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

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!

-mike

 

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!!

 

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

 

PM sent Mike

 

did someone say 29246?

 

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).

 

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.

 

yeah, a better angle is all it takes. k

 

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

 

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.

 

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

 

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!

 

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......

 

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

 

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?

 

"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

 

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......