Q&A / Tech

Ask Away with Jeff Smith: Understanding Vacuum Advance and Why You Need It

I’ve always thought that vacuum advance was something just for stock engines and something you didn’t use with a performance engine. In fact, I’ve heard guys say they’ve locked out their advance and just run a fixed timing amount. Is this a better way to go with a street engine? I have a small-block 350 Chevy in a ’67 Chevy pickup with a TH350 trans and 3.73:1 gears. It’s a nice little cruiser and not really a hot performance engine. Right now the engine has an Edelbrock Performer intake, a 600 cfm Holley four barrel carb, and an HEI distributor. What kind of timing should I be running and should I disconnect the vacuum advance?


Jeff Smith: You’ve addressed several issues, but they are all related to ignition timing. First, on a street-driven engine there is no good reason to lock out your ignition timing. If we had to use a generic maximum advance that seems to work for most pump gas engines, somewhere around 34 to 36 degrees before top dead center (BTDC) is a good number. But with this much initial timing in the engine it will be difficult for the starter motor to crank the engine. You can get around this with an ignition cut-out so that the engine cranks and then you hit the ignition circuit, but this is unnecessarily complex for a street car.

The better plan is to start with a decent initial timing of roughly 10 to 14 degrees. This can be checked with the engine running at idle with a timing light. Make sure the vacuum advance connection is removed, and now rev the engine up to around 2,500 to 2,800 rpm. Ideally, the timing now should be somewhere around 34 to 36 degrees to total advance. With 34 degrees of total mechanical advance and 14 degrees initial timing, you have 20 degrees of mechanical advance—14 + 20 = 34.

Now connect the vacuum advance from the carburetor to the distributor and read the timing on the harmonic balancer while revving the engine to 2,800 rpm. This number will now be greater than 34 degrees with the addition of the vacuum advance. Let’s say it now reads 49 degrees, which would mean the vacuum advance is worth another 15 degrees. These are typical numbers.

First, let’s look at the mechanical advance portion of the timing—the 34 degrees total. This is determined by the weights and springs spinning around inside the distributor. This establishes the amount of timing the engine will see at wide open throttle (WOT). As you are probably aware, at part throttle the engine will create a certain amount of vacuum in the intake manifold. This is because the throttle is mostly closed and the engine is pulling against this restriction. Because the throttle is mostly closed, very little air is moving into the cylinders. So at light cruise, such as running down the highway, the engine is making much less power than it would at this same rpm at WOT.

With less air and fuel in each cylinder, the air-fuel mixture is not as densely packed compared to WOT. This less-dense mixture requires more ignition timing to complete the combustion because it takes longer to complete the combustion process. So we need a way to increase the amount of timing based on the load on the engine. This is how vacuum advance works. At part throttle, high manifold vacuum moves the diaphragm in the vacuum advance canister on the distributor to add more timing. But at WOT, the vacuum drops to near zero and vacuum advance is removed and the total timing then is established by the initial plus the mechanical advance.

So there are significant advantages to retaining the vacuum advance on your distributor. In the case of the HEI, you can actually purchase an adjustable vacuum advance canister that will allow you to custom-tune the amount of timing advance. For example, Pertronix offers an adjustable vacuum advance canister that bolts in place of the stock canister. Using a 7/32-inch Allen wrench placed in the vacuum nipple, you can change the amount of advance. Generally, the canister comes adjusted with roughly half of the total advance possible. The canister easily installs with two screws and then you can drive your truck with the unit adjusted as delivered. Each clockwise turn of the Allen wrench will add about 1.5 degrees of additional advance above 5 to 7 inches of manifold vacuum. The maximum is about 14 degrees of vacuum advance.

If too much advance is added, the engine will either start to knock or ping or perhaps it may surge slightly at very light throttle opening with high vacuum. If so, back the adjustment (counterclockwise) one to three turns and you will be very close to the ideal timing at part throttle. Once tuned properly, you may notice the engine does not require as much throttle to cruise at the same speed. This tuning should produce slightly better fuel mileage—assuming you can keep your foot out of the throttle now that it runs better!

So based on this, you can see that having a curve in the distributor along with vacuum advance is a good thing as it reduces low-speed timing where the engine might detonate with too much timing but it can also benefit from more timing at part throttle than what is required at wide open throttle.

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  1. Great article! Many people do not understand this and you explain it well!

  2. Fantastic! You just nailed it. Great article.

  3. i have a question I started up my 1970 ss chevelle it has a 350 create motor ,I have a loud whiney sound coming from my distributor ???? Bering maybe or Whatever ,Have you ever had this problem.BTW its a Mallory 85 series distributor. Ricky SS

  4. Ken Chorney says:

    I believe the old Crane adjustable vacuum advance cans for GM V8 point distributors used the allen wrench through the nipple to adjust the spring tension (i.e. the rate of advance vs. vacuum) and also included a limiter plate that installed under the end mounting screw that determined the total amount of advance the vacuum unit could add.

  5. Lots of good info, but one very important point left out of the article, was the source of the vacuum. In order to function properly, in the manner stated in the article, the vacuum canister should be connected to a *manifold* vacuum port (sourced from below the carb throttle plates), NOT “ported” vacuum (above the throttle plates). Manifold vacuum is highest at idle and part-throttle cruise – precisely when the engine can use the additional advance. Despite the misinformation rampant in aftermarket parts instruction manuals, ported vacuum is not the proper source for vacuum advance on a performance street engine. It’s purely to reduce emissions, is only in effect just off idle, and does nothing to aid engine performance or fuel efficiency.

    • My 1950 MoToRs Repair manual says ported distributor vacuum connection (that is, below throttle plates) is designed to achieve a steady idle. Way, way, before federal emissions mandates. It also has listings in the specs going back to the 1930s. Some manufacturers used ported connections, some didn’t. It is true that engines were detuned severely in the 1970s, and retarding ignition timing was part of that.

      They DID make an ironic kluge concession to manifold vacuum – when the engine overheated, a solenoid would swap the distributor back over to manifold vacuum, in an attempt to keep the motor from burning itself up.

      Manifold and vacuum connections aren’t that much different in practice, at least in a stock motor. Once the idle RPM is brought above stock idle RPM, the vacuum advance will start to pull in anyway. This is why most people like a bit higher idle – it smooths out. 30° BTDC is typical at a ported connection at 600 to 650 RPM.

      It is basically identical at all other driving situations, particularly at steady cruise. You can see this for yourself by plumbing a vacuum gauge to both ported or “timed” carburetor connection and a manifold connection. The effect on the vacuum advance is consequently the same.

      The important thing is to use vacuum advance, and adjust it correctly. Most V8 will happily cruise in high gear on level ground at 50° to 55° BTDC ignition timing. Vacuum advance was the only way to achieve this, centrifugal weights and springs alone can’t do it. I’m constantly amazed at how clever those old school engineers were!

  6. Robert Fortier says:

    great info.had many questions regarding my vac. advance ajust. Pertronic dist.,ford 302 crate
    in MK4 Cobra (manifold vac.&ported vac.) erratic at low rpm,in first gear,hope it solves my
    problem. Thanks Robert

  7. Good article. But you the way you are describing setting your timing confuses people. You need to set total @4000 rpms, with vacuum advance disconnected (to make sure mechanical advance is all in). Then your initial will be = total-mechanical. Only way to change the mechanical amount is to swap out for a shorter plate, weld your current plate, or buy aftermarket that has mechanical adjustability.
    Then hook up vacuum advance and adjust it like you mentioned.


  8. Have to dis-agree. More dense fuel, earlier the timing. Less dense fuel does not need more advance.

    • Actually Steve, I was talking about the density of the air in the cylinders. At part throttle, the reduced density of the charge in the cylinder needs more time to combust. That’s why we add timing at part throttle – and why tuning with vacuum advance will improve part-throttle performance and mileage. Not all engines need a lot of timing. Some might need as much as 45 -48 degrees at part throttle. Others may need very little – it all depends on cam timing, mixture distribution, and about a dozen other variables. Hope this helps…

      • I disagree with you about a less dense charge of fuel mix needing more time to burn. this is backwards physics. The engine needs more advance to burn the enriched charge at the press of the accelerator. (more dense air/fuel) Less fuel/air, less timing.

        • You are a moron

        • Jeff Smith says:

          You are certainly allowed your opinion, but let’s look at this in a more application-specific sense. You contend a denser amount of air-and fuel in the chamber needs more timing (up to a point, that’s true this may be where the misinterpretation begins). But if that were true, then why do we need a load-based timing advance (vacuum advance)? The reason is that with limited throttle opening – the less-dense air in the chamber requires additional timing (greater advance) to produce peak cylinder pressure at the proper time to make the most torque possible at any given throttle opening. I think if you are willing to be open to the concept – you will see that this is why vacuum advance was created. The same is true with electronic control of timing. If you look at a typical timing map for EFI engines, you will see MORE total timing at partial throttle and LESS total timing at WOT at any given rpm.

        • the richer mixture takes less time to complete the burn because the oxygen is completely consumed before the fuel is all used. less time is required and is then referred to as ‘faster’. rate of flame front travel during the combustion event is another story and confuses the fast/slow conversation. air/fuel charge density is mostly a function of manifold pressure (WOT creates higher manifold ABSOLUTE pressure in the induction manifold) the term ‘vacuum ‘ might better be referred to as ‘pressure differential’. if not, the effect of ambient pressure on the calculation is missing. think about air/fuel charge density at WOT in Denver!

        • Vacuum advance works best when hooked up to ported vacuum, it comes in as soon as you touch the pedal, and stays in through entire rpm range…..sorry but only someone who doesnt know would hook it up to manifold vacuum…lol…rookie

  9. I have a problem that’s driving me crazy. I have a mint 77 Monte Carlo with a 3.73 rear & a 700R4 tranny built to withstand 475 HP. The cars weight is 3,900 lbs. I was running a stock HEI distributor wired into an older style MSD timing computer, 6AL, & blaster 2 coil . I had engine problems so I just ordered a new crate engine which is my garage on a stand. This engine was dynoed before it shipped and is rated at 454 HP at 5,800 & 477 torque at 4,200 RPMs. The cam’s lift is 225/231 at 050. The carb is rated at 750 cfm. They set the base timing at 12 degrees BTDC & total timing is 34 degrees BTDC. I’m ditching the MSD timing computer & will leave the 6AL box & separate coil. The distributor they gave me is the MSD Streetfire. I spoke to the engine builders & they said to run without vacuum advance. Well after reading all your expert articles, I know that’s a mistake. Here’s my dilemma & question. I wanted to purchase the MSD pro billet 8360 distributor. I feel I can easily wire this into my previous MSD equipment. The problem is this distributor has a vacuum advance locked in at 16 degrees with no way of adjusting it, even with after market parts. is this setting to high? If no, then what should I set my base & advanced timing to? If you think having a non adjustable vacuum advance is not a good thing then what distributor should I get to go with my set up? Thank you, Jamie

    • That 16 degrees is a good place to start. As an example, at a cruise rpm of around 2500 rpm if the mechanical advance is nearly all the way in, you might have 30 degrees of total timing – adding that vacuum advance of 16 would produce 48 degrees of timing which is a good place to start. You might find that it’s near perfect and should improve your highway fuel mileage. The vacuum advance should also improve part-throttle response. I would just duplicate the advance curve from the previous distributor and go from there. Likely it will be very close to what you need.

    • Hook the vacuum advance up to venturi vacuum, that way as soon as you touch the pedal you have advance, which means you can dump more fuel to it, and make more power….if you get 16 of vacuum advance + 12 initial= 28…adjust your mechanical advance accordingly….dont go over 36 total timing…..I dont know why all these clowns plug vacuum advance into manifold vacuum….it doesnt make sense….I’m a Ford guy, and that’s how they came from factory, some had too ports on the advance canister, one on either side of the diaphragm…one got venturi vacuum to advance the timing, and one got manifold vacuum to retard the timing when you let off the throttle

      • Matt, what? no carburetor today uses venturi vacuum. You have ported vacuum or manifold vacuum.

        You are trying to throw your weight around pretending to know what you are talking about by using the term venturi vacuum when you don’t even know that is not even used.

        My holley 4000 carb, you know a carburetor used on 55-56 Fords and supercharged 57 Fords use venturi vacuum. Reason for this? the distributor on these engines were Holley Load-O-Matic distributors that had no I repeat no mechanical advance. The venturi vacuum actually provided increasing vacuum the faster the engine runs no matter how hard you hit the throttle to provide a vacuum signal to give ignition advance.

        There was the use of a spark delay valve that would kick in under heavy acceleration to provide a tip in on the second advance can on the distributor to provide a quick retard to the ignition system under heavy loads to prevent ping.

        This venturi vacuum was only used in this case and is not used on modern carburetors let alone aftermarket carburetors.

        So please if you are going to give advice please be sure that you know what you are talking about before you go trying to show people up while showing the world just how little you know.

        As far as vacuum advance goes, you want manifold vacuum. It provides vacuum at all times when you are at no or little load which is what you want. You lose vacuum when you go under load which removes vacuum advance and returns you to total timing.

        Also Ford did not always use vacuum as you put it. Ford for a long time ran manifold vacuum for vacuum advance then during the emission era their quick fix to try and reduce emissions was to switch to ported vacuum to drop advance down to clean up the exhaust which it really didn’t help it actually resulted in a increase in emissions in the engine vs having it attached to manifold vacuum.

        What you said about the two connector vacuum advance is also wrong. My ’56 for example has a dual advance unit as it is a pure vacuum controlled advance as there is no mechanical advance. The primary advance can connects straight to manifold vacuum. The secondary advance can connects to venturi vacuum not to be confused with ported vacuum they are two different things. When you cruise manifold vacuum is high you get advance added to the advance provided by the venturi circuit. You step down on the throttle hard enough manifold vacuum goes away along with ported vacuum but venturi vacuum remains so it provides a quick retard to the timing as now you remove the extra advance being provided by the manifold vacuum source.

        So I kindly request you do not provide information as your spreading of disinformation is what causes people that need help to see conflicting information that does nothing to help them but causes them to dwell on issues. It is what I am dealing with myself trying to figure out how much advance I need to put in my dist on my recurve for my engine and I keep seeing contradictory information being provided.

  10. Correction on my question above. The distributor I wanted to purchase with the non adjustable vacuum advance is the MSD 8360.

  11. Wow that’s twice. I meant the 8361.

  12. Ross York says:

    Contact MSD and they can get you answers on any timing issue. Their website also has great videos with David Frieburger on timing.

    • Ross York says:

      MSD vacuum advance information is bassackwards though.

      • Michael Gillean says:

        Mallory Tech (MSD) claims the opposite directions for vac adjustment. They say, “CCW increases the advance, CW decreases the advance degrees”. So which is it?

  13. Great write-up. I understand vacuum advance for example cruising and light throttle. What actually happens on the deceleration for example down a hill and no throttle but the revs remain high. Is Vacuum advance still present? I understand with late model EFI cars is that advance retard occurs on Deceleration. Thanks

  14. Hi Jeff, I have a new rebuilt 383 and I am having trouble getting it to run crisp and with good throttle response I am trying to determine how much machanal I need so I use the right machanal advance stop bushing could shed some light on this for me thanks

  15. This is all good and well, but as a modern parts guy, I know for a fact that many units are faulty out of the box, or the fuel is not up to the octane numbers that they used to be. I understand that many guys on these forums go for the $400+ units, but the daily customers that I work with don’t have that kind of bread, and they are left with little knowledge and a shabby running engine.

    In my daily driven 383 I do not use timing advance. It has 202 valves, Holley 700dp, warm plugs, & HEI. Driven on the street and cross country I have no issues with cruise idle, throttle response, fouling, pinging/detonation, or other drivability problems.

  16. If you’re running 10.5:1 compression you will detonate with vacuum advance hooked up. On pump gas and 10.5:1 compression you can’t have a max total timing more then 28 degrees. I have a cracked cylinder in my garage to show you what happens when you allow the advance to reach 48 degrees or more on a high compression engine. If you have a high performance engine you don’t want to ever see more then 38 degrees. Look up the timing on a 2002 z06 vette. Total max timing on it is 28 degrees and it has 10.5:1 compression.

  17. Pingback: Ask Away! with Jeff Smith: Vacuum Advance and How it Relates to Part-Throttle Timing - OnAllCylinders

  18. Michael Gillean says:

    Apples and Oranges. Modern computer controlled spark and fuel is not the same as old-school distributor and coil. I have a Ford 460 with 10.5 compression, and it will ping like hell without the Vac advance, using manifold vacuum.

  19. I actually got in a discussion with a Edelbrock carb tech who insisted that ported vacuum was the most efficient, and claimed I didn’t know what I was talking about when I told him ported was emission related. I even referred him to an earlier post by you… He just refused to accept he might be wrong. Oh, the Edelbrock AFB instructions say to use “ported” vacuum… 🙁

    • Of course you would use venturi vacuum…..manifold vacuum is at its highest when you are idling, as soon as you touch the pedal your manifold vacuum begins to disappear…why would you hook advance up to it…..hook it up to venturi vacuum and as soon as you touch the pedal your vacuum increases and stays there, just like you want your advance to do….lol

    • The Edlebrock guy was right
      Do you think you know more than the carb manufacturer…..the book even tells you how to hook it up…lol

    • Ted, kindly ignore what Matt says, he knows what he does not talk about.

      ported vacuum was an emission option. Before emissions manifold vacuum was used.

      He keeps using venturi vacuum in reference to ported vacuum because he does not know the difference in them and think they are the same when they are not.

      Venturi vacuum is situated high up in the throat of each barrel. It is a signal that is created by air rushing past it. Think of how fuel is pulled into the carb through the boosters as air flows past the booster it creates a low pressure point that pulls fuel in. Faster air flows more fuel pulled in. Venturi vacuum functions the same way. Faster the air flows across it more vacuum is created. Ford used venturi vacuum on their Holley 4000 carbs for use with the Holley Load-O-Matic distributors that had no mechanical advance.

      Ported vacuum is situated just above the throttle plates and as you apply more throttle vacuum comes in off idle. It does not increase it remains a steady vacuum signal off idle.

      Manifold vacuum is situated below the throttle plates and maintains vacuum at idle and off idle.

      So for you, attach your vacuum advance to manifold vacuum. your engine will thank you as it will idle smoother. When your engine is under low load situations you are able to run more advance. When an engine is at idle what load is there on there. There is no reason to run ported vacuum on distributor advance can unless you have to meet emission requirements. Outside of emission requirements it should be avoided.

  20. The question is, what did cars run before emissions came into play?
    I run my hot rod with manifold vacuum.

    • Mainly manifold vacuum and venturi vacuum, not to be confused with ported vacuum like this Matt individual keeps telling people to run which no carb has venturi vacuum today nor in the last 50 years.

  21. Pingback: Ask Away! with Jeff Smith: How to Set Up an Advance Curve Without a Distributor Machine

  22. HI JEFF,


    • The one furthest from the distributer housing gets venturi vacuum to advance your timing, and the one closest to the housing gets manifold vacuum to retard the timing when you let off the pedal…..some are even adjustable, the end of the vacuum canister threads and it has a plastic stopper, and spring…u can use shims, and try different rate springs to get your advance coming in quicker, and you can control how much advance it’s getting by using thin flat washers as shims……I have mine getting 12 degrees of vacuum advance as soon as I touch the pedal

      • First he didn’t ask you, secondly for everyone that sees this ignore his use of venturi vacuum.

        No carburetor has used venturi vacuum in over 50 years.

        There are three forms of vacuum that has been used on carburetors.

        Manifold vacuum : vacuum at idle and off idle.

        Ported vacuum : vacuum off idle only

        Venturi vacuum : vacuum that increases as engine speed increases

        Venturi vacuum was used before distributors had mechanical advance as it was a increasing vacuum signal as the engine ran faster and faster. Ported vacuum was the same amount of vacuum at 1,000 rpm as it was at 2,500 rpm.

        As far as Artie`s question goes look and see if there is an emission decal available for your ’71 429 CJ, it would tell you. I couldn’t say but if Ford kept with the style of previous dual vacuum advance cans like my ’56 uses which I don’t think they did as yours has mechanical advance and mine is vacuum only. Mine how ever is manifold vacuum on the outter can and the inner can is venturi vacuum. Mine is done this way so when you apply hard throttle and loose manifold vacuum the outter advance is slammed back via spring pressure creating a instant retard to prevent ping under load. Yours will be the same, only difference is one will be manifold vacuum and the other will be ported vacuum so when you apply heavy throttle the manifold vacuum going away will result in a instant retard of your ignition timing to prevent ping under load.

  23. Ron Skye says:

    Jeff…trying to replace distributor pick up in ford 5.0….but vacuum advance arm is riveted to bottom of pick up coil….what am I up against here….I can’t find a place that sells a unit with the pick up coil and vacuum advance….thanks in advance

    • Matt,
      You must be the only person on the planet who has not read the excellent and correct paper “Timing 101” written by a GM engineer who spent his career working on ignition systems. No where does he mention “Venturi vacuum” but he does mention MANIFOLD VACUUM when says to use it and it only.

  24. I dont know who monitors this discussion, but are you all serious…do you understand how carberatur’s work….or for that matter how a V8 engine functions….do you guys just drive your vehicles, and have shops do all the real work?

    • Its very obvious no one monitors this considering your complete lack of understanding on this matter spreading disinformation as fact when you sir are in fact incorrect and wrong.

      I still cant believe how many times you used the term venturi vacuum when venturi vacuum is not used, it hasn’t been used in some 50 years. We only have two forms of vacuum, manifold and ported. Ported vacuum was used for emission reasons to prevent too much advance at idle as it was believed to increase emissions. In fact it created a worse situation as it actually increased emissions slightly over manifold vacuum.

      engines require manifold vacuum for vacuum advance because you run more advance under light load conditions and idle is a light load condition.

      You cited Ford numerous times of running ported vacuum as the cause for why its right. You fail to grasp that Ford used venturi vacuum first, then manifold vacuum, then they went to ported vacuum during the emission era.

      If you are running your Ford on ported vacuum I feel sorry for your engine and how you are gimping it by using the emission era form of vacuum advance over the proper non emission era manifold vacuum.

  25. Hi Jeff, I have a question that no one can seem to answer. I’m setting up a 1970 Ford 302 with a later “Duraspark” ignition system. I get about 25 degrees advance from the centrifugal mechanism, and about 45 degrees on top of that from the vacuum. So with 10 degrees initial, I’m seeing about 80 total at 3000 rpm. I appreciate any advice!

    • I’m not sure why you would get anywhere near 45 degrees from the vacuum advance. But under the assumption that this is accurate, best thing to do is to remove the vacuum advance unit and braze or weld or use JB Weld to limit the amount of vaccum advance. There is normally a rod that travels in a slot. Limit the length of the slot and this will limit the vacuum advance. But before that – check to make sure you ahve the correct advance unit – It could be the one you have is incorrect. Limit the advance to around 10 to 15 crankshaft degrees.

  26. Thanks for answering me Jeff, there’s no doubt that the vacuum solenoid is adding 45 degrees, with the line disconnected I get 35 total advance, and about 80 when it’s hooked up. I have another distributor that appears the same as the one installed. I’ll try that one and see where I’m at, and if need be I’ll rig up something to limit it like you said. These are both stock Ford duraspark era units. I really appreciate the help on this!



  28. I have a 1972 mach 1 Q code, looking for advice to adjust single port vacuum advance on distributor, bought a new vac advance and wondering if it needs to adjusted before I install it, I,m more of a backyard mechanic. How many turns ccw to do that?

    • I would just start with how it came out of the box rex.
      Start off by checking and confirming your total advance with the vac canister disconnected and the line to the carb plugged. Since you’re trying to dial in your vac advance I’d say check your total at about 2k rpm.

      That’s part throttle, so you should be close to 30 degrees total mechanical.

      Once you’ve confirmed the mechanical advance, hook the vac advance canister back up to the manifold vaccum port on the carb, and check your total timing at 2k again. Take that number, and subtract the total mechanical from earlier from it to get your vaccum advance number.

      That way you can see how many degrees of timing your vac advance adds.

      Now. As a general rule of thumb, we usually look for a vaccum advance to take the total timing to between 45 and 50 ish degrees when active.

      So if it’s not at that number you can adjust the vac can to allow more or less advance as needed.

      Now bear in mind, all engines are different. So you may need to adjust it to a different number.

      At the end of the day, give your engine what it wants. Set a certain degree of timing to begin with, then take the vehicle and test it. If you notice any Detonation (loud pinging) then you have too much for your engine.

      If I’m wrong in any points y’all, feel free to correct me. I’m no expert by any means, just tryna help

  29. don’t know if received my ? re: 1972 Q code Mach 1

  30. William Griffin says:

    I have a 63 1/2 falcon sprint
    The return to idle spring is not bringing the excelerator linkage back to full idle position. I understand that I can hook up a spring nn The front side
    That would return the gas excelerator back to full idle position. Is there a name for that spring?

  31. Mark Neitzel says:

    Hi Jeff, thanks for all of the great articles. On this article, could you verify that you are talking about manifold vacuum or ported vacuum? Thank you.

  32. Rob Watson says:

    What does the letter F represent on vacuum advance distributors with an arc, followed by another arc stating Dwell? I am trying to help someone set up a 123 electronic distributor for a Chrysler 180 (2155cc) Solex carb engine and the closest thing I could find in Ducellier distributor specs is for the 2.0L (1981 cc) Chrysler 180 version. Thx

    F 55 , 61 +/- 3 degrees Dwell , points gap 0.45mm , points 0 I am hoping the 123 program will allow it to be set close enough to start.

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