[455abody]

It started a easier today when cold but had to feather the gas for a couple minutes it was wanting to die. Once warmed up i hooked my vac gauge up and the most manifold vac I could get was 15 by adjusting the bottom carb screws. Advanced my timing to 12 or 13 baseline took for a drive I could hear some light pinging when I stepped on it. I then backed off to 8-10. I think I need a pro to check out.

[Will]

Originally Posted by moontower69

That's my experience. Ported is bs emissions creation of early 70s and will just increase your operating temps at the stoplight big time. Ported takes a good running engine and makes it run hotter and hurts throttle response.

Like Cliff said, once you get it tuned properly, manifold should be fine unless cam selection is way off (ie, too much duration for the compression ratio and combo of car and gearing)

I think you may have misinterpreted what Cliff wrote. This was discussed rather extensively pretty recently here on the forum so the thread shouldn't be too hard to find.

Your experience and mine are somewhat opposite. I've used ported vacuum successfully on several engines and prefer it to a manifold source. Never have had overheating or any other issues.

[steve25]

When are you hearing this light pining, when your stepping on it enough to be into the secondary's?

Since you have a vacuum gauge install a longer hose on it so you can read it while driving and report back as to the vacuum reading you see when the pinging starts to take place.

I would wager a bet that you need a tad fatter jetting, not less timing!

[tc]

Originally Posted by 455abody

It started a easier today when cold but had to feather the gas for a couple minutes it was wanting to die. .

Now you need to adjust the open distance of the choke blade and fast idle rpm.

As for having advance vacuum at idle, I adjust my Holley carbs for a square transition slot opening before I place the carb on the engine. This carb needed more throttle opening without the advance vacuum. Having the added degrees of timing at idle leaves the throttle blades showing a square of the t slots.

[moontower69]

"I've used ported vacuum successfully on several engines and prefer it to a manifold source. Never have had overheating or any other issues."

I don't dispute your experience at all. I'm simply saying it's a verifiable fact if you "A-B test" that ported leads to higher idle temps v manifold.

Why, because less timing at idle increases engine temp. That's all. You have way more knowledge and experience than me and I'm not trying to question that or start the endless debate about ported v manifold.

Your preference for ported is valid but it doesn't change fact that ported increases idle temps. Not an ego issue for me, just factual statement.

[steve25]

Fact! Only a mis tuned/ Camed motor would need any sort of vacuum advance added in at rpms below 1200.

Fact! Added timing put in by means of vacuum advance will always make a motor run cooler and efficient fuel wise at part throttle cruse !

Part throttle is the reason , at part throttle above idle the minimal charge takes much longer to burn hence the need for more timing advance.

[Kenth]

If you run around cruising at idle speeds you certainly can have a heating problem with a ported source for the ignition vacuum advance.
Most of us are running above idle speeds going somewhere and can enjoy normal engine temperatures due to an activated vacuum advance, ported or constant vacuum source.
The only difference is that using a ported source and a properly adjusted carburetor idle/low speed circuit you don´t have to chase a certain ignition vacuum advance curve at idle speed due to all sorts of engine modifications, hotter camshafts etc.
Above idle speed and part throttle you will always have an activated vacuum advance.

[ta man]

Originally Posted by moontower69

"I've used ported vacuum successfully on several engines and prefer it to a manifold source. Never have had overheating or any other issues."

I don't dispute your experience at all. I'm simply saying it's a verifiable fact if you "A-B test" that ported leads to higher idle temps v manifold.

Why, because less timing at idle increases engine temp. That's all. You have way more knowledge and experience than me and I'm not trying to question that or start the endless debate about ported v manifold.

Your preference for ported is valid but it doesn't change fact that ported increases idle temps. Not an ego issue for me, just factual statement.

Every engine is different..mine will not tolerate manifold vacuum..But I've had other engines that like manifold vacuum ..there are no rights or wrongs.

[Will]

Originally Posted by ta man

Every engine is different..mine will not tolerate manifold vacuum..But I've had other engines that like manifold vacuum ..there are no rights or wrongs.

If my engine likes 16 degrees timing at idle there are a couple ways I can get there. I can run a long mechanical curve that starts at 6 degrees and add 10 degrees of vacuum advance at idle with a manifold source or I can run a shorter mechanical curve that starts at 16 degrees with a ported vacuum source. Engine idle temps will be the same either way.

I've not had an engine that wanted more than 16-18 degrees at idle but I've never run what most would consider a "big" cam either. 232 @ .050" in a 400 is as big as i've gone.

Just curious here - how many people have hooked up a gauge and measured vacuum at idle and found that vacuum increased beyond about 16 degrees of timing? If you did see an increase, where did it stop increasing and what was the combination?

I submit that if your engine runs cooler at idle with the timing advanced well beyond the point where vacuum stops increasing your carb probably needs some work.

[Kenth]

Most 1967 and earlier Pontiac V8´s (and others) run with constant vacuum to the ignition vacuum advance, 6° initial + 20° vacuum advance, total 26° at idle from factory.
These engines also got a somewhat quicker centrifugal advance, to 2-3000 rpm´s, than later engines to keep the engine happy at part-throttle.
Camshafts used produced enough vacuum to keep the vacuum advance happy fully activated at idle.

[steve25]

Unless you have a very under Camed big cid motor you will find that most Cams with a Intake duration of above 228@ .050" will make more vacuum above idle, then at idle and that is the determining factor along with compression ratio for being able to run full Manifold vacuum or not.

[Cliff R]

"Every engine is different..mine will not tolerate manifold vacuum..But I've had other engines that like manifold vacuum ..there are no rights or wrongs."

My current and last 455 were not fond of running a lot of timing at idle speed. They roar to life instantly after shut-down and idle smooth, produce plenty of vacuum and the tach barely moves when the trans is placed in gear. If I move over to manifold vacuum to the advance they start to "skip" and "miss" slightly and generally just unhappy with all that timing on them.

I can re-adjust the mixture screws slightly and idle speed but then I'm left with more drop in RPM when the trans is placed in gear and slight "skip" in the exhaust note that is not present when running less timing at idle speed.

The engines that have been brought here for running issues that liked, wanted and responded well to a butt-ton of timing at idle speed have all been lower compression and "hefty" sized cams in them. So there may be scenarios where you can improve things some running the timing off the scale at idle speed. The problem one quickly runs into when doing this is that the VA being used will require a very low cut-in point as engines with low compression and hefty cams may not produce enough vacuum at idle speed to effectively operate most VA units. They make VA units that are adjustable for spring tension and cut-in points for this purpose. I don't like or use them here, I much prefer the VA units that control how much timing is added instead, but they are becoming more difficult to find these days......Cliff

[ta man]

A good idle in park with an easy transisition into gear with a strong idle in gear is what I try and achieve.
More often than not on these threads we see guys discussing trying to achieve a "good idle" when they are in park and in a garage.
To me a good idle is when you are sitting at a stop sign or a traffic light and the engine has a strong idle not barfing all over itself or surging.
My normal routine is get the car running decent pull it out of the shop..drive a mile to the stop sign readjust idle speed and idle mixture screws if needed. Drive another 10 miles get some heat into the engine at highway speeds. Pull into my neighborhood Coin wash..readjust again. Sometimes it takes a bit especially if tuning an engine for the first time.

[Cliff R]

You nailed that deal down, and why I use and prefer to run less timing at idle and throttle plates opened a tad further. I can superficially tune most set-ups for smoother idle by adding a butt-ton of timing at idle, but many will have the throttle plates so far closed that it becomes difficult to establish a stable idle in and out of gear when a slight load is placed on the engine. I want to see very little RPM drop when the trans is placed in gear, and rock solid idle quality in gear when the vehicle is stopped. This only happens when final tuning is done with the engine fully heat soaked.

One major problem I see here with troubled vehicles brought for custom tuning is some "creep" in the advance curve right at idle speed. If you have spring in the mechanical advance that are allowing any timing in at idle speed it's going to be difficult to tune and you will see much greater RPM changes in and out of gear. You'd be surprised how many vehicles have been brought here for carburetors issues that were fixed by putting stronger springs in the distributor or throwing all that imported advance curve crap out and putting stock parts back in them......Cliff

[RWS]

The factory port for vacuum on a Q-Jet is located at the bottom of the back of the carb, and that's probably the best place to attach the hose from the distributor.

If I recall correctly, though, Q-Jets don't have ported vacuum at all, and so no matter where you plug in the hose you are running vacuum advance all the time unless your engine has a functional TCS solenoid, in which case you'd only get vacuum advance in certain gears (third and fourth on a manual transmission car).

[455abody]

My newly rebuilt poncho is coming along,I was able to get the factory choke working that was a relief. The timing curve seems off with the new engine.I am running a Mallory Unilite distributor. I ordered a new spring set to install some lighter springs. Then I found out it would take an act of God to install them.You have to dismantle the whole bottom part of the unit to put in a stop bushing and new springs.Plus it is 20 years old so I made the call to DUI and ordered a custom unit from Steve for the 455. The car is running and starting smooth but lacking the punch of the old motor with bearings that were worn to the copper. I hope the new DUI distributor can help me in the power department. It sure nice to have it back in the garage. Thanks for all your good advice, Brian

[Will]

Originally Posted by RWS

The factory port for vacuum on a Q-Jet is located at the bottom of the back of the carb, and that's probably the best place to attach the hose from the distributor.

If I recall correctly, though, Q-Jets don't have ported vacuum at all, ...

Some Q-jets most definitely do have ported vacuum ports. Cliff wrote rather extensively about ported vacuum and where the source for it is on a Q-jet in a post not long ago on this subject.

[Greg Reid]

They definitely have ported vacuum. I'm using it on my car and have used it on the last two Q-jets I used. Front lower passenger side.

[Kenth]

Front lower passenger side in throttle plate is the EGR port, this port is too weak for the ignition vacuum advance since it is a bleed off port.
However, it can be used if the bleed off hole/slot is closed.

The regular port for ignition vacuum advance is on drivers side, right above the idle mixture screw.
This port is for most units ported/timed, and for others it is a constant vacuum port, check location of slot, if below blade it is constant, if above it is ported.

[Formulajones]

Originally Posted by Kenth

Front lower passenger side in throttle plate is the EGR port, this port is too weak for the ignition vacuum advance since it is a bleed off port.
However, it can be used if the bleed off hole/slot is closed.

The regular port for ignition vacuum advance is on drivers side, right above the idle mixture screw.
This port is for most units ported/timed, and for others it is a constant vacuum port, check location of slot, if below blade it is constant, if above it is ported.

Correct, there are some units that simply don't have a ported source for the vacuum advance.