1969 Stage 1 7029246 - Does it have a fixed Idle-Air Bypass System?

Discussion in 'The Venerable Q-Jet' started by schwemf, Aug 30, 2019.

  1. schwemf

    schwemf Mike Schweitzer

    Does the 1969 Stage 1 Qjet (7029246) have a fixed idle-air bypass system, as do the 1970 BBB Qjets? I'm using a 1969 GS400 QJet, 7029242, that does not have this system, on a '69 GS400 Stage 1. When I remove the vacuum gauge from the QJet's manifold vacuum port, idle increases by 100 RPM!

    Last edited: Aug 30, 2019
  2. schwemf

    schwemf Mike Schweitzer

    Someone must have a '69 Stage qjet, 7029246, that they can turn over and inspect the throttle body for idle-air bypass! I'd sure appreciate it!!
  3. Daves69

    Daves69 Too many cars too work on

    My 69 400 4 speed (non stage1 ) carb does not.
  4. Buicksky

    Buicksky Silver Level contributor

    PM sent Mike
  5. 2manybuicks

    2manybuicks Platinum Level Contributor

    20200216_180735.jpg 20200216_180725.jpg 20200216_180825.jpg did someone say 29246?
  6. schwemf

    schwemf Mike Schweitzer

    I've uploaded a color picture from Cliff Ruggles' book, and a black-and-white picture from Doug Roe's book showing Idle bypass air components.

    I've also taken pictures of this system on a 1970 Buick 455 auto QJet (7040240).

    IMG_20200217_064952891.jpg IMG_20200217_065009229.jpg IMG_20200217_065150455_LI.jpg IMG_20200217_065458420.jpg

    Attached Files:

  7. schwemf

    schwemf Mike Schweitzer

    Thank you, 2manybuicks, for posting those pictures. It clearly shows no fixed idle-air bypass system.

    All I can make out is a "292" and an April 1969 Julian date "1059". I'll assume you know this carb to actually be a 7029246.

    Thank you again.
  8. 2manybuicks

    2manybuicks Platinum Level Contributor

    yeah, a better angle is all it takes. 20200219_211413.jpg k
  9. Cliff R

    Cliff R Well-Known Member

    Just a note about the black and white photo above.

    #1 is the idle mixture screw discharge

    #2 is the idle bypass air

    #3 is the idle/off idle transfer slots

    The Buick 246's do not use idle bypass air that I've seen......Clff
    schwemf and techg8 like this.
  10. schwemf

    schwemf Mike Schweitzer

    Thank you Cliff for clarifying the black and white photo.

    Thanks again, 2manybuicks, for digging out your carburetor and taking another picture!

    I installed bushings on the primary throttle shaft, so perhaps now the primary throttle blades sit too centered in their bores (i.e. too little space around the blades when seated, resulting in too little bypass air at idle?) . Doug Roe cautions about this, that Rochester purposely set "space" around the blades, so he advises not to eliminate it.
  11. Cliff R

    Cliff R Well-Known Member

    That makes no sense to me at all. Both the primary and secondary throttle plates are designed to fit tight in the bores when fully closed. They need to be centered and indexed when installing bushings.

    The primary plates are open at idle speed so there would be plenty of space around them anyhow. You have no choice but to install them centered and even so they seal up tightly. If you try to install them in any other fashion they will hang up and bind at low throttle angles and may not allow things to close down far enough to get a low enough idle speed.

    Not having idle bypass air on a 246 carb isn't a problem unless a larger than aftermarket camshaft was installed with more overlap and vacuum at idle is down some requiring more throttle angle to keep it running. Lowering compression can do the same thing, and going to a bigger cam and lowering compression for sure is going to make the factory carb unhappy without modifications in most cases........Cliff
  12. schwemf

    schwemf Mike Schweitzer

    I see my error. Doug Roe was referring to adjustments to the throttle stop screw (please see attachment, paragraph DESIGN FEATURES/Idle System Throttle Stop). When Doug says "the primary throttles should never be readjusted to seat in the bore", I now believe he could have added "...by fully backing off the throttle stop screw".

    I believe to solve my original issue, which prompted this post, "When I remove the vacuum gauge from the QJet's manifold vacuum port, idle increases by 100 RPM", I simply need to open the primary throttles more and lean the idle mixture screws to obtain the desired idle RPM.

    Thank you again Cliff!

    Attached Files:

  13. Cliff R

    Cliff R Well-Known Member

    While on the topic of seated primary throttle plates I see folks having issues all the time with that deal.

    For some reason folks think or are led to believe that you have to run manifold vacuum to the vacuum advance on many of these engines. On some set-ups that works very poorly and it becomes difficult to close down the carb far enough to get the idle speed down, and/or good sensitivity from the idle mixture screws.

    For the most part the factory used ported vacuum to the advance, not manifold vacuum. They also INCREASED the base timing at idle on many later model engines and also many came with very small emission oriented camshafts in them. When vacuum at idle is strong without a lot of base timing there is no need to run the timing clear off the scale using manifold vacuum to the advance, and it can make the engine pretty unhappy and difficulty tuning idle speed and mixture.....FWIW......
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Mid to late 60's Buick engines used manifold vacuum to the advance. If you look at the ignition timing specs for these engines, they specify 0* (TDC), or single digit initial timing. The distributors fitted to those engines from the factory had 26-34* of mechanical advance hence the very low initial timing specified.


    Using manifold vacuum let the engine idle at 14-18*. If you use ported vacuum on an engine that idles at TDC, the engine will run very hot due to retarded idle timing. A lot of guys are using junk yard big cap HEI distributors. The HEI's frequently had large amounts of mechanical advance leading to the same problem, the necessity to use low initial timing and the resultant hot running due to retarded ignition timing at idle. In the 1970's, GM went to ported vacuum for the vacuum advance. This was due to emissions regulations. Retarded idle ignition timing gave cleaner exhaust (oxides of nitrogen). Again the engine had the tendency to run hot. The fix for this was a thermovacuum switch in the intake manifold that switched the vacuum advance to manifold vacuum whenever the coolant temperature climbed above 220*. This increased idle speed and advanced the timing to cool the engine off.

    TCS System.jpg

    Obviously, if you are building performance into your Buick engine, you will need the correct ignition timing, something many guys do not understand or concern themselves with. You can't just run any distributor and any initial timing you want without looking at the timing at all RPM's. Any moderately sized cam is going to want more initial timing at idle. It will also like a faster rate of mechanical advance in most cases. This helps with the low end loss from the cam itself as well as the aforementioned cooling issue. None of this is a problem with most aftermarket distributors as they have fully adjustable mechanical and sometimes vacuum advance. Like I stated though, if you are just stabbing an unknown distributor in there, it will likely need a recurve to get the best out of it. At part throttle, the amount of degrees available from the vacuum advance canister will be the same whether you run manifold or ported vacuum. The difference will be at idle. Some cams will not produce enough vacuum to run a stock canister at idle, and as Cliff said, the engine will have fluctuating timing and may not be happy as a result. There are adjustable canisters out there that can help. IMO, vacuum advance is important for any street car. You just need to experiment to see what works best for your combination. Some engines will like manifold vacuum to the canister, others may prefer ported vacuum, try them both, it's quick and easy to do so.
    Last edited: Mar 4, 2020
  15. Cliff R

    Cliff R Well-Known Member

    Just a note on distributors and distributor tuning.

    Several years ago I opened up weekends to have troubled vehicles brought here to custom tune and correct running issues with their engines. It is done by appointment and I've had scores of really nice older muscle cars brought here to date, including at least half a dozen 455 Buick powered cars.

    Most were thought to have carburetor issues and most did. However, almost every single one required distributor work as well. This comes from "old school" methods of installing the lightest possible springs in the advance to get the timing "all-in" right off idle. Most also either disabled or weren't using the vacuum advance.

    Without the vacuum advance your engine is missing timing at light engine load. A street engine needs vacuum advance to be efficient and effectively burn leaner mixtures. It is a LOAD SENSING device, and there are absolutely zero issues from using, only negatives if you don't.......FWIW......Cliff
  16. schwemf

    schwemf Mike Schweitzer

    Here are the specs for this motor:
    Buick 400 cid, .030 overbore, Poston Stage 1 cam ground to "factory" specs.

    All measurements taken at 5300 feet above sea level (in Denver)

    Vacuum in Park is a steady 14" Hg
    Vacuum in Drive, at idle, 12" Hg, 620 RPM
    Timing at idle 12 degrees BTDC

    Ported vacuum advance with "B1" canister, vacuum advance limited to 10 distributor degrees via block-off plate.

    1971 Buick 350 11112037 distributor w/ 24 degrees max advance, all "in" at 3000 RPM (12 initial + 24 centrifugal). Distributor completely rebuilt including polished shaft, new bushings, new grease, .015" end play between gear and housing.

    Flooring accelerator from a stop results in tires breaking loose (radials P215-70/R14). This is the best this car has ever accelerated, and I've owned it since 1982.

    I just don't understand why the idle increases by 100 RPM at idle when I unplug my vacuum gauge from the Quadrajet's manifold vacuum port? To me that implies something is still wrong?
    Last edited: Mar 6, 2020
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Why? All it means is the idle mix is on the rich side which it usually has to be with most cams. The vacuum leak leans it out and the idle increases. I don't think that means anything is wrong.
    schwemf likes this.
  18. Cliff R

    Cliff R Well-Known Member

    "Using manifold vacuum let the engine idle at 14-18*. If you use ported vacuum on an engine that idles at TDC, the engine will run very hot due to retarded idle timing."

    This is not always accurate. I've tuned THOUSANDS of these engines using ported vacuum to the advance and never once found that I could get one to run cooler adding more timing via manifold vacuum to the advance.

    Of course I'm NOT setting the initial timing clear down at 0 to 4 degrees, we're typically setting it at 10-14 degrees (sometimes even higher with really "low" compression engines using "big" cams in them) and adding 20-24 degrees with the mechanical advance.

    I suppose IF you set the timing clear down at zero or close to it, and didn't add timing via the vacuum advance you may see some running hot or overheating issues, but it would be foolish to set one of these engines up like that in the first place.

    One also has to remember here that there are several long/lengthy "reads" on the Internet on the topic of vacuum advance and distributor timing and not everything mentioned there is entirely accurate, especially when it comes to ported spark advance. I recently got into some pretty heated debates on that topic with a resident "guru" on another site. Despite claiming to be an "engineer" at GM decades ago and having several degrees that person does not understand or really know how ported vacuum works, so still putting out old/outdated/inaccurate information on the topic.

    The factory was very specific with source location when they set up a carb to use ported vacuum to the distributor. With the very slightest throttle movement off idle FULL MANIFOLD VACUUM is applied to the advance. When the throttle returns to idle no vacuum is added. At any engine load, vehicle speed or throttle position off idle BOTH sources, manifold and ported apply the same vacuum to the vacuum advance. Ported vacuum does NOT increase with throttle position or engine speed/increased load.

    So the only real difference with using ported vs manifold vacuum to the advance is coasting and at idle when the ported source is not uncovered and vacuum to the advance is at zero.

    As for which one to use one will find that with very well thought out engine builds (cam optimal for the CID and compression ratio) a lot of timing at idle will not be required. It also helps to use tight quench and improved combustion chamber shapes (modern chambers). In other words the more that is done to increase combustion efficiency the LESS timing (and fuel) is needed at every RPM.

    One also needs to realize here that when you put a "hefty" cam into one of these engines and don't increase the compression ratio to compensate, or the LSA is so tight it's reducing vacuum at idle too low with "normal" initial timing in the engine, you may need to increase timing via to vacuum advance and in all cases will need to modify the carb to add adequate fuel at a relatively "low" vacuum reading. This may require an "adjustable" vacuum advance so the spring tension can be set to all the timing is applied at the "low" vacuum reading the engine is producing. Most factory VA units will not be adding timing below about 10" vacuum and many are set even higher.

    While on the topic using heavy weights and really light springs in your distributor (common practice in this hobby) can have the distributor adding some timing at idle speed. This can make idle tuning difficult, if not near impossible especially if you have an auto transmission car. When the trans is placed in gear and load put on the engine some of the timing that is "in" at idle can fall out, reducing idle speed and often making the engine not only slow down, but run rough and even stall out (sound familiar).

    Kind of interesting but for the past several years I've opened up weekends to custom tuning troubled set-ups and have had vehicles brought here from as far as 1600 miles away. More times than not the problems I've been hired to correct were actually distributor issues caused by folks removing VA and putting some cheap POS offshore spring/weight kit into their distributor!.....hope this helps some.......Cliff
  19. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Cliff, all I am saying is that if you sit in traffic with the timing at TDC, the motor will have a tendency to run hot especially with a stock type 2 or 3 row radiator. That's exactly what you would have using the distributors for 65-69 with VA connected to ported, as they specified 0 -2 1/2* initial timing. In those years, most had VA connected to manifold vacuum. Starting in 1970, VA got hooked to ported. I'm not saying one is better than the other, just what it was way back when. Once we are talking modified away from stock, everything changes anyway. Some guys with GM HEI big cap distributors have the same situation as a lot of those distributors had a lot of mechanical advance, so if they set the total to 30-34* without VA, they end up around 0*. They might benefit from manifold vacuum in that case. The ideal option is to modify the mechanical advance so you can run more initial timing. Then they can hook up the VA to whatever they want.
  20. Cliff R

    Cliff R Well-Known Member

    Correct Larry.

    It really depends on the engine, but yes, many engines with a LOT of quench in them, and chitty "open" or poor combustion chamber shapes will tend to run pretty hot without some timing in them at idle.

    One common denominator I've seen with many troubled running hot/overheating engines is having a LOT of quench in them. For engines built here NEVER over .040" quench for any reason. Matter of fact the tighter the quench the better, just shy of the pistons smacking the heads is ALWAYS better than having the pistons .030" in the holes then stuffing a .050-.060" thick head gasket on it. I've not had a single engine brought in here that was even slightly impressive with the latter set-up...FWIW

    Cam timing is another "player" in that deal. I've actually moved camshafts a few degrees on a few very unhappy "fresh" engine builds that were running consistently over 200 degrees and times over 230 and had trouble getting them up to temp after the revised cam timing.

    Of course it should go without saying that making good choice in terms of static compression, camshaft specs, and very close control of timing fuel curves will ALWAYS yield a much better end result at every level, especially how much heat is transferred to the coolant instead of making power with it.....FWIW......
    Last edited: Mar 9, 2020

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