Ported Vacumme
 

Hello there I just got my 455 back in my car and trying to work out the bugs. My question is where is the best port to plug the distributor vacumme advance hose to? I thought I had read the manifold port is best but not sure. Does anyone know the best manual out there for engine hose routing. Thanks Brian

Depends on how the idle quality is and how the distributor is set up. I have 14 degrees initial ( no vacuum hooked up ). Then I plug the hose into the manifold, having the vacuum canister set for 10 degrees max, giving the engine 24 degrees at idle. The engine idles better at 24 than 14. Also have 20 degrees in the mechanical advance. So 34 degrees open throttle and 44 cruising down the highway.

I have a Mallory Unilite distributor after it warms up it is running decent. But it is hard starting when cold my carb has been redone by Sean Murphy. I think I need to play around with the timing and check some different vacumme options.

Choke?

Also, lower initial timing is easier to start the engine. Might need to check the carb out, as posted above. Check to see if there is accelerator pump gas when cold. Sometimes just learning the procedure, as in gas pedal pumps before starting, etc, takes time.

Yes I have the divorced choke and its hooked up.

How is the choke set up? Vacuum pulloff hooked up and adjusted?

...and does the choke blade actually CLOSE?

What carb?

Forget about vacuum advance until you have the engine well tuned and running right.

Vacuum advance will not help a cold motor in starting!
The choke being set up to be closed off all the way when first cranking the motor and added initial base timing above 10 degrees along with a starter, cable and battery set up to spin the motor fast enough are key parts, also a winter blend fuel depending on where you live.

Actually using manifold vacuum to the advance can make some engine harder to start warmed up. When you fine tune the idle speed and mixture screws using manifold vacuum advance the throttle plates in the carb are closed down more. I've ran into a few combinations here, usually with larger cams in them that refused to start if/when we were adding a lot of timing at idle via the vacuum advance on manifold vacuum.

Keep in mind that until the engine fires and starts making enough vacuum to apply the advance the timing is retarded considerably from where you fine tuned things fully warmed up and heat soaked.

I put up some good information on the topic of vacuum advance recently to help folks understand how it really works. Even many "guru's" and resident "experts" do not understand vacuum advance and continue to put up inaccurate information about it. Probably doesn't help that decades ago some very lengthy articles were written about it and they are still copy/pasted today. Although much of that information is helpful there are several flaws with it and it only serves to further confuse folks when they are trying to set timing and tune their engines.

What has thrown another monkey-wrench into the equation here is that cam companies have been pushing tight LSA so folks can have the "bling" factor when they idle thru the parking lot at your local cruise night or car show. Combine that with the proverbial brick wall of 9.5 to 1 compression for pump gas and many of these engines end up with too much overlap and not enough compression to offset the negatives involved there. The poor signal produced at idle from tight LSA and increased overlap require MORE timing and fuel to make the engine happy. Where an engine will a very well chosen camshaft for the CID and compression will make good vacuum and fine with 8-12 degrees (or less) initial timing at idle it's not uncommon these days to see folks having to use upwards of 18-24 degrees and a LOT of idle fuel available to get their new engine to be happy below about 900-1000rpm's.

Not really a big deal if you have good tuning skills and I'm not condemning tight LSA, just pointing out that it's easy to run into tuning issues if/when the engine isn't making good vacuum at low RPM's because of the cam choice made during the rebuild.......Cliff

"...But it is hard starting when cold..."

Just cold; or cold from sitting a few days?

The gas evaporates out of my Qjet if I let it sit for 2-3 days. Then it takes a lot of cranking for my mechanical pump to refill the carb.
If my car sits, I use an eyedroppers worth of gas in the carb for prime so I don't have to crank so much...

Today's 10% cut pump fuel also makes a non fuel injected motor 10% harder to light off, especially when temps fall below 40 degrees!

Why does it take "a lot of cranking" to get the carb to refill?

Why would you have to put any fuel into the carb to get it to fire off?

The accl pump does that for you and the fuel under the pump is trapped in a confined space and you still should get several shots of fuel to the engine even if the fuel level in the carb is low or empty.

In many of these cases where we see folks have difficult cold starts the accl pump seal is defective and weak or no pump shot, or it has to get wet again with fuel before it swells back up and starts working.

A good modern accl pump assembly with a flouroelostamer seal and garter spring to keep in in contact with the pump bore will not swell up, shrink, fail or not provide a positive pump shot during a cold start.

Another problem we see with poor or lack of good accl pump function is failing to gently seat the check ball during the rebuild to make sure it's not allowing fuel above it to drain back some.......Cliff

https://cliffshighperformance.com/Qu...-pump-assembly

I think I need to figure out how to adjust the choke. When the carb came in I hooked the small rod to the choke lever and that was it. It keeps the plate about 3/4 shut. I have a service manual I am going to see if it has any info on that. I am thinking it is choke issue and base timing issue. I am going play with this weekend, Brian

The divorced choke should lift the fast idle cam to the highest position and tightly close the choke flap when you move the throttle once on a cold start. At the same time the accl pump will put a shot of fuel into the intake.

If the choke flap isn't going completely closed the link is too long or the choke coil isn't working correctly. Could also be a linkage issue with the carb but the first statement assumes the carb is correctly assembled.......Cliff

Cliff would you recommend bending the choke rod so the throttle plate is closed at startup? Thank you for your info! always on the money

Yes, if the choke rod has Bend in it to absorb lenght then bend it out some to get the choke fully closed when you have stabbed the gas pedal once, as that's why the bend is there.

Then you will likely have a far more happy to Fire over motor when it's cold!

This will work fine as long as the choke will still open all the way when hot and not drift back to a somewhat closed position.

Manifold vacuum generally best for a driver
 

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)

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.

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.

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!

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.

ported idle temps
 

"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.

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.

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.

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.

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.

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.

"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

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.

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

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).

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

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.

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.

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.

Yes, sorry, I should have been more specific: my Q-Jet is a 1970 RA III carb, and it does not have ported vacuum, but I do appreciate some later carbs that came on engines with EGR systems do.

Interesting, did not know that. I can learn something here every day.

Sent from my moto g(6) play using Tapatalk

No 1970-74, except 1971 455H.O., Pontiac Quadrajet uses the "ordinary" ported source for the ignition vacuum advance above drivers side mixture screw.
Instead these engines uses the TCS system to control the vacuum advance.
No vacuum advance at idle, only with cold or to hot engine and when final transmission gear is used.

Ported Vacumme
 

I wanted to give an update on my 70 Goat.I have been doing some tuning on my car when I got back from the engine shop the mechanic had some vac lines going to the wrong places. He had the distributor vac line going to the choke pull off. Once I sorted that out it started much easier with the choke hooked up. Then I had the distributor vac advance hooked to a vac port in the back of the carb. Needless to say still running rough a drinking gas. Today I hooked vac advance line to manifold vac wow made a nice difference. It ran smoother more responsive and better gas mileage. Took a winter 40 mile drive temp never got past 160 oil pressure 60 pounds.car was running sweet. Cant wait to get my new DUI distributor installed next week. Thanks for all the helpful input. Brian

Benefits of manifold v ported
 

That's good news.

Looks like a test case for why I said that manifold (unless a big duration cam) is how these motors were intended to run. Manifold vacuum leads to better throttle response and lower overall temps (especially siting in traffic) v ported. Ported was the engineers response to EPA regulations and not at all a performance enhancement.

I think of it as the timing is already there as you give it part throttle rather than that half second delay if running ported before the dizzy advances. (Obviously on hard/full throttle there's no difference between manifold and ported because vacuum drops, and just get the mechanical advance.). But most folks drive their cars in traffic and there manifold is typically better on a stock-ish setup. That's my experience.

"Then I had the distributor vac advance hooked to a vac port in the back of the carb."

Any port on the back of the baseplate would have been manifold vacuum. There are no ported sources available on 1970 Pontiac carburetors that I am aware of. The rear of the baseplate has 1/8"NPT threads for various fittings used by the factory, all would be manifold vacuum.......Cliff

The only port I see Cliff is the tree coming out the back for my power brakes other tree port is capped off. I am using the port on the intake manifold seems like the engine likes it. Does the vacumme on the intake manifold port decrease as the rpm goes up? Thanks Cliff

RPM has nothing to do with vacuum production falling off far as manifold or ported. Light engine load produces high vacuum. Vacuum changes or drops off quickly with increased throttle angle/engine load. The harder you load the engine the less vacuum is produced. At very heavy and full throttle the vacuum available above and below the throttle plates is pretty much the same, or zero.

ALL sources below the throttle plates will show basically the same readings and operate the vacuum advance in the same fashion.

This includes the "Tree" in the rear of the baseplate or any other source in the intake manifold.....Cliff

I would avoid using a source on a single runner for the ignition vacuum advance, like the one on the #1 runner on a Pontiac intake, as the signal pulsates and are somewhat weaker than any plenum source under the carburetor throttle blades.

Do we know how the factory had the vacumme routing going to the intake manifold vac source? My 70 assembly manual does not show---car seems to like the vac advance going to the intake manifold vac source.