours or parts you throw at the engine.

***
Worn Timing chain will give you a dancing timing marker and the ripple affect in timing will be experienced throughout all rpm ranges.
Sometimes a decent running at idle or under load but never consistent.
***

***
Low or uneven compression will cause the fall on the face issue too but likely low power since the block is tired and can't give you the guts to go. The fall on the face thing can go away as it warms up as the compression will likely increase enough to mask the bigger issue of low compression.
Likely not your problem here but thought I would mention it since it is all a combination of all things here to make an engine run well or as expected.

***
Carburation issues should be sorted out is a huge factor in this equation as well as mention by Wildbunch02
Bad choke setup, if it comes off too early you will get the fall on your face scenario as noted
Dirt in the carb clogging up different circuits in the delivery when warming up
Internal leakage will cause the fall on your face drivability issues when between cold to warm or running in general.

***
Worn distributor will cause you all sorts of grief as well.
Dwell, point gap, misfires, falling on the face, lack of power, etc

I've personally not played with weights and springs like in the old school days for quite some time.
I have used different spring tension springs, different butterfly weights, & vacuum advances that allowed for setting the internal bellows so the vacuum advance came in at different rpm's.
It was all trial & error.
Once I found the sweet setup I would transfer it to different engines as needed.

Finally if all above checks out and you get it running with the stock setup that makes you happy, to save aggravation, if I were you, I would go with the new electronic systems that allow you to dial in your own setting via computer using the tach signal not any vacuum advance.
I'm not sure who offers these but I'm sure it is out there with all the EFI conversion systems.
there is bound to be a distributor that has these features.

I personally would stay as stock as possible, get the engine running as good as possible then go forward playing with timing as you have calculated so precisely after making many notes on how it got to run good then if things go horribly wrong go back to the stock setup that works.

Good luck in your quest for the perfect running engine

Keep us posted on how things go.

You are absolutely correct! On multiple accounts. Yes, my avatar is a picture of my '67 Newport with 383 BB and Original Distributor. Yes, I'm 100% sure that there's something wrong with the carb. It's not working as good as it could be. Yes, I had my compression checked when they changed the plugs this spring. All were above 100 psi dry. Yes, timing chain was never checked but the car only has 39 000 miles and low sludge when we popped the valve covers. Yes, distributor play and wear were checked this spring when we replaced spark plugs, plug wires, new set of contacts, new coil, new condenser, set the dwell, gap and all that. Yes, original gas tank may have some rust in it by now, I'll change the fuel filter although the fuel grade hose has a DOT stamp from 1990 so why wouldn't they have changed the filter at that time but I'll change it anyway.

Yes, I would like to keep it stock as well. There is a certain gratifying feeling when you take a lot of time to get something perfect, especially using original components. A sort of "better than new" feeling as is the same with a continuous project like owning a classic car. In my opinion, much more fun then throwing an electronic something-or-other in place and calling it a day.

Thanks! I'll take not of these for next year so I can hit the ground running.

 

[Mr onetwo]

Read this article over @ Slantsix.org DusterIdiot’s distributor rebuild/recurve tutorial lots of good info that is applicable to V8's also.Also check out the ignition section of FABO

 

[CBODY67]

(Fast-forwarded through all of the replies. My comments might duplicate some, btaim!)

Once the throttle plates are opened, you ARE getting manifold vacuum to the distributor, although it might be 1-2" Hg less than if it was tapped in an intake port (as the power brakes are). But you should notice that the max vac adv happens at less than 18" Hg, too.

When you switch from ported to manifold sources, you automatically get full vac advance at idle, which your current curve is NOT set up for. The additional advance (normal base timing plus full vac advance is the reason you're feeling the improved off-idle response AND also it should also make any tendency to clatter much greater in intensity and happen sooner.

Our '66 Newport 383 2bbl (regular-fuel rated motor) NEVER DID LIKE REGULAR, period. We got it with 7100 miles on it. We ran premium in it from then on! According to the factory specs, it should have run well on the 94-95 RESEARCH Octane fuels of the later '60s, but it didn't. Those fuels would approximate the 91 PUMP Octane fuels of modern times.

An arbitrary 1/8" movement at the perimeter of the distributor cap would be close to 10 degrees change in timing, I suspect. Rather than do the inches, GET A DIAL TIMING LIGHT AND KNOW where things are!

So, put it all BACK to factory specs and start over! In those earlier times, Chrysler's B/RB distributor timing was pretty decent as it was, from what I can see. Base centrifugal timing starts at about 1000rpm. Total timing (base + full centrifugal) usually is close to the desired 38 degrees BTDC total (2bbls especially, 4bbls sometimes closer to 31 degrees), than not. Any re-curve would just quicken the top advance numbers so they happen at a lower rpm, closer to 3000rpm than 4500rpm.

So, not a whole lot to gain there. I just bumped the '66 383 2bbl from 12.5 degrees BTDC base timing to 15 degrees BTDC, which was a small movement to make happen. Tweaked the idle speed down about 25 rpm. All was good.

ONE thing about cars with manual adjustments, YOU learn what they like and "make them happy". That means learning their little quirks and working around of with them rather than not! That way, you're both happier! Just as with the human body, mechanical mechanisms need their "care and feeding" to operate in the most optimum manner, too. Even if it might mean using better fuels in them!

In current times, it's not unusual to see OEM engines with 11.0:1 mechanical compression ratios that will operate on "junk" gas. You ought to see the modern electronics it takes to make that happen! Plus the slightly altered camshaft timing specs, too. Getting an older engine to do the same with just a few minor adjustments just will NOT happen--period.

As for the spark clatter, you claim that higher octane fuels didn't make any difference, which should NOT have been the case--period. Check the rear exhaust manifold bolts to see if they are tight, for good measure. What was the prior history of your vehicle? What OTHER performance-related items might be on it?

As for that "idea" to get phenomenally better cruise fuel economy and power with the vac advance working on manifold vacuum? Forget it!!!!! It will NOT work with your carb and such, UNLESS you go to a detonation limiter electronic ignition system. Think mid-'80s GM pickup with "electronic spark control". This was a free-standing system which used a detonation limited screwed into the pass side block drain plug hole. The "box" was mounted behind the glove box, with wiring that went to the HEI distributor (with a special module in it). That's ONE option.

My '77 Camaro 305 2bbl came from the factory using manifold vac to run the distributor vac advance. With the egr deactivated (for testing purposes), it has great off-idle response and would get 20 mpg on the road (2.56 axle, F70-14 tires). That was good, but I thought it should be higher, all things considered. But it worked well as it was.

After it got closer to 40K miles, I did some "testing" with adjusting the idle mixture and speed with the vac advance hooked up and not hooked up. Even tried putting it back to ported vacuum. It all made not real difference, other than I had to open the throttle plates at idle more with the ported source. THAT is why it takes a carb designed for the vac advance to be manifold if that's how the distributor is set-up! It's the relationship of the throttle plates to the idle and transition ports in the throttle bores! There is a certain relationship between the throttle blades and the transition (between idle fuel system and main fuel system) that greatly affects off-idle performance (as in what you have experienced some of, possibly).

I also tried re-setting some "ported" cars I have (Chrysler products) to manifold and it did NOT work. Had to close the base idle throttle amount too far closed to keep the base idle speeds down! That put the plates our of whack with the transition ports and drivability suffered. Sags, hesitations, etc, which the accel pump would not cover. I put it back AND gave the Chrysler engineers credit for knowing what they were doing in making things work as well as they did.

In general, Chrysler products were more optimized back then, unlike Fords and Chevies which needed some tinkering to make them run better, by observation. Trying to use the same techniques (GM) to make a Mopar run better, you might find the Mopars are already there.

I respectfully advise you to PUT IT BACK TO STOCK and fine tune it from there. Things can work much better, for you AND the car. Put higher-octane fuels in it (NOT octane boosters with cheaper fuels--not economically good for your pocket). Ensure the existing timing advance is where it needs to be (according to factory specs), verifying what your existing distributor has in it (where the dial timing light comes in handy, plus a portable tach). Make sure the point adjustment in the distributor is in spec (gap AND dwell), which can affect ignition timing, too! Get the dwell in spec FIRST, then set the base timing, IN THAT ORDER.

For good measure, with the #1 cylinder at TDC, the keyway in the crankshaft nose should be parallel with the #1 cylinder connecting rod, pointing at the piston pin of #1 piston's centerline. Then look at the timing mark on the crankshaft balancer to see what it indicates on the timing tab. This can then indicate if the balancer outer ring has slipped and could need replacing.

Now, there can also be an adjustment in the vac advance itself! Put an Allen wrench through the nipple and see if it will index with an inner "nut" that will turn. Turn it 1/4 turn CW to tighten the spring, delaying vac advance amounts (slows down the curve, so to speak). See if THAT helps your clattering problem. If there is no internal adjustment, then you can get one that has it. Might even be that the one in there is wrong?

As for your initial inquiry, you'll need a mechanical timing advance curve that is WAY slower than you've got now to run the vac advance from manifold vacuum. There's got to be enough mechanical advance to still allow good WOT power, too, which might cause part-throttle issues as you now claim to have. OR the detonation limiter set-up.

Ford ran some of their earlier-'60s six cylinders on manifold vac advance. Didn't work too well, as I understand it. They did things differently after those few years. Few people were buying 6s anyway, back then.

Right now, you're wanting to get rid of the 35mph-region ignition clatter, which should be related to fuels/ignition timing. Yet you're wanting to put more ignition timing in the motor to stop that? And on 87 octane fuel???? Not going to happen, in the manner you propose.

The vacuum advance is there for part-throttle cruise efficiency. Without it, you're running on just mechanical advance. Perhaps that's the issue? A prior owner put a "quick" curve kit in it and didn't bother to see if it was too much for the engine? Another reason to verify where things are!

Respectfully,
CBODY67

 

[Nicholas Gromak]

(Fast-forwarded through all of the replies. My comments might duplicate some, btaim!)

Once the throttle plates are opened, you ARE getting manifold vacuum to the distributor, although it might be 1-2" Hg less than if it was tapped in an intake port (as the power brakes are). But you should notice that the max vac adv happens at less than 18" Hg, too.

When you switch from ported to manifold sources, you automatically get full vac advance at idle, which your current curve is NOT set up for. The additional advance (normal base timing plus full vac advance is the reason you're feeling the improved off-idle response AND also it should also make any tendency to clatter much greater in intensity and happen sooner.

Our '66 Newport 383 2bbl (regular-fuel rated motor) NEVER DID LIKE REGULAR, period. We got it with 7100 miles on it. We ran premium in it from then on! According to the factory specs, it should have run well on the 94-95 RESEARCH Octane fuels of the later '60s, but it didn't. Those fuels would approximate the 91 PUMP Octane fuels of modern times.

An arbitrary 1/8" movement at the perimeter of the distributor cap would be close to 10 degrees change in timing, I suspect. Rather than do the inches, GET A DIAL TIMING LIGHT AND KNOW where things are!

So, put it all BACK to factory specs and start over! In those earlier times, Chrysler's B/RB distributor timing was pretty decent as it was, from what I can see. Base centrifugal timing starts at about 1000rpm. Total timing (base + full centrifugal) usually is close to the desired 38 degrees BTDC total (2bbls especially, 4bbls sometimes closer to 31 degrees), than not. Any re-curve would just quicken the top advance numbers so they happen at a lower rpm, closer to 3000rpm than 4500rpm.

So, not a whole lot to gain there. I just bumped the '66 383 2bbl from 12.5 degrees BTDC base timing to 15 degrees BTDC, which was a small movement to make happen. Tweaked the idle speed down about 25 rpm. All was good.

ONE thing about cars with manual adjustments, YOU learn what they like and "make them happy". That means learning their little quirks and working around of with them rather than not! That way, you're both happier! Just as with the human body, mechanical mechanisms need their "care and feeding" to operate in the most optimum manner, too. Even if it might mean using better fuels in them!

In current times, it's not unusual to see OEM engines with 11.0:1 mechanical compression ratios that will operate on "junk" gas. You ought to see the modern electronics it takes to make that happen! Plus the slightly altered camshaft timing specs, too. Getting an older engine to do the same with just a few minor adjustments just will NOT happen--period.

As for the spark clatter, you claim that higher octane fuels didn't make any difference, which should NOT have been the case--period. Check the rear exhaust manifold bolts to see if they are tight, for good measure. What was the prior history of your vehicle? What OTHER performance-related items might be on it?

As for that "idea" to get phenomenally better cruise fuel economy and power with the vac advance working on manifold vacuum? Forget it!!!!! It will NOT work with your carb and such, UNLESS you go to a detonation limiter electronic ignition system. Think mid-'80s GM pickup with "electronic spark control". This was a free-standing system which used a detonation limited screwed into the pass side block drain plug hole. The "box" was mounted behind the glove box, with wiring that went to the HEI distributor (with a special module in it). That's ONE option.

My '77 Camaro 305 2bbl came from the factory using manifold vac to run the distributor vac advance. With the egr deactivated (for testing purposes), it has great off-idle response and would get 20 mpg on the road (2.56 axle, F70-14 tires). That was good, but I thought it should be higher, all things considered. But it worked well as it was.

After it got closer to 40K miles, I did some "testing" with adjusting the idle mixture and speed with the vac advance hooked up and not hooked up. Even tried putting it back to ported vacuum. It all made not real difference, other than I had to open the throttle plates at idle more with the ported source. THAT is why it takes a carb designed for the vac advance to be manifold if that's how the distributor is set-up! It's the relationship of the throttle plates to the idle and transition ports in the throttle bores! There is a certain relationship between the throttle blades and the transition (between idle fuel system and main fuel system) that greatly affects off-idle performance (as in what you have experienced some of, possibly).

I also tried re-setting some "ported" cars I have (Chrysler products) to manifold and it did NOT work. Had to close the base idle throttle amount too far closed to keep the base idle speeds down! That put the plates our of whack with the transition ports and drivability suffered. Sags, hesitations, etc, which the accel pump would not cover. I put it back AND gave the Chrysler engineers credit for knowing what they were doing in making things work as well as they did.

In general, Chrysler products were more optimized back then, unlike Fords and Chevies which needed some tinkering to make them run better, by observation. Trying to use the same techniques (GM) to make a Mopar run better, you might find the Mopars are already there.

I respectfully advise you to PUT IT BACK TO STOCK and fine tune it from there. Things can work much better, for you AND the car. Put higher-octane fuels in it (NOT octane boosters with cheaper fuels--not economically good for your pocket). Ensure the existing timing advance is where it needs to be (according to factory specs), verifying what your existing distributor has in it (where the dial timing light comes in handy, plus a portable tach). Make sure the point adjustment in the distributor is in spec (gap AND dwell), which can affect ignition timing, too! Get the dwell in spec FIRST, then set the base timing, IN THAT ORDER.

For good measure, with the #1 cylinder at TDC, the keyway in the crankshaft nose should be parallel with the #1 cylinder connecting rod, pointing at the piston pin of #1 piston's centerline. Then look at the timing mark on the crankshaft balancer to see what it indicates on the timing tab. This can then indicate if the balancer outer ring has slipped and could need replacing.

Now, there can also be an adjustment in the vac advance itself! Put an Allen wrench through the nipple and see if it will index with an inner "nut" that will turn. Turn it 1/4 turn CW to tighten the spring, delaying vac advance amounts (slows down the curve, so to speak). See if THAT helps your clattering problem. If there is no internal adjustment, then you can get one that has it. Might even be that the one in there is wrong?

As for your initial inquiry, you'll need a mechanical timing advance curve that is WAY slower than you've got now to run the vac advance from manifold vacuum. There's got to be enough mechanical advance to still allow good WOT power, too, which might cause part-throttle issues as you now claim to have. OR the detonation limiter set-up.

Ford ran some of their earlier-'60s six cylinders on manifold vac advance. Didn't work too well, as I understand it. They did things differently after those few years. Few people were buying 6s anyway, back then.

Right now, you're wanting to get rid of the 35mph-region ignition clatter, which should be related to fuels/ignition timing. Yet you're wanting to put more ignition timing in the motor to stop that? And on 87 octane fuel???? Not going to happen, in the manner you propose.

The vacuum advance is there for part-throttle cruise efficiency. Without it, you're running on just mechanical advance. Perhaps that's the issue? A prior owner put a "quick" curve kit in it and didn't bother to see if it was too much for the engine? Another reason to verify where things are!

Respectfully,
CBODY67

Thanks CBODY67! Will check those manifold bolts and I've never used octane booster. Only full tanks of 94 octane all summer and only started pinging after driving for 30 minutes or more (guess that's the warm up time). I read this on Thursday: http://moparconnectionmagazine.com/gallery-distributor-recurving-for-improved-fuel-economy/. Also I know you said base timing comes in at 1000 rpm but according to my service manual it says for the stock distributor, timing starts coming in at 450 rpm.

The guys in the website link I added were experiencing the same part throttle detonation like me. Sadly, they fail to state the octane rating of the fuel. They also fail to say if they're using ported or manifold vacuum to run the distributor. Anyways, they say "The advance was too abundant, and there was a persistent light-throttle rattle (detonation) at about 17” of vacuum." Their overall solution was to get rid of the "engine's 1970's emissions laden" components, delay the mechanical advance until 1750 rpm (up from 700 rpm) and reduce the vacuum advance rate using an allen wrench.

Before mods, they had 0* initial (for emissions) and 33* mechanical coming in at 700 up to 4000. After they had 15* initial and 18* mechanical coming in at 1750 up to 3000.

They state an average increase of 26% in fuel economy (16mpg compared to 21mpg) and an increase in horsepower of 15-20% all over the rpm range.

For my car I should stiffen up the springs to make it come in later, reduce travel in the slots to no go too high too soon and do something with the vacuum can to get my economy. I'm averaging between 9 and 10 mpg with my ported setup.

I'll let you know if I find a screw in my vac can.

 

[CBODY67]

In looking at theFSM page above, remember that the distributor runs at 1/2 crankshaft speed, hence the notation of "distributor degrees" in the spec. These are the numbers that you'd see if you put the removed distributor into a Sunn Distributor Machine to check its function. Our old Chrysler dealership had one and used it for EACH tune-up (they physically removed the distributor and put it onto that machine. Much better to set the points with and such, rather than leaning over a customer's fender!).

Notice too, the mechanical advance numbers. At total advance, there's 21-25 degrees "in the distributor". Add that to the initial 12.5 degrees BTDC and you get an "on the engine" advance of 33.5-37.5 degrees total advance "on the engine". That pretty much where it needs to be for best WOT power.

As for the vacuum advance, notice that it's "all in" at 13.5" Hg. Guess what? If the engine is working decently well, it's got more than that for intake manifold vacuum at cruise. So, no need to even entertain the idea of swapping the dist vac to "manifold" for better fuel economy.

The Stromberg WWC carb is a pretty good one. That's what was on my '66 when we bought it. There is ONE problem with them and some other 2bbls of that era, but NOT related to carb fuel metering. It ran and operated well, by observation. When I replaced with a '70 spec Holley 2210, performance did improve a bit, as did fuel economy on the road. The Holley eventually had the same problem, but Chrysler/Holley had a "Bridge Kit" to address that particular issue (not related to fuel metering).

An emerging issue with many older car fuel/ignition parts is that the OEM-specific-spec parts vanished years ago. IF the ones with rubber diaphrams and such are around in NOS, putting them with the modern fuels might not work well too long, a variable situation. The ones which now claim to "fit your application", have a more generic approach so they will fit many more models. You're still getting "generally" what you need, but as for specs, not specifically what the OEM item was. The good thing is that many of these (recent manufactured) things will work just as well with little performance difference. So, just wanted to mention this to temper your "want to keep it stock" orientation a bit.

As mentioned above, you have to tweak things a bit sometimes to get to that "sweet spot" of what the engine actually wants. This doesn't involve massive changes, just a little bit more here, there, or a bit less. For example, my devised method of idle mixture/speed adjustment does not really duplicate any one method, but can incorporate several things PLUS "smell".

Starting with the engine fully warmed and at base idle. Fully apply the parking brake with the foot brake fully applied, also. Trans in "P". Then put it in "D", slowly release the foot brake and see if the car moves forward. If it does, do it again.

With the trans in "P", with a dwell tach already hooked up, do the basic carb idle adjustments, to spec. As for the mixture, lean the mixture 1/4 turn at a time, until a 20 rpm drop is noticed, then return to the adjustment with optimized rpm. Do this with each idle mixture screw. Retweak the idle speed as necessary.

Put the trans in "D" and check for idle speed drop and smoothness. With the vehicle reliably stopped with the parking brake, check the rear exhaust pipe flow for pulses (in gear). If you can feel anything but a smooth flow, add about 20rpm to the idle speed screw adjustment. Recheck. When the flow just smooths out, that's the target idle speed. Then put the trans back in "P" and recheck the idle speed mixture, for each screw. It could well be that you're already where you need to be, but this verifies it. THEN return to the exhaust pipe and put the palm of your hand into the flow. After about 2 seconds, remove it and smell for hydrocarbon smells (think rich mixture). You can only do this once, maybe twice, before any smells need to be washed off! If everything's right, you'll have minimal hydrocarbon smell on your hand. This should result in the cleanest and smoothest you can get the engine to run at idle. Of course, you do these things at your own risk, but I used it many times to get my desired base line numbers for future times.

I will admit that when I prototyped this procedure in the later '60s, we had "leaded" and "low-lead" fuels, rather than universally unleaded. Ethanol was just an octane enhancer and a very small part of the fuel blend. What I did notice was that the exhaust temp on the '66 was less than it was on the '72 400-2bbl in our '72 Newport Royale! But "the smell test" still worked. I haven't needed to see how it all works with modern fuels, yet.

But even when the cars were new, sometimes a bit of this and less of that worked better for the individual vehicle. It's probably just more necessary to do the "try it and see" tweaks now than it was back then.

CBODY67

Otherwise, you can find an air/fuel ratio meter and go that way.