Vacuum Advance w/ HEI

[lintcollector]

So the truck has been idling high, and then a neighbor came over and listened a bit, then reached in and pulled off the vacuum tube off the carb coming from the vacuum advance. It idled nice & low, sounded great. he put the hose back on and it started to increase the idle.

I am trying to find a new vacuum advance to work with the HEI on the truck. In reading through prior posts, I see some folks saying I don't need one, just to plug the hole on the carb. I guess, do I need a vacuum advance?

This is for a 72 C10, 402cbb. The truck is for my daughter who just drives it local and may get up to 40mph, but I'd like to hear her squeal the tires on occasion and not have it stall.

Do I need a vacuum advance, or should I jsut get a fancy adjustable one and then start to play with it until it sounds good?

Thanks in advance for helping the idiot again.

[geezer#99]

Keep the vac pot just swap the hose from your manifold vac port to a ported vac port.
Ported port has no vacuum until the throttle opens.
Likely you have a ported port on the drivers side front of the carb.

[HO455]

The vacuum advance plays a huge role in gas mileage, so yes you do want it. It sounds like your advance vacuum source is coming directly from the intake. Try to find a ported source on the carburetor base to supply the vacuum. A ported source will not provide vacuum at idle but will when the throttle is opened slightly.
Can you post a picture of your current source of vacuum and the base of the carburetor? It will make this easier to describe.

[lintcollector]

Thank you both, I will look at the carb and see what I can find. If I can't find anywhere to port the hose, I'll post some pics.

[68Gold/white]

Post 2 and post 3
X2

[Sheepdip]

This is a cut and paste from another forum with origination from a GM Engineer.

It's worth the read, it puts it in laymen's terms.


TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car"

[68 P.O.S.]

Originally Posted by Sheepdip

This is a cut and paste from another forum with origination from a GM Engineer.

It's worth the read, it puts it in laymen's terms.

This!!! ^^

[68 P.O.S.]

Another thing, which carb are you running? When you run the vac advance off of manifold vac (as you should), you have to make sure the transition slots in the carb are adjusted to form a square. I'll post a pic later to show, unless someone beats me to it. With the transition slots squared, total timing set at 36 degrees, and vac advance connected to manifold vac, your idle will fall around 750 rpm. It's pretty cool when everything is setup correctly together how it all kinda falls into place. Almost like GM designed it that way!

[lintcollector]

Originally Posted by 68 P.O.S.

Another thing, which carb are you running? When you run the vac advance off of manifold vac (as you should), you have to make sure the transition slots in the carb are adjusted to form a square. I'll post a pic later to show, unless someone beats me to it. With the transition slots squared, total timing set at 36 degrees, and vac advance connected to manifold vac, your idle will fall around 750 rpm. It's pretty cool when everything is setup correctly together how it all kinda falls into place. Almost like GM designed it that way!

Thank you both Sheepdip and 68POS. I now have a better understanding of the vacuum system. I knew there was a purpose, I just had no idea how all of that former "witchcraft" worked together.

I have one or two terms I didn't follow, such as "SBC" and "Hg", but I completely understood and fall into the "retarded slightly" terminology.

68POS, if you can post a pic of the "transition slots in the carb...to form a square" I would be most appreciative.

I'm running a Quadrajet 4 barrel currently, but I did just buy a Holley carb and it's sitting in the garage. If making squares in my carb works, I'll happily return the Holley.

Thank you so much guys, I really do appreciate it.

[RichardJ]

>>Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements <<

That is blatantly FALSE.

Many of you are familiar with the Edelbrock carbs and sadly Edelbrock spews the same nonsense. Actually that does make sense, because it is my understanding that Edelbrock doesn't make the carbs they sell to you.

Edelbrock carbs are made by Weber (Magneti Marelli) who bought the patents from Carter so they are the same basic carb, starting in 1957. Pre emissions. Magneti may have been bought by someone else.

The point is the pre-emission Carters(Edelbrocks) DID NOT HAVE A MANIFOLD VACUUM PORT for the distributor. The vacuum source for the distributor was ported vacuum.

#1 This first AFB is from 1960. The ported vacuum port is threaded for a steel line which was common in those years on all carbs.

#2 The second is a 1964 AFB. it also has a threaded vacuum port and both are small throat for the small air cleaner used in those days.

#3 Next is a 1966 AFB with large throat with newer style ported vacuum port and NO manifold port for distributor. Large vacuum port is for PCV as with more modern Edelbrock carbs.

#4 Old Rochester 4 bbl with old style threaded, ported vacuum port for distributor. No manifold vacuum port.

#5 Rochester B used on these 6 cyl Chevy Trucks from the '40s up until 1967. Again it has a ported vacuum port and NO MANIFOLD VACUUM PORT.

In '68 the inline 6 started using the Monojet on Emissions equipped engines. Monojets typically have so many vacuum ports, they look more porcupine than carburetor.

I have a picture of an early Quadrajet with a ported vacuum port for the distributor and NO MANIFOLD VACUUM PORT.

Please stop spreading this ignorance about ported vacuum ports introduced for emissions carburetors.

[HO455]

Originally Posted by RichardJ

>>Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements <<

That is blatantly FALSE.

Many of you are familiar with the Edelbrock carbs and sadly Edelbrock spews the same nonsense. Actually that does make sense, because it is my understanding that Edelbrock doesn't make the carbs they sell to you.

Edelbrock carbs are made by Weber (Magneti Marelli) who bought the patents from Carter so they are the same basic carb, starting in 1957. Pre emissions. Magneti may have been bought by someone else.

The point is the pre-emission Carters(Edelbrocks) DID NOT HAVE A MANIFOLD VACUUM PORT for the distributor. The vacuum source for the distributor was ported vacuum.

#1 This first AFB is from 1960. The ported vacuum port is threaded for a steel line which was common in those years on all carbs.

#2 The second is a 1964 AFB. it also has a threaded vacuum port and both are small throat for the small air cleaner used in those days.

#3 Next is a 1966 AFB with large throat with newer style ported vacuum port and NO manifold port for distributor. Large vacuum port is for PCV as with more modern Edelbrock carbs.

#4 Old Rochester 4 bbl with old style threaded, ported vacuum port for distributor. No manifold vacuum port.

#5 Rochester B used on these 6 cyl Chevy Trucks from the '40s up until 1967. Again it has a ported vacuum port and NO MANIFOLD VACUUM PORT.

In '68 the inline 6 started using the Monojet on Emissions equipped engines. Monojets typically have so many vacuum ports, they look more porcupine than carburetor.

I have a picture of an early Quadrajet with a ported vacuum port for the distributor and NO MANIFOLD VACUUM PORT.

Please stop spreading this ignorance about ported vacuum ports introduced for emissions carburetors.

Thank you!!! Finally someone else who has had enough of this hogwash. This person may have been an engineer but they were not an internal combustion engineer. Their basic premise that you must be lean at idle shows they just don't understand what is going on inside the combustion chamber.

Virtually every automotive manufacturer used ported vacuum advance sources even on vehicles that never saw the highway. They didn't do that out of concerns over the environment and they certainly weren't going to spend any money they didn't have to.

So obviously there must be valid reasons for ported vacuum advance. Let's look at a few from the standpoint of the automotive manufacturers.

To make the newly manufactured automobile run perfectly without having to tune each car individually to make it acceptable to their customers.

Having repeatable idle speeds with changes in atmospheric air pressure.

To avoid hunting idle caused by the mechanical and vacuum advance systems chasing each other.

Preventing timing advance when driving down long inclines in gear to achieve the advantages of compression braking. ( Not as common today but prior to disc brakes and automatic transmissions it was a must do to prevent loss the of brakes due to overheating.) The engine RPM being controlled by the drive train can cause manifold vacuum to be significantly higher than normal driving conditions resulting in the engine backfiring.

Less wear on the vacuum canisters and the associated mechanical parts in the distributor.

So yes by all means use a ported source for your vacuum. I personally know that the engineers who designed these carburetors and engines are wayyyy smarter source on their worst day than I will ever be so I will rely on their knowledge and use the ported vacuum source they provided me.
Thank you! I will now put my soap box away.

[tim_mc]

Perfect "timing" of this response as I was going to play around with my timing tomorrow. I have a fairly stock 350 SBC with an Edelbrock 1406 and Standard Motor Products VC-204 vacuum can, which I bought new since I didn't know what the PO had on there (and was unplugged anyway). I *think* an SMP VC-204 is the same as an Echlin VC1838, but I can't find the cross-reference. This summary on vacuum advance might be of interest to someone. HEI is discussed further down with specific can part numbers and specs by year/model.

[tim_mc]

Found the cross-reference. Standard Motor Products has a slick x-ref on their eCatalog site at: https://ecatalog.smpcorp.com/STD/#/v...parts/num/DG-3

The SMP VC-204 is the same as an Echilin VC1838/AR12, which was recommended in the article as a user-friendly can for a good performance street engine (starts @ 7-9Hg, 7 max dist. degrees @ 10-12Hg).

My 2c: I've read mixed reviews on adjustable cans. If you don't know what the current can came off of, you might want to buy a new one that's for a similar big block w/HEI application. RockAuto shows applications and often shows the specs:
STANDARD MOTOR PRODUCTS VC246 {#19110656, 1973519} Advance Starts @ 3.0-5.0" Hg, max 9 dist. degrees @ 6-8Hg.

The SMP VC246 is a match for the Echlin VC1825/AR1, which is listed in the article for '75 454 Caprice/Chevelle/Monte/Suburban & '76 454 Caprice/Impala. Per RockAuto the VC246 is listed for nearly every Chevy/GMC truck between '75-'79 (assume big block). That might be a good starting point, but this is just my opinion and your mileage may vary.

[sloGMC]

One line from that letter, "if you choose to run manifold vac advance.... you must select vac. advance that pulls in all advance at vacuum lower than 2" below idle" -- that doesn't make any sense. The only time vacuum is higher than at idle is during deceleration. If anything, ported vacuum would "not allow" the canister to ever achieve full advance because it is, by design, not exposed to manifold vacuum until the throttle blades are cracked open, which reduces vacuum. His logic would dictate doing the opposite.

As for above mentioned arguments, to avoid mechanical and vacuum advance "hunting"-- if your engine varies at idle enough to trigger a significant change in mechanical advance that then creates a significant change in vacuum, I would guess you have another problem.
For the record I also find the argument that any OEM from the 60's still knows best today to be weak.

I find this conversation somewhat ridiculous as we are splitting hairs with regard to when vacuum advance is applied (say, 700rpm vs 900rpm), so the power and fuel economy benefits of manifold vacuum advance vs ported are extremely limited. I personally use full manifold vacuum for the above mentioned benefits and have zero issues. I did this based on conversation with, yes, a combustion engineer I worked with at my last employer.

To the original poster: there are many things that go into tuning a vehicle. Your high idle speed could be a vacuum leak, high idle speed screw, or high cold (fast) idle screw setting, among others. Assuming you have no vacuum leaks, with your vacuum canister disconnected you should: set your initial timing (mechanical advance) to 8-12 degrees or whatever is listed on your emissions sticker, adjust your idle speed screw to your preferred idle rpm or whatever is listed on your emissions sticker, adjust your idle mixture screws for max rpm or vacuum. Then, reconnect your vacuum advance ported vacuum and do nothing, or alternatively, connect it to full manifold vacuum and once again, lower your idle speed, as engine speed will increase with more advance. There is no reason you should need to get an adjustable advance canister.
Finally, SBC = small block chevy. Hg = mercury. Vacuum advance is measured in inches mercury (inHg) for automotive purposes.

Cheers.

[toolboxchev]

Originally Posted by sloGMC

One line from that letter, "if you choose to run manifold vac advance.... you must select vac. advance that pulls in all advance at vacuum lower than 2" below idle" -- that doesn't make any sense. The only time vacuum is higher than at idle is during deceleration. If anything, ported vacuum would "not allow" the canister to ever achieve full advance because it is, by design, not exposed to manifold vacuum until the throttle blades are cracked open, which reduces vacuum. His logic would dictate doing the opposite.

As for above mentioned arguments, to avoid mechanical and vacuum advance "hunting"-- if your engine varies at idle enough to trigger a significant change in mechanical advance that then creates a significant change in vacuum, I would guess you have another problem.
For the record I also find the argument that any OEM from the 60's still knows best today to be weak.

I find this conversation somewhat ridiculous as we are splitting hairs with regard to when vacuum advance is applied (say, 700rpm vs 900rpm), so the power and fuel economy benefits of manifold vacuum advance vs ported are extremely limited. I personally use full manifold vacuum for the above mentioned benefits and have zero issues. I did this based on conversation with, yes, a combustion engineer I worked with at my last employer.

To the original poster: there are many things that go into tuning a vehicle. Your high idle speed could be a vacuum leak, high idle speed screw, or high cold (fast) idle screw setting, among others. Assuming you have no vacuum leaks, with your vacuum canister disconnected you should: set your initial timing (mechanical advance) to 8-12 degrees or whatever is listed on your emissions sticker, adjust your idle speed screw to your preferred idle rpm or whatever is listed on your emissions sticker, adjust your idle mixture screws for max rpm or vacuum. Then, reconnect your vacuum advance ported vacuum and do nothing, or alternatively, connect it to full manifold vacuum and once again, lower your idle speed, as engine speed will increase with more advance. There is no reason you should need to get an adjustable advance canister.
Finally, SBC = small block chevy. Hg = mercury. Vacuum advance is measured in inches mercury (inHg) for automotive purposes.

Cheers.

X2 on this, it gets complicated. Most would agree with the following:

Every engine is different, wear, Orignal parts, Aftermarket parts, HEI and so on.

Tuning each different engine is different. Street, Daily, Strip, Towing, and so on.

The full port, and partial port is important as far as what YOU want.

I run with the full port, the weights and springs take over once I am in the throttle.

Here is what I have found: Running today's pump gas (87 Octane) generally likes more timing with large chamber heads (76 cc) I get 1 to 4 mpgs more this way. I cannot for the life of me make it knock with too much timing. 15 intial, 20-22 vac advance, my total is most likely much more than 32-34.

Now if I had high compression heads with a large cam I would really be looking to have heavier weights and springs in the distributor, and much less vac advance and total. At this point, performance is tuned and mileage goes out the window.

I run a (fancy adjustable unit) by either Pertronix or Accel. About 30 bucks average. I have seen them on Ebay for 10 bucks a piece. That's not that fancy.

You will get lots of arguments here on this subject, and it is based on what the individuals' engine wants, needs, likes. And of course, the tuner telling the engine what he wants it to do.

[lintcollector]

Originally Posted by sloGMC

To the original poster: there are many things that go into tuning a vehicle. Your high idle speed could be a vacuum leak, high idle speed screw, or high cold (fast) idle screw setting, among others. Assuming you have no vacuum leaks, with your vacuum canister disconnected you should: set your initial timing (mechanical advance) to 8-12 degrees or whatever is listed on your emissions sticker, adjust your idle speed screw to your preferred idle rpm or whatever is listed on your emissions sticker, adjust your idle mixture screws for max rpm or vacuum. Then, reconnect your vacuum advance ported vacuum and do nothing, or alternatively, connect it to full manifold vacuum and once again, lower your idle speed, as engine speed will increase with more advance. There is no reason you should need to get an adjustable advance canister.
Finally, SBC = small block chevy. Hg = mercury. Vacuum advance is measured in inches mercury (inHg) for automotive purposes.

Cheers.

Originally Posted by tooboxchev

The full port, and partial port is important as far as what YOU want.
You will get lots of arguments here on this subject, and it is based on what the individuals' engine wants, needs, likes. And of course, the tuner telling the engine what he wants it to do.

Originally Posted by HO455

Thank you!!! Finally someone else who has had enough of this hogwash. This person may have been an engineer but they were not an internal combustion engineer. Their basic premise that you must be lean at idle shows they just don't understand what is going on inside the combustion chamber.

I assumed there were some differences of opinion on the subject as I couldn't find anything definitive in past discussions. I think the various replies have proved that theory correct.

I am looking to have an around town, no towing, kind of truck. I think I'll just swap out the Quadrajet to the Holly and then play with the direct port for a while, and then try the ported method.

I also think I'll buy the fancier $30 Accel vacuum advance so I can play with it and be able to try different options.

Sometimes, being an accountant who likes to tinker doesn't quite get the truck to where it needs to be like someone who turns wrenches and understands carbs. Just wish I was a smart enough accountant to be able to figure out all the minutia of my 72!

Thanks all for your help & input

[68 P.O.S.]

Lintcollector, here's a link to a great thread from Gregski that shows the transition slots. He begins to talk about it on post 37 and the following posts. The whole thread is worth a read.

http://67-72chevytrucks.com/vboard/s...=713728&page=2


And some more fuel for the fire; here's 2 links for articles written by John Hinckley:

http://www.camaros.org/pdf/timing101.pdf

http://www.stl-vettes.com/65Vette/co...ng_Advance.pdf


Take all the above comments under advisement as it's good to know both sides of the coin. However, I tend to follow the advice of the actual engineers who were part of the development vs any armchair engineers.

[lintcollector]

Originally Posted by 68 P.O.S.

Lintcollector, here's a link to a great thread from Gregski that shows the transition slots. He begins to talk about it on post 37 and the following posts. The whole thread is worth a read.

http://67-72chevytrucks.com/vboard/s...=713728&page=2


And some more fuel for the fire; here's 2 links for articles written by John Hinckley:

http://www.camaros.org/pdf/timing101.pdf

http://www.stl-vettes.com/65Vette/co...ng_Advance.pdf


Take all the above comments under advisement as it's good to know both sides of the coin. However, I tend to follow the advice of the actual engineers who were part of the development vs any armchair engineers.

Thanks for the follow up 68POS, I appreciate it and will be getting to the articles & pics after work today.

And like they say, get 20 mechanics together and you'll get 25 opinions out!

[rsgt]

I wish my Quadrajet had a ported vacuum port, but it doesn't so I limited my vacuum advance to not give so much advance.

[68 P.O.S.]

Perfect, that's what your supposed to do

[RichardJ]

Post #19
>>I run with the full port, the weights and springs take over once I am in the throttle.
Here is what I have found: Running today's pump gas (87 Octane) generally likes more timing with large chamber heads (76 cc) I get 1 to 4 mpgs more this way. I cannot for the life of me make it knock with too much timing. <<

Manifold vacuum is HIGH at idle. As soon as you start opening the throttle, manifold vacuum DROPS. Timing is Retarding

Off-idle or just above idle, the mechanical advance is Advancing the Timing and the reduced vacuum is Retarding the timing. Both at the same time. Net gain in timing = 0 or less.

Manifold vacuum is dependent on engine load. The off-idle vacuum drop is greater with the engine in gear pulling from a stop than what you see leaning over the radiator.

Tape a vacuum gauge to your windshield with a long hose connected to manifold vacuum or tee'd into distributor vacuum hose. Drive around. Everytime you see the vacuum drop, the timing is retarding.

Using manifold vacuum is the real "Band-Aid"
Manifold vacuum will usually reduce or eliminate pinging. That's why it is so popular.