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#1 S3 Stromberg Bypass Valve

Posted: Sun Jul 02, 2017 4:37 am
by MontanaDiver
I am overhauling all four carbs on my US S3 V12 E type. I removed the left rear first and discovered upon disassemble a gasket cut and installed to block the airflow for the bypass valve. I am overhauling them one at a time to prevent the STUPID factor from creating the wrong parts installed, i.e. throttle shafts.

Any logical reason to block off both ports around the valve?


#2 Re: S3 Stromberg Bypass Valve

Posted: Sun Jul 02, 2017 8:12 am
by MarekH
Dear Dennis,

It cuts off a source of vacuum leaks if the bypass valves have perished over time prior to replacement. The failure mode is that they become brittle and crack due to heat/time.

You can either leave them blocked off and plug the vacuum lines at both ends or replace them and make sure they don't leak air when not active. All they do is admit a bit of extra air into the plenum when vacuum is very high, e.g. on overrun when the throttle is snapped shut at high rpm. Put another way, they don't really make any useful difference to the running of the car but do have a downside potential of introducing vacuum leaks if they fail.

kind regards

#3 Re: S3 Stromberg Bypass Valve

Posted: Sun Jul 02, 2017 2:03 pm
Hi Marek,

do any problems occur when they are permanently blocked?
I'm also just overhauling all carbs of my 71 S3 and found that the new gaskets are slightly different from the old ones.
The old, installed ones have a bypass cut away so that air can permanently go through the valve even if it's closed..

Parallel I'm thinking about blocking the temperature compensators in closed position like a well known carburettor specialist has recommended to me.

What do you think? :seeingstars:

Best regards


#4 Re: S3 Stromberg Bypass Valve

Posted: Sun Jul 02, 2017 2:50 pm
by MarekH
People block off the bypass valves because they discover the source of a vacuum leak and don't much want to strip anything down further or spend any money on something that doesn't affect normal running.

If you have the parts in the rebuild kit and have already taken everything apart, then it is just as easy to put it back together properly. The bypass valves will only do anything once you have snapped the throttle shut but the carburettors have not yet reacted to the high vacuum situation.

The temperature compensators bleed air, dependent on temperature. Hot air is thinner than cold air and so the mixture will be leaner at hot temperatures. To sort these out, you'd want to take all four off and look how they open and close by putting them in the oven. You want them all to open and close the same amount at the same time.

If you remove them and plug the holes then your engine will run slightly leaner when under bonnet temperatures soar.

kind regards

#5 Re: S3 Stromberg Bypass Valve

Posted: Sun Jul 02, 2017 4:23 pm
by MontanaDiver
Thanks for the information. :thankyouyellow: Us regulations certainly take their toll on the operation and efficiency of cars.

I am starting to doubt the abilities of the last rebuilder. I took lot's of pictures to prevent errors and found the float installed upside down. Not certain of the critical nature of doing it incorrectly, however it does point out the value of not screwing things up by taking my time. :fingerscrossed:

Proof will be when I get it running again! :swerve:


#6 Re: S3 Stromberg Bypass Valve

Posted: Sun Jul 02, 2017 5:03 pm
Hi Dennis,

you are not alone!

I also found 2 of 4 floats beeing upside down.
Choke cables secured with wire instead of clips.
All idle screws completely screwed out and the pre owner was wondering why the car was only running on choke.
All 4 carbs had different settings of the jet needles and the needles where completely stuck so that you could not adjust the mixture any more.

Now my ultra sonic cleaner is running for hours to get the dirt of the last 46 years out :hammerdrill:

Greetings from Germany Sven

#7 Re: S3 Stromberg Bypass Valve

Posted: Wed Aug 02, 2017 3:24 am
by MontanaDiver
She's all back together and starts so much better with the choke when cold. I now have to back off the choke setting immediately after starting but she runs great. I attempted to sync the carbs and got the two that have the chokes set up similar, however the other two would not lift the ball. She smells a little rich, probably try to set up with shop to check the HC levels. Still a little bit of a hot soak restart issue, but it now starts with a 4-5 second crank time when hot. Had her out in 95 degree weather.

I also tackled the brake drag and replaced the reaction valve disc. Broke the reaction valve while taking it apart, however the brakes are great.

Thank you to all the members who share their wealth of knowledge on this forum.