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Old 12-20-2006, 03:21 PM
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No ported vaccum port in my Q-jet, help

I did a carb overhaul in my C30 dually, I just got help from some guys from the forum about improving the torque and the thing that worked best was the carb service, I installed smaller jets according to the altitude here, also new primary rods, now works great, improved power and fuel economy, BUT, I discovered the cause for bad idle and pinging, the carb has NO ported vaccum port, the vac advance was connected to full manifold, I left it disconected, works great, stopped pingin, but I'm concerned about if this is correct, HEI's are suposed to work with vaccum advance, am I wrong? can I run it without vaccum advance? how should I set the timing?
Vaccum advance is supposed to be an aid to improve fuel economy while at part throttle, but I very seldom have part throttle, our roads are twisty and lots of hills, most of the time the gas pedal is being worked, tranny shifts a lot, please suggest what to do and how much timing should I run, I'm at 6.600 feet of elevation.
Thanks.
Augusto.

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Old 12-20-2006, 03:25 PM
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How much vacuum advance do you use? You can limit this with a small bracket on the can.
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Old 12-20-2006, 03:49 PM
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Hows the weather up there?

Whew you're up there! I would say that as long as you have good power and the pinging has stopped. Leave it be!! The centrifugal weights under the rotor cap may be doing all you need. see What others think I am much closer to sea level but that is what my instinct is telling me. Good luck. Brian
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Old 12-20-2006, 06:39 PM
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Add one Its really not tough. Chances are you actually already have the nipple for it, its just that the throat of the carb doesn't have the hole drilled. Take a look at the pictures below. One shows a carb without it (which I added later) and the other shows a factory Qjet with a ported hole. (You're looking at the top of the throttle body, by the way.) In the first photo, the blue arrow is the idle transition slot. The center arrow is the ported vacuum source. forget about the other red arrow in the right. The second photo shows just the idle transition slot and no ported hole.

Remove the TB, put a small piece of masking tape on the leading edge of the throttle blade (to hold it open ever so slightly) rest an .018" drill bit on the throttle blade, and drill into that cavity. All the Qjets I've seen have the proper cavity, but double check first to make sure its not open to manifold somewhere else.

if you don't have a dummy nipple already in the fuel bowl casting, you can drill and add one with epoxy and some brake line tubing.
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Old 12-21-2006, 04:12 PM
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I found the following post at www.camaros.net it is very informative and answers my question perfectly, I wanted to share it with all you guys.
Thanks for helping.
Augusto.




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As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

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" parts.
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Old 12-21-2006, 04:53 PM
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Wow, that's a ton of good info.

And to Curtis, I never knew that about the qjets. I'll keep that info filed away too, thanks.

Mark
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Old 12-23-2006, 09:04 PM
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Wow

I just thought I understood vacuum advance!!!!This Guy rocks!!! Brian
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Old 12-23-2006, 09:10 PM
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I would set the base timing at a lower setting then connect the Vac advance, or if you wanted to limit the vac advance, you can always place a stopper on it, the plastic bushings that come in HEI advance kits, you can slide it over the tang on the end of the advance unit, so when it closes it limits the amount of advance. On every vehicle I ever tried to use ported vacuum the engine was a non responsive slug, if your carb is a factory carb and doesn`t have a ported source then it wasn`t meant to use ported vacuum. My SM Q-jet doesn`t have a ported source either, so I tried it with and without, it has better response and power with it.
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Old 12-26-2006, 06:51 AM
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I was giving this subject a second tougth and realized that old service manuals instructed to "disconect and plug the vacuum line at the distribuitor before setting the initial timing" why if it's ported? there is no vaccum at idle I said, wrong, the vacuum always was full manifold in the pre-smog era, I'm gonna look over some old books I have, to be sure.
Augusto.
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Old 12-26-2006, 10:04 AM
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lotsa things get copy/pasted wether right ir not.
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Old 12-26-2006, 12:42 PM
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Amen, johnsongrass.

I've read that timing article so many times and there are so many things I disagree with, but as soon as I open my mouth someone automatically sides with that guy because he has a title beside his name.
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Old 12-27-2006, 05:04 PM
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I connected the vacuum line to full manifold and the idle speed went up a lot, I backed of the idle screw 'til it fell off but no luck, could not find any vacuum leaks in the carb or elswhere, now I'm confused, any ideas?
Augusto.
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