[Cliff R]

Kenth, great post! I am in complete agreement with everything in it until the last paragraph. You DO NOT need the vacuum advance connected to full manifold vacuum to enjoy peak engine performance, drivability and efficiency in a street driven car. Aside from some potential benefits at idle a strong ported source will mimic the manifold source (both locations are below the throttle plates at cruise). The only real benefits would be having a lot more timing during decelleration and at idle. I've heard time and time again that adding more timing at idle will help with engine cooling. I can't qualify that statement as we've done LOTS of testing with troubled running hot/overheating/detonating engines over the years. In not one single instance did switching the vacuum advance over to manifold vacuum help anything. If they ran hot after extended cruising and the temps climbed even higher after slowing to a stop, they just kept right on going up regardless how much timing we added in at idle.

The flaw I see in your post is recomending that manifold vacuum ALWAYS be used or inferring that it has all of these benefits. It's not a matter of being right or wrong, but a matter of taking a stand on one side of the fence and trying then start selling. I know LOTS of engine combinations that WILL NOT RUN WELL using manifold vacuum to the advance. These are usually engines where the owner/builder has chosen a pretty healthy cam with late intake closing events, tight LSA and lots of overlap (probably too much for the static CR of the engine). This in almost all cases causes an immediate deficiency in the carbs idle system to supply the needed fuel at the relatively LOW vacuum present. So the mixture is lean...and...no doubt about it raising the initial timing either by static, manfold vacuum to the canister or both will immediately help. It does not in and of itself cure the real problem and in most cases causes several other problems to occur.

Remember my Olds 442 customer above? Both he and his helpers were dead set on using a manifold vacuum source to the advance. This smoothed out the idle and provided a suitable tempory "fix". The engine ran OK aside for the canister adding too much timing and causing light throttle pinging. The biggest problem that developed was the engine vacuum falling off just a tad when a load was placed on the engine at idle speed. When the car showed up here the idle speed was noticably high and dead smooth. It checked at just over 1000rpm. When the trans was placed in gear, the vacuum fell off enough to pull out some timing...and...the engine speed slowed down considerably. When I lowered the curb idle speed to realistic specs, 750 rpm out of gear, the engine faltered and died out when it was placed in gear as the manifold vacuum fell off enough enough that the vacuum advance spring overcame it and the timing retarded too much. At this point we had a choice to make. Purchase an adjustable or different advance with a spring setting below about 8" of vacuum or switch the source to the advance over to ported and increase the throttle position a tad. We chose scenario #2, although we could have acheived decent results by changing the advance and doing some more tuning. What we ended up with was a stable idle both in and out of gear as the timing didn't change.

I run into this all the time. Cams a tad too big for the static CR and stock ignition/carb specs just don't work. Please don't think I'm being to critical of your comments. It's the selling the one and only way to do that I'm not in agreement with. I feel that the owner/tuner should set up his/her combination with what works the best.

This topic is no doubt the most mis-understood of any. Even many of the "experts" don't really understand it. Your post should help to clear things up some, it is great information......Cliff

[PMDRACER]

Kenth and Cliff R -

Thanks for the informative reading! I have the scenario mentioned above about ping at the top of the gear at WOT and light-throttle ping, especially when the motor gets a little on the warm side. Now I know what I'm doing this weekend. 8^)

Hopefully more folks here on the Boards will read this and also notice that two guys can get into a debate without resorting to insults.

Kudos to both of you!

[buwalda]

Once again, I completely & 100% agree with Kenth & speak with 40 years of automotive experience.

Cliff,
I really am surprised by some of your observations.
Firstly, you ask why would you add 'a bunch of timing' when stock figures work so well. Well, I thought the answer was obvious: some of the components [ the cam, for one] are not stock anymore, so it is logical to assume that ignition timing, idle speed, jetting etc may need to be changed. We change the centri timing, so why not the initial? [ by way of adding full manifold vac adv.]
Secondly, you say your friends Olds 442 couldn't hold the vac adv when dropped into gear. I'm not surprised with the stock vac adv can! The spring is too strong. The bigger cam probably needs a higher idle speed, so why wouldn't it need a a different vac. adv can as well? And that why is Crane, Accel, Echlin, Mr. Gasket & probably a whole lot of others make adjustable vac adv units for this very purpose: to give adjustability so that you can tailor the ignition requirements to your particular combination.
While the sound of a 'lope' might sound nice, it is actually the engine telling you it is 'unhappy'. The engine is jumping around because of poor & unequal commbustion in each cylinder. It can be made more 'happy' by giving it the extra timing it needs at idle so that more complete & eficient combustion is obtained. The engine will also run cooler now because it is running more efficiently. Throttle response from idle & off idle is also enhanced, compared to ported vac adv, because the engine is at a higher level of efficiency. Throttle response: If the engine makes 40 ft/lb at idle with full man vac adv, & then the right foot foot demands 60 ft lbs, that will be easier for the engine to deliver, compared to a 30 ft lbs starting point with ported vac adv. Giving more timing at idle increases idle speed; the increase in idle speed happens because the engine makes more power with the increased timing, BUT without an increase in throttle opening. Again, this proves that the the increased timing benefits the engine. This matter of timing is really just the basics of how the engine works & what it's ignition requirements are. It's not rocket science.
Full manifold vac adv that is set up to work properly is NOT a 5 minute job. It will take trial & error. But once it is dialled in, it makes a big difference & those that have taken the trouble to do so will be well pleased. There is no good reason to be without it.

[Cliff R]

The power off idle issue is a mute point. With a well chosen ported vacuum source you will have all of the timing from the vacuum advance with just the slightest throttle movement. Any heavy/full throttle from idle and you loose the timing from the advance and power production becomes a product of the mechanical advance curve. This is the part of this issue that is completely misunderstood by most. Once you move the throttle plates off idle, far enough open to sustain vehicle cruising speed(s), the amount of vacuum at all points under the throttle plates is the same, despite whether the source is considered manifold vacuum or ported. If it's under the throttle plates at cruise it operates the vacuum advance in the same manner.

My question is this. If I can simply reduce the initial timing on the Old's engine described previously to 8 degrees BTDC, reduce the amount of advance supplied by the canister to 12 degrees....and....end up with a stable idle in and out of gear, no detonation issues at light or heavy throttle, no more difficult hot restarts.....then....why would I want to mess around with "special" vacuum canisters that work with vacuum readings less than 7-8"? Point is, if I get the desired results under all engine operating conditions the problem(s) are fixed, no need to continue to "tune".

Your not showing me any benefits despite the well written comments other than the theory that I'm going to have improved engine cooling. which, from my 25 plus years of testing and tuning many hundreds of high performance engines this JUST DOESN'T HAPPEN. In 100 percent off all cases if the engine ran hot/overheated during extensive driving under various conditions we couldn't help squat by adding in a bunch of timing at idle via the vacuum advance. Maybe if I just started up the engine and sat at a stoplight or stuck in traffic someplace at idle there could be an improvement....but....if you're up and running your not going to see any running cooler advantages by using a manifold source to the vacuum canister vs ported.

I beleive we've beat on this topic pretty hard and it's been kicked around on this and other websites quite a bit. I love to read the well written comments....but....disagree with advocating a single solution or inferring that it has to be done only one way to get the best results. If more tuners would go after the carb instead of trying to run tons of initial timing to correct deficiencies in idle fuel delivery this topic wouldn't get so much attention......Cliff

[buwalda]

Cliff,
That 'slightest throttle movement' is throttle respnse. You may only have to move the pedal a little bit with ported vacuum to enable it, but you STILL have to move the pedal some. With manifold vac adv, the engine is already at a more efficient point, & therefore requires less throttle movement: Throttle response.
I do not know why there is a perception that 'lope' is good. Sure, we love it & it sounds great!! But the engine hates it!! It hates that big cam we just rammed down it's throat; & it is telling us by the 'lope', which is incomplete combustion .
Cliff, in one of your posts, you said you got a smooth idle, or close to smooth, out of the 442 Olds. Well, that is what is best, not a lope, which is a sign of distress in the engine.
Over the years, I have suggested this simple test to non-believers of manifold vacuum advance: If you have a big cam & the stock 8-12 degrees initial timing, loosen your dist clamp & advance the dist while the engine is running. The idle speed will increase & idle quality will improve.
The speed:
increases because the engine has produced more power from the extra time given to complete combustion.
The idle quality:
improves because combustion has been improved & there is less power variation now between cylinders.
The only way that our old technology engines can have enough useful initial timing & not be hard to crank, is by using manifold vacuum advance. The stock vac adv may or may not work; that is why it is best to get an adjustable unit, because you can custom tailor it to your engine.

[Cliff R]

Finally! We've cut to the chase! What it really boils down to is that I prefer to hear that big nasty cam thumpity, thimpity, thumping away in the engines we send out of here! Why install a cam with over 80 degrees of overlap and have it sound like an 066 grind?

The off idle power issue is insignificant as we choose a well located ported source and tune the carb so the correct amount of transition slot is exposed and any throttle movement brings on the vacuum advance.

Before adding the Rhoad's lifters to my own engine it was near impossible to use a manifold vacuum source for the advance. The vacuum in gear at 700rpm hovered between 6-7". If a manifold source to the advance was used, the timing moved up to 20 degrees at idle, rpm's up to about 1000. The throttle plates had to be lowered and the mixture leaned out. Placing the trans in gear at this point would have the engine falter and die out as the weak signal was not enough to keep the vacuum advance on.

Anyhow, after adding the Rhoad's lifters either ported or manifold vacuum to the advance can be used. I've driven the car extensively using both sources. The idle quality in and out of gear is more stable NOT using manifold vacuum....I LOVE to hear the cam lump away....mileage, power, throttle response, etc equal using both methods....so I use the ported source...end of the story.....Cliff

[Junkyard Dog]

Cliff
Geoff
Kenth
Would any of you say anything differently for the real men out here who drive stick shifts? I would like to hear from each of you on how this would change (or if it would change) your advice.

[Doug Christie]

This is a GREAT post! Here's a problem I have. In another post a while back I mentioned that I had failed getting my car through AirCare here in BC. It failed at idle with readings of HC 805 ppm, allowed is 386. CO% was 5.71, allowed is 4.15.

I don't have vacuum advance hooked up, but after reading this post went and hooked it up. Initial timing is 12*. Hooked vac. adv. up to manifold vacuum and it went up to about 32*, idle went up and smoothed out. So now I reduce idle and adj carb. Idle screws go way in and are very responsive. Great! Now for the test. I put it in gear and it falls on it's face. So now I'm back to no vac. adv. and a nice lopy sounding cam! I like the lope too, but AirCare doesn't.

The dist. is stock so I guess I should be getting an ajustable vacuum adv.. Vacuum at idle is 14" hg. It did increase to 15 with full vacuum adv. Cam is 224/234, 465/488 lift, 112*ICL.

Also, at cruising speeds HC is 105 ppm, allowed 265, CO% .20%, allowed 2.16%, and NOx 111ppm, allowed 2576 ppm.

My question is, is there a "best" place to pick up the vacuum signal if you want to pass an AirCare test, ported or manifold? Will vac. adv. solve the high numbers at idle and effect the low numbers at cruise speeds?

[grandville455]

Doug we are both running the same cam now and with my rhodes lifters i am seeing 15* at idle in park and 14* in drive!

[WCWhitey]

I have thoroughly enjoyed this post. Well written, thought out and backed by experience. I just have one thing to offer that may or may not be of relivance. The sorce of the vacuum is very important also. I run ported with my XE256 cammed 406 (small cam). Originally, I had the vacuum line hooked up to the front intake runner of a 69 stock intake and my vacuum jumped from 16-18" vacuum of idle at 850 out of gear, however it was an erratic pulse so with my vacuum advance adjusted to give me 10 deg and an initial set at 10 it would bounce back and forth up a degree and down a degree giving an ok idle but still sounded like a missing engine. After hooking the advance up to the base of the Holley which provided a constant 18" the idle smoothed out nicely (not much lope with that cam anyway), the timing does not vary and the carb is very responsive to any adjustment of the idle mixture. Just that simple swap has really sweetened the motor out. I have come to the conclusion that the stock runner location is not the best sorce of vacuum for advance, possibly because of the turbulance inside a single runner (only a quess). This is just something I stumbled upon and I hope it helps someone in a similar situation.

[krisr]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by JunkYard Dog:
Cliff
Geoff
Kenth
Would any of you say anything differently for the real men out here who drive stick shifts? I would like to hear from each of you on how this would change (or if it would change) your advice. <HR></BLOCKQUOTE>

How about those real men who use stick shifts AND MSD pro-billet distributors with no vacuum advance facility

[Malky]

Great info from both Cliff and Kenth. One point I find very interesting from the posts is that with a moderate cam and the can connected to manifold vacuum, so about 25 deg advance, you can almost completely close the throttle plates because the combustion is super-efficient FOR THAT NO-LOAD CONDITION.
I assume that adding the additional load of going into drive stalls the car because the engine cannot draw the aditional fuel and air it needs through the closed throttle plates. So by running ported vacuum to the can, with only about 12 deg advance, it seems you are deliberately setting up a somewhat inefficient no-load idle, with the throttle plates open more than they need to be, so that the loaded idle combustion is efficient.
Hope I got that correct.

[Cliff R]

JYD, recently, while on the phone with a customer I asked him if he wanted to include an electric choke with his car. He replied: "real men do use chokes"!. I guess they drive 4 speeds too!

As far as using a manual transmission and vacuum advance, we still will typically hook them up to a ported source. It is not uncommon these days to get engines in here for tuning that use aftermarket distributors that don't have a vacuum advance. Not using one is really no big deal, but it does have benefits as mentioned in previous comments. On an engine sporting well chosen parts and modifications to improve combustion efficiency we don't see nearly the improvement that one would on a basically "stock" engine. What we have found in recent years is that less ignition lead is required. My own engine falls into this catagory. It doesn't want any more than 30 degrees total timing and about 10 more degrees from the advance at cruise. On occassion I've forgot to hook up the advance and don't even notice it till the next time I test a carb. The engine is dead smooth off idle with no noticable transition between the idle and primary main system even if the canister isn't hooked up. It is difficult to say if fuel mileage is effected as it becomes difficult when cruising around not to do a few full throttle blasts before a tank of fuel has been consumed! In addition, who installs a 455 capable of mid 7 second 1/8 mile runs and cares about gas mileage anyhow?.....Cliff

[Cliff R]

Malky, I've ran into quite a few combinations that didn't want a lot of timing at idle. My own 327 cid powered 67 Impala SS was like that. With it's just over 10 to 1 CR and stock 300hp cam it had a strong signal at idle. With the stock initial setting of 8 degrees BTDC it idled perfectly with about 15" of vacuum. If the advance was moved to a manifold source the additional timing not only smoothed the engine out but put a noticable "miss" into it. It would begin to shake and "buck" at idle speed. At this point lowering the idle speed would have the carbs plate almost completely closed and the mixture screws lost effect and could be nearly seated. Anyone tuning this engine would have quickly realized that it didn't like all that timing at idle speed. I installed a 4L60 OD trans into that vehicle and got as high as 25mpg's on the highway and about 18 mpg overall......Cliff

[wheelspin]

Outstanding thread!

Les

[buwalda]

Well,
It has been mentioned about 3 times already, but some still seem to miss it: YOUR STOCK VAC ADV UNIT WILL BE UNLIKELY TO WORK WITH A LARGE CAM.
AN ADJUSTABLE AFTERMARKET UNIT WILL MOST LIKLEY BE REQ'D. Ofcourse a stock unit will 'fall on it's face' with a large cam, when put into gear. So will a Holley carb with a 10.5" power valve 'fall on it's face' if engine vacuum is 9". Do you still leave in the 10.5" p/valve??
Do you use stock valve springs with that 0.550" lift cam, or the stock 2" exh system? Ofcourse not! So why would you expect the stock vac adv unit to work??
With an aftermarket vac adv unit, the engine will NOT fall on its face when put into gear because the softer spring in the vac can is made to work for just this very purpose.
Rhoads lifters reduce the duration of a large cam at low rpms, so it behaves like a smaller cam; so, ofcourse it idles better. But comparing that function to that of vac adv is comparing apples to oranges. I have both Rhoads type lifters AND vac adv on my engine.
The object of the exercise of hopping up an engiune is to
1. Get the most power.
2. With the best reliability.
3. Using the least amount of fuel.

Using a large cam without vac adv hooked to manifold vacuum requires more throttle opening, uses mnore fuel & often requires the IFR's to be increased or the idle air bleeds to be decreased or both. By giving the engine the extra timing it NEEDS at idle, it produces more power on less throttle opening.
Nobody would debate the great sound of a big cam, but it is NOT best for the engine by denying it the additional timing it requires at idle & low rpm. Vac adv hooked to full manifold vacuum is the best practical way to accomplish this. The bigger the cam, the greater the benefit will be to idle smoothness, cooling, mileage & throttle response. It works just as well on manual cars, but the effect will be less pronounced because the engine is not being loaded by a torque converter.
Krisr, who posts on this board, has been in my car & can comment on idle quality etc.

[buwalda]

I forgot to add something to my last post [ It's 1 am here! ].
The extra throttle opening that I mentioned can be reduced back to, or close to, the stock position in the throttle bores [in relation to the transfer slot ], by drilling holes in the throttle blades for the additional air. This allows the blades to be reset. However, the engine will STILL require extra fuel with this extra air because of the inefficient combustion, as a result of insufficient initial timing. While inefficient in itself, this extra can also cause plugs to load up because of the extra rich idle & a smell of gas.

[Kenth]

Since ignition and valve timing and carburation needs to be tuned as a system:

CARBURETOR TUNING 101

From ROCHESTER CARBURETORS By Doug Roe:

These engine factors are often overlooked when troubleshooting problems originally blamed on faulty carburation, and neglected before selecting a replacement carburetor. Review them before investing time, effort and dollars in carburetor changes. The aggravation and money you save may be your own. Carburetor performance is affected by *Spark timing, *Valve timing, *Temperature, *A/F density, *Compression ratio, *Intake manifold design.
Most vehicles have labels that specify engine idle speed and spark-advance settings. These are calculated by OEM engineers to ensure the engine combination will produce minimum emissions within specified limits. The trend is to use a retarded spark at idle and during the use of intermediate gears. Some smog systems that prevent vacuum advance in intermediate gears have a temperature-sensitive valve that allows spark advance if the engine starts to overheat and during cold operation. Many carburetors have ports for timed spark and actual manifold vacuum. A retarded spark limits oxides of nitrogen by keeping peak pressures and temperatures at lower values than those caused by advanced spark settings. It also reduces hydrocarbon emissions. This reduces emissions - but affects economy, driveability and coolant temperature. Fuel is being burned in the engine, but its energy is largely wasted as heat flow into the cylinder walls and as excess heat in the exhaust manifolds - wich also heats the engine compartment. Consequently, the cooling system has to work harder. In essence, fuel is still burning as it passes the exhaust valve. Thermal efficiency of the engine is less because there is more wasted energy. A retarded spark requires richer jetting in the idle and main systems to get off-idle performance and driveability. A tightrope is being walked here because the mixture must not be allowed to go lean or higher combustion temperatures and more oxides of nitrogen will be produced. If mixtures are richened too far aid driveability, CO (carbon monoxide) emission increase. Because retarding the spark hurts efficiency, the throttle plate must be opened more at idle to supply enough mixture for the engine to run. This fact must be considered by the carburetor designer in positioning the off-idle slot. It must also be considered because high temperatures at idle, due to the retarded spark and high idle speed settings, promote dieseling. On carburetors equipped with timed spark advance - no advance at closed throttle - a port in the throttle bore is exposed to vacuum as the throttle plate moves past the port - usually slightly off idle. The spark-advance vacuum versus airflow calibration is closely monitored in the production of the carburetor because it has a substantial effect on emissions. The distributor advance, once held to be so important for economy, has now become an essential part of reducing emissions.
Valve timing has the most effect on idling and low-speed performance. Adding valve overlap and lift with a racing cam allows the engine to breathe better at high rpm. But it worsens manifold vacuum at idle and low speeds - creating distribution and vaporization problems. The engine becomes hard to start, idles roughly - or not at all - has a bad flat spot coming off idle, and has very poor pulling power (tourqe) at low rpm. The conditions are especially noticeable when a racing cam is matched with a lean, emission-type carburetor. Because manifold vacuum is reduced, the signal available to pull fuel mixture through the idle system is also reduced - the mixture is leaner. The throttle must be opened farther than usual to get enough mixture into the engine for idling. This can place the off-idle slot/port in the wrong relationship to the throttle. Now there is insufficient off-idle fuel to carry the engine until main-system flow begins. When manifold vacuum is reduced, the power valve may operate, or at least open and close, because vacuum varies wildly. This is not a valid reason to remove the power assembly, but it may require altering the valve spring. This will help close the valve at lower vacuum and stabilize it. A racing cam may magnify these problems to the point where a street vehicle becomes undriveable for anything except competition. This is especially true if the carburation capacity has been increased to match the better breathing of the cam. There have been cases when a racing cam was installed at the same time a carburetor was changed. If the mechanic did not understand what was occuring in the engine, the carburetor was often blamed for poor idling and the bad flat spot at off-idle. The real culprit was the racing camshaft!
TUNING FOR RACING CAMSHAFTS
Radical camshafts sometimes require carburation changes to get satisfactory operation below 2000 rpm. A racing camshaft with lots of valve-timing overlap can cause seemingly insuremountable carburation tuning problems. It takes some extra effort to insure that the engine will idle at reasonable speed and will not load up the plugs when driving warm up laps, caution laps or running back to the pits after a drag run.
Preliminary Inspections - Check the accelerator-pump setting and make sure the bowl screws are tight. Look at the underside of the carburetor with the throttle lever held against the curb-idle stop (not against a fast idle cam). Note the position of the primary throttle plates in relation to the transfer slots or holes. This relationship has been established by the factory engineers to give the best off-idle performance.
Baseline Measurements - Check the clearance between the throttle plate and bore with a feeler gage or pieces of paper as you hold the throttle lever against the curb-idle stop. Note this clearance in your tuning notebook. Record every modification as you proceed, regardless how good your memory may be. Install the carburetor and the start the engine. If you have to increase the idle- speed setting to keep the engine running, note how many turns or fractions of a turn, are needed to open the throttle to this point. Adjust the idle-mixture screws for the best idle. If the mixture screws do not seem to have any effect on the idle quality, note that fact. Use a responsive (not highly damped) vacuum gage to measure the manifold vacuum at idle. If the engine idles with the manifold vacuum occasionally dropping to a lower value than that required to open the power valve, you will need to install a valve that will remain closed at idle before proceeding.
Precautions - Because you will be drilling very small holes, requiring a "wire drill" set, you must proceed in very small increments. Even a 0.002-in. increase in an idle-feed restriction of 0.028 in. increases area and flow by 15%. It is easy to drill holes, but there is no easy way to get back to the starting point. Zealous drilling may require a new throttle body. Wire drills, incidentally, are not used in a power drill but must be held in a pin vise. Pin vises are available where you buy wire drills - at precision tool supply houses or model shops. An adequate accelerator-pump setup usually eliminates any need to work on the off-idle transfer fuel mixture, wich is controlled by the idle-feed restriction. The mixture can be checked by opening the throttle with the idle screw until the main system just begins to start, then backing off the screw until it just stops. If idle and off-idle performance turn out to be unacceptable, increase the idle-feed restriction slightly.
Modifying Throttle Plates - (on carbs wich don´t use the idle air bypass system, such as the Q-jet). Take the carburetor off the engine. Turn it over and note where the throttle plates are in relation to the off-idle (transfer) slot. If you can measure more than 0.040 in. of the slot between the throttle plate and the base of the caburetor, drill a hole in each of the primary throttle plates on the same side as the transfer slot. If holes already exists in the throttle plates, enlarge these holes. Small-displacement engines require smaller holes than big-displacement ones. Start with a 1/16-in. drill on your first attempt and then work up in 1/32-in. steps. Before reinstalling the carburetor, reset the idle to provide the same throttle-plate-to-bore clearence that you made as a baseline measurement. Start the engine. If the engine idles at the desired speed, the holes are the correct size. Too slow an idle indicates that the holes needs to be larger; too fast an idle indicates the need for smaller holes. If holes have to be plugged and redrilled, either solder the holes closed or close them with Devcon "F" Aluminium. When you have the holes at the correct size, wich may require the use of number or letter drills to get the idle where you want it, note the size. Where there are two throttle plates on the primary side, both plates should have the same size hole. On a Q-jet you will achieve the same results drilling the idle-air-bypass ports in the throttle body.
Modifying Curb-Idle Discharge Ports - Do the idle mixture screws provide some control of the idle quality? That is, do they cause the engine to run rough as the idle needles are opened? If not, check the size of the curb-idle discharge ports into the throttle bore. You can check this by unscrewing a mixture needle and using your number drill set. If the ports is not within 0.030-in. of the largest part of the needle, say a 0.095-in. port with a 0.125-in. needle, drill out the discharge ports for each barrel equipped with a mixture screw. NOTE: Carburetors on emissions-controlled engines may have small curb-idle ports. Before increasing the idle-feed orifice to make the carburetor work with a racing cam, always open the curb-idle ports to their size before emissins control. This is about 0.095-in. for the Q-jet.
Modifying Idle-Feed Restrictions - If this does not give you idle control, then open the idle-feed restrictions approximate 0.002 in. at a time until some control is achieved. Correct control is indicated when the engine runs as smoothly as possible, and turning the idle-mixture screws either way causes rpm to drop and roughens the idle as the mixture is leaned or richened.

I have practiced this myself and can verify that this works.
However, in "Modifying Throttle Plates i have added information on the Q-jets idle air bypass system.

...........Kent

[krisr]

That is true, Geoff's 66 is definately a head turning winner!. It idles, drives perfectly smooth and has stacks of power. The funniest thing though is that his 455 has better economy than my 389! So definately a person I'll turn to for advice when my 389 is back together again and its time to tune it up.

[Cliff R]

Geoff and Kenth, good information, glad to see a topic get the thorough coverage that this one has! I think we've shown that there are more than one way to set these engines up. I'll make one last brief comment, then I'm done!

With this hobby, nearly every single combination is different. It is quite rare when one gets to tune the same combo. This is the scenario that I see time and time again with vehicles brought here. Too much cam for the static compression ratio, stock advance canister adding 25 or more degrees of timing at idle, no control over the mixture screws and runs like CRAP everywhere. In the search for high performance, most particularily with street engines a larger than stock camshaft is chosen. In the vast majority of cases, the stock cam would have provided better overall vehicle performance with few or any negatives. (Believe it or not, the factory did extensive testing with camshafts and was pretty darned good at choosing them for their engines.) Once the move is made to make more HP, nearly everything behind the engine gets changed based on the new found, and at much higher rpm, USABLE power. Big lobes, tight LSA's and incresed duration are the trend with most cam selections. One must realize that this is nearly always going to MANDATE changes to both the carb and distributor. I would emphasize that, just hooking up your distributor to a manifold vacuum source is seldom if ever going to get the desired result. In almost all cases, the vacuum advance unit will need to be modified to limit the amount of advance added AND the vacuum rate at which it is applied. This is thoroughly explained if one takes the time to read this thread in detail.

I would add that the approach outlined by both Geoff and Kenth does work quite well. They have taken a stance that their methods are the best and only way to get the desired result, which I don't fully agree with, but that's what makes this hobby so interesting. I can take nearly any engine/drivetrain combination brought here, no matter how poorly the components were chosen and make it work without running the initial timing off the scale at idle. Several comments were made about my own vehicle. What I didn't mention is that some time I was sent a personal email by one of the major contributors to this thread, thoroughly outlining turning the distributor on my engine to smooth the engine out, then note the setting, and add in the needed amount of timing from the vacuum canister using manifold vacuum. I did EXACTly that, and ran the engine extensively on the street and track. What I found was that the engine idled DEAD SMOOTH if manifold vacuum were used vs ported vacuum. No other measureable difference. I didn't see any off idle smoothness benefits or better mpg's. The one and only negative from using manifold vacuum to the advance was having so much timing applied when coasting, the vehicle pretty much lost any engine braking effect at lower speeds, which is pretty much a mute point. What it boiled right down to was engine idle quality. Since I LOVE to hear the cam, I went back to using ported vacuum to the advance and less timing at idle speed. This puts a very slight "lope" into the engine at idle and virtually no engine rpm change in or out of gear. I get LOTS of compliments on the engine sound and the numbers obtained from the car speak for themselves. With a basically "stock" engine I push a 3805lb car to mid 7 second times and can drive it anywhere, anytime in any weather on pump fuel only. Recently, at our local 1/8 mile track I made 5 consecutive runs in the heat without shutting down the engine. All runs were between 7.75 and 7.78 @ 90mph in full street trim. I seldom check actually fuel mileage, but it goes approximately 200 miles on a tank of around 14 gallons.....unless I'm running my Holley carb, closer to 160-180 miles per tank.....Cliff