[66sprint6]

Our '67 LeMans has a carburetor from a 1970 GTO. I'd have to look at the number, but I know that that's the application it is used for. It was nicely rebuilt from a known reliable vendor around here. It works great, but one thing that I've noticed is that is has no connection for ported vacuum. The typical location to the right of the fuel inlet is blank. I believe that the factory setup uses the TVS switch on the intake and the hoses routed to manifold vacuum, ported vacuum, and another hose to the distributor. in any event, it has a hose that's supposed to go to a ported connection. So the question is, if there is no connection to a ported outlet to the right of the fuel inlet, and none on the side either, where does a '70 GTO get the ported vacuum source? I was thinking of just getting an earlier carb with the ported connection to the right of the fuel inlet, but I wanted to check first. Thx.

[steve25]

As opposed to years previous to 1970 and except for the 1970 455 HO motor, ported vacuum was not used on all other 1970 Pontiac V8 motors including the 2BBl motors.

The TCS system used full manifold vacuum.

Not sure if you can drill the carb and press in a steel or brass tube.

[Kenth]

Originally Posted by steve25

As opposed to years previous to 1970 and except for the 1970 455 HO motor, ported vacuum was not used on all other 1970 Pontiac V8 motors including the 2BBl motors.

The TCS system used full manifold vacuum.

Not sure if you can drill the carb and press in a steel or brass tube.

Not true at all!

1970-71 and 73-74 engines, except 1971 455 H.O., uses the Transmission Control Spark system, TCS.
1971 455 H.O. uses a ported vacuum source with a TVS to control the vacuum advance.
1972 uses the similar Speed Control Spark, SCS.

FWIW

[steve25]

Well then this info out of the winter 1994 issue of Pontiac Enthusiast magazine is wrong, is that what your saying Ken?

[PontiacHO]

Originally Posted by 66sprint6

Our '67 LeMans has a carburetor from a 1970 GTO. I'd have to look at the number, but I know that that's the application it is used for. It was nicely rebuilt from a known reliable vendor around here. It works great, but one thing that I've noticed is that is has no connection for ported vacuum. The typical location to the right of the fuel inlet is blank. I believe that the factory setup uses the TVS switch on the intake and the hoses routed to manifold vacuum, ported vacuum, and another hose to the distributor. in any event, it has a hose that's supposed to go to a ported connection. So the question is, if there is no connection to a ported outlet to the right of the fuel inlet, and none on the side either, where does a '70 GTO get the ported vacuum source? I was thinking of just getting an earlier carb with the ported connection to the right of the fuel inlet, but I wanted to check first. Thx.

Have you tried running live vacuum to the advance can? If it runs well then you’re done. Most likely will run cooler at low-idle speeds anyway.

[steve25]

This is an option and is worth giving a try, since there’s no harm, but most times there are two issues that take place from this.

1) the rpm goes up much higher then needed, this in turn makes you back down on the idle speed such that you then can end up with a off idle stumble due to the transfer slot’s and or the accelerator pump now being misadjusted .

2) if the above does not take place then you can end up with a ruff idle due to excess timing.

[Formulajones]

I run my 70 (original carb) off full manifold vacuum, bypassing the TCS setup that originally only supplied vacuum to the distributor in 3rd gear. Now with vacuum all the time it does better on gas around town in stop and go traffic and runs a little cooler as you would expect when the engine gets a little more part throttle and idle timing, it's just happier.

This would not work well however if you have a vacuum advance that adds a ridiculous amount of timing, some as much as 20 degrees, would not work in this situation if you have any decent amount of curve in the distributor.

So I've modified the vacuum advance to only provide about 12 degrees of timing when it's at full vacuum. This gives me great drivability, mileage, good stable idle with more idle vacuum, just an all around happier engine.

Up to you, just something you have to experiment with and spend some time dialing in.

[Kenth]

Originally Posted by steve25

Well then this info out of the winter 1994 issue of Pontiac Enthusiast magazine is wrong, is that what your saying Ken?

Pontiac Enthusiast is correct and your description was NOT.
BTW, my name is Kenth, thank you.

[Kenth]

Originally Posted by steve25

This is an option and is worth giving a try, since there’s no harm, but most times there are two issues that take place from this.

1) the rpm goes up much higher then needed, this in turn makes you back down on the idle speed such that you then can end up with a off idle stumble due to the transfer slot’s and or the accelerator pump now being misadjusted .

2) if the above does not take place then you can end up with a ruff idle due to excess timing.

IF this was true, just about EVERY engine pre-1968 would suffer from these issues, as most of them uses manifold vacuum to the ignition vacuum advance.

[steve25]

The only thing wrong in my post was my comment in regards to the pre 1970 vacuum arrangement..

[Kenth]

No 1970 Pontiac carb uses a ported source for the ignition vacuum advance, not even the 455 H.O.

The TCS/SCS systems does not provide full manifold vacuum at idle at operating temperatures.

FWIW

[Shiny]

Did the TCS provide ANY vacuum to the distributor advance below high gear or was the TCS solenoid an "on/off" valve?

[steve25]

A solenoid can only be on or off, no in between unless it’s malfunctioning.

[Shiny]

Understand the solenoid part, just wondering if there was something else built into the housing or fittings that "leaked" or otherwise provided vacuum advance at idle or in lower gears?

I suspect not, which means a 70 V8 had ZERO vacuum advance at idle, correct?

[65 Lamnas]

Originally Posted by 66sprint6

Our '67 LeMans has a carburetor from a 1970 GTO. I'd have to look at the number, but I know that that's the application it is used for. It was nicely rebuilt from a known reliable vendor around here. It works great, but one thing that I've noticed is that is has no connection for ported vacuum. The typical location to the right of the fuel inlet is blank. I believe that the factory setup uses the TVS switch on the intake and the hoses routed to manifold vacuum, ported vacuum, and another hose to the distributor. in any event, it has a hose that's supposed to go to a ported connection. So the question is, if there is no connection to a ported outlet to the right of the fuel inlet, and none on the side either, where does a '70 GTO get the ported vacuum source? I was thinking of just getting an earlier carb with the ported connection to the right of the fuel inlet, but I wanted to check first. Thx.

You can add a ported vacuum advance port if you want one.

[Formulajones]

Originally Posted by Shiny

Understand the solenoid part, just wondering if there was something else built into the housing or fittings that "leaked" or otherwise provided vacuum advance at idle or in lower gears?

I suspect not, which means a 70 V8 had ZERO vacuum advance at idle, correct?

The way it worked a couple things had to happen. It ran through a transmission control spark solenoid, and the vacuum was provided by a temp controlled switch on the intake manifold. The carb in 1970 had no provision for ported manifold vacuum.

I'm trying to remember if it provided vacuum at all to the distributor when it was cold, I think it might of to aid with cold start, it's been so long since I disconnected mine. But what I do know is once it was up to operating temp, the only time vacuum would be applied to the vacuum advance on the distributor is when the transmission was in high gear only. So 2 criteria had to be met, operating temp at the manifold vacuum switch, and a switch on the trans that told the solenoid it was in high gear. Both automatics and 4 speeds had this setup.

So to answer some of your question, you are correct in thinking there is no vacuum advance working at idle or low speed (gear 1 and 2) conditions as long as the engine is up to temp. So if you are doing a bunch of stop and go driving the engine is not seeing any vacuum advance at all unless you get up into 3rd gear, or 4th gear with a manual.

It was an emission compliant deal, as it adds vacuum advance hydrocarbons go up is how it was explained to me. California in later years didn't even like the factory setup as I had to have my 70 Formula e-checked and even with the TCS system functioning they wanted the vacuum advance unplugged from the distributor at all times and even stuck a decal on the inner fender stating such. I guess if you got pulled over for one of those random road side checks they were doing at the time. Of course as soon as I left the station I plugged it right back in as the car didn't run as well without it and mileage suffered.

These days I don't use the TCS setup at all and run the vacuum advance straight from manifold vacuum as the car just runs better all around with it in all conditions, idle, traffic, cruise etc....

[OG68]

I think that if the car was overheating, the temp sensor would send a signal to open up the TCS vacuum valve allowing vacuum at all times, increasing the idle at lower speeds. Same principle as the 68-69 models with the temperature vacuum switch.

[Joe's Garage]

Originally Posted by Shiny

Understand the solenoid part, just wondering if there was something else built into the housing or fittings that "leaked" or otherwise provided vacuum advance at idle or in lower gears?

I suspect not, which means a 70 V8 had ZERO vacuum advance at idle, correct?

At normal operating temps, vacuum was off unless in high gear.

Below 85* (cold) vacuum was applied to help with cold idle speed.

Above 220* (hot) vacuum was applied to help with cooling.

Very simple system when it all works.

Thanks to Kenth for the documentation. I click 'SAVE' on almost everything he posts.....

[Cliff R]

Ported vacuum advance can be added to a 1970 factory Q-jet as Jim mentions.

However this is NOT just as simple as drilling the main casting for a vacuum fitting. The baseplate has no source to supply that port so it has to be added. The factory used a "slot" to provide ported vacuum and it is very precisely located so that with the slightest movement of the throttle right off idle you get FULL MANIFOLD VACUUM to the advance.

Folks who claim their engines run cooler need to go back and read that statement again. I see this highly inaccurate statement (switching from ported to MVA to cure overheating) "regurgitated" over and over and over again on just about every blog and Forum in existence. That is followed with more highly INACCURATE info about how ported VA is simply for "emissions", not good for your engine, adds timing at heavy throttle and high RPM's, etc, etc. etc.

A correctly located PORTED vacuum source for the distributor does EXACTLY the same thing as a MANIFOLD located source except idle and coasting. So the ONLY way it could possibly help or eliminate a running hot or overheating issues is if and when the driver of the vehicle idled everywhere they went.

With that said you may see a superficial improvement using manifold vacuum to the advance, but rest assured if and when you start driving the vehicle it's going to run just as hot as it did before you made the switch. I guess one could throw in some statement or scenario where they were idling in a parade all day and saw an improvement, but I can tell everyone reading this that I've tuned more running-hot/overheating Pontiac engines than 99.9 percent of the folks who will read this and switching to a manifold source to the VA didn't cure any of them.

Matter of fact most were completely cured simply by fixing issues with the carb and distributor and getting the timing/fuel curves spot on. Those I couldn't help simply had WAY too much quench in them and/or very poor camshaft choices for the application.

I helped/cured quite a few pre-1968 set-ups by speeding up the water pump by going to the later set-ups. Several others by removing those POS stamped steel waterpump impellers and installing a factory cast one. A few times going to HUGE radiators and fans with enough blades on them to just about suck you thru the radiator helped, but in ALL of those cases you are simply treating the symptoms and not the disease........FWIW and IMHO.......

[Kenth]

Ported or full intake vacuum for vacuum tuning has been discussed to death. And yet this is something that only affects the idling characteristics, certain exhaust emissions and the character of the engine and how it is built.
For those who are tired of this, look at what the car manufacturers' engineers did, they had the knowledge, believe nothing else.

For others, the reason for delayed ignition timing (ported vacuum) was emissions requirements from the authorities.
A delayed ignition timing so that the combustion disappeared burning out through the partially open exhaust valve, heating the cylinder heads so that the HC content would be reduced, so it is not surprising that people report reduced operating temperatures in the engine at idle or in traffic jams when they connected the vacuum advance to full intake vacuum on their cars from 1968 and later.
Many people think that the engine sounds "more aggressive" with ported vacuum and do not understand that the engine burns fuel past a partially open exhaust valve then, and that the larger throttle opening required when the engine is running less efficiently causes the engine to "dieseling" when shut off.
Here the use of the idle solenoid comes in as an answer from the manufacturer.
Ported vacuum is good, IF the engine has a camshaft that is so hot that it does not generate a strong enough vacuum signal so that it can fully activate the vacuum advance unit at idle.
With such a hot camshaft, problems such as "wandering" and weak idle with a large difference in rpm between Neutral and Drive can be experienced if you have the vacuum advance unit connected to full intake vacuum.
The cure here is a ported connection, earlier initial ignition, recalibrated low-speed circuit in the carburetor with added idle bypass air which is needed for a delayed ignition at idle.
There is really no reason to rebuild the entire car/engine for this, maintaining the old engine the proper way will go a long way.
At least, this is what i have found.

FWIW