Ported Vacuum Source

[MontanaDiver]

I lost the fuel pump on my S3 several weeks ago. It sat in the shop while repairs were being done for several days with no fuel in the carbs. After the fuel pump was repaired, I presume the inlet valve started sticking on at least two carbs. I was able to tap the carbs and remedy one, however she still is flowing over and making the hot soak start more difficult. She also seems to be loading up, overrich at lower RPM's. Presume it is the same problem. Fuel pressure is approximately 2 PSI. I have ordered a kit for the carbs and am anxiously awaiting the repair.

On the top of the carb at the manifold side is a crosspipe. I cannot find any information about the purpose of this, wondering if this is a ported vacuum source. I replaced the distributor last Fall and would like to switch to an advance instead of the retard that is on it now. This leads me to finding a ported vacuum source.

The attached photo, one I stole from another topic has the blue line leading to the pipe. Is this a ported vacuum source? I cannot find any information in the parts catalog or the service manual detailing what this pipe does. Seems to go nowhere, just connects the two carbs. Can I adapt this for the distributor advance?

Additionally, is their a particular vacuum advance unit that anyone has found that is better than any other units?

Of course this is on a US LHD S3 OTS.

Dennis

[angelw]

Dennis Wrote: The attached photo, one I stole from another topic has the blue line leading to the pipe. Is this a ported vacuum source? I cannot find any information in the parts catalog or the service manual detailing what this pipe does. Seems to go nowhere, just connects the two carbs. Can I adapt this for the distributor advance?

Additionally, is their a particular vacuum advance unit that anyone has found that is better than any other units?

Hello Dennis,
The pipe you refer to is parts of the Emission Control System and is plumbed into the rear Exhaust Header on each cylinder bank. You could disconnect this and use the ports for vacuum. However, in original spec, the S3 used a vacuum retard and if you converted to vacuum advance, the same vacuum source can be used.

Regards,

Bill

[MarekH]

No you can't "use the same vacuum source".

The port at the left hand rear carburettor is designed to give you a vacuum signal at idle and no signal at all as soon as you are off-idle. The intention is to provide vacuum control at idle only.

"Ported vacuum" is where you don't care about signal at idle but do have a signal at partial throttle, which then tails off at significant throttle opening. The intention is to provide vacuum control at partial (light) throttle openings only.

The two are not the same and are not interchangeable.

The tan coloured US Owners Manual shows the original application for the EGR ports. Others have used the EGR vacuum port as a ported vacuum source.

Total advance at any point will by the static advance plus centrifugal advance from the distributor plus the advance or retard from any attached vacuum capsule, so you may need to adjust your static advance and check your centrifugal advance if you wish to alter the overall advance. Adjusting advance by merely substituting in an advance capsule for the retard capsule may lead to undesired results outside the operating range of the capsule.

Some cars of the period had capsules with two stubs for vacuum signals on opposite sides of the diaphragm so that they provided retard at idle an advance above idle.

kind regards
Marek

[MontanaDiver]

Most of my work on cars is limited to cars manufactured during the emission control era. I have never used manifold vacuum as a source for the vacuum advance. From some of the articles and research that I have read about usage of ported vacuum, it was partially to assist in the off idle idle stumble as engines were engineered to pass an emission test.

Unfortunately my experience is with US V8's. The retard idea is very foreign to me. I have little experience with jaguar so far. I rebuilt a Triumph motor and overhauled the carbs when I was younger and not fearful of creating a nightmare.

I will need to drag out the timing light and look at some numbers before I modify the system. I was looking for a little better fuel economy without loss of power or creating a spark pattern that burns a piston.

Hopefully, I will be able to glean some sage advice from those that are much more experienced than I.

Thanks for your assistance.

Dennis

[SRK]

Dennis, hope you don't mind but i'd like to pile onto your post.

My car had no vacuum line attached to the dizzy when I bought it. I've now replaced the complete ignition system with a SNG Barratt system and have adjusted everything with no vacuum line attached. The engine is running great, smooth idle, excellent acceleration. Have not tried any high speeds since I live in Hawaii and about 50mph is pretty much where we top out in normal driving. So my question to the forum is: can anyone think of anything terribly wrong with this?

Thanks

Steve

[42south]

My car had no vacuum line attached to the dizzy when I bought it. I've now replaced the complete ignition system with a SNG Barratt system and have adjusted everything with no vacuum line attached.

Steve

Hi
Nice location, always one of my favourite destinations.
Your car will be better off without the vacuum attached, as the SNG replacement for the S3 is a vacuum retard system, as per the original specification for the car.
This was done to meet the US emissions regs back in the day, but it hurts performance and fuel economy.
Just make sure you blank off the vacuum port underneath the left rear carb, it's there on the RHD cars, maybe the other side on yours. It's also a good move to blank the port on the vacuum unit under the dizzy.
There is lots of info in the archives, try a search.

[Adamski]

The factory fitted retard unit makes the spark fire closer to TDC and the purpose was to make it warm up quicker,burn hotter at idle and meet better emissions. As soon as you take off it is inoperative as there's not enough vacuum to power it. Early cars had a temperature controlled vacuum valve that cut the vacuum once warmed up. Again these become inoperative with age and are unavailable These capsules deteiate with age and become porous which is why many have blanked the supply. I have tried with and without and finally am running with the theory being that I get better idle.
Bottom line is that I've used disproportionate amount of time to get it to run right and have come to the conclusion that it's not bad but will never be as good as it was when it used 5 star leaded fuel. I have posted my experiences if you search Stromberg.
Many have upgraded to injected engines and I can see the sense in this. I have also seen SU conversions but have no experience in this. In the UK we didn't have all that emission pump stuff and I'm guessing it complicates matters even more. Getting the cooling system 100% perfect is critical, modern fuel burns so much hotter.

[Woolfi]

"Many have upgraded to injected engines and I can see the sense in this."
I planned in the past, to mount a injection from a Series II XJ12 on the EV12 motor. After I have done two acceleration tests against injected EV12 I stopped my planning. From 2000 to 4000 rpm the Stromberg Motor had significant more torque than the injected motors. We presssed the pedals in 4th gear at 2000 rpm and waited , what happened. The car with the Stromberg left the Injected car quickly. This was a big surprise and against common thinking.

I think at more than 4000rpm the injected motor is stronger than the carbed motor. Also the gas consumption of the injected car will be lower, most in the city.

A car with a advance capsule, which is connected to the correct vaccum port, takes less gas, than a car with retard capsule. That was the reason, why carbed carsfrom the 50ties to the eighties had
advance capsules.
Regards Wolfgang Gatza

[jagwit]

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