The Stovebolt Site Forums The Shop Area Engine Shop vacuum line for vacuum advance

Hi folks,

I have a 1956 Chevy 3100 with a rebuilt 235. My fist build ever. The modifications I added is a stock 261 camshaft, dual 2 bbl. progressive Carter / Weber carbs, Mini HEI distributor, bored out .030 with an "848" head, Fenton 2x1 intake with water heat to the manifold and a Fenton split manifold headers. I am trying to hard pipe everything with min. rubber hose. I am trying to make a real neat look.

Here is where I am needing assistance. The Fenton intake has a port between the carbs that is on the side not the top where the carbs sit. I was told that I could hook up a 3/8" vacuum line to from that port to the new frame mount brake booster that I have installed.
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I was thinking of maybe installing a "T" so that I may also run a smaller line to the distributor. Is this a good idea? Is it better to run the vacuum advance line off one of the carbs? If so which carb or does it matter? The mini HEI and the carbs where purchased from Stovebolt Engine Company. The Mini HEI is from a G.M. 2.8L “S” truck 1982-1984. I do not know if all the info is needed but I just want to throw it out there.

Lastly, as I am learning how everything works I know gas feeds from the carbs in thru the intake to the cylinders but how does it keep from going into the vacuum port off the Fenton intake? I guess what I am asking how does gas not get into the brake booster? The black port on the booster is that a check valve? If I was able to use that port also for the distributor how would that keep gas from going to the distributor?

thanks guys just trying to figure it out.

http://i1291.photobucket.com/albums/b554/stovebolt56/engine2_zpsadd372c4.jpg

http://i1291.photobucket.com/albums/b554/stovebolt56/2013-06-13_01-36-16_906_zps6c51e675.jpg

http://i1291.photobucket.com/albums/b554/stovebolt56/BetAftersandandpowder_zps27a7513f.jpg

TinyJaime,

Here is how I plumbed the "ported vacuum" to the distributor on dual Carter-Webers.

There are ported and no-ported vacuum ports on the side of the Carter-Weber carburetor.

Simple definitions:
ported vacuum - above the throttle plate/flapper in the carburetor - little or no vacuum at idle - this is where the vacuum advance should be connected
nonported vacuum - below the throttle plate flapper in the carburetor - vacuum at all times when the engine is running

For the heck of it, I ran a tee-ed hose w/fitting from both carbs to the distributor vacuum advance unit (I thought it would look nice).

You can also see the port on the side of the intake manifold that runs to the wiper motor. This is non-ported vacuum (it has vacuum all the time) - this is not appropriate for the vacuum advance.

The gas going into the combustion chamber and flowing as exhaust gases out the exhaust creates the vacuum (another simplistic explanation) - vacuum sucks (sucks gas/etc towards the combustion chambers in the block). That is why gas does not go into the wiper/brake-booster - the vacuum is sucking from those devices. When you get a back fire on the carburetor side of the valves (as apposed to an exhaust-side backfire), that back-fire will blow towards the wiper motor and towards the vacuum brake booster (the vacuum is gone).

Nice looking motor.

Thanks Don. Tim that gives me a great idea now. You have a great looking motor. From what i understood since i have the electric wipers i do not have to worry about vacuum like on the older models but i should use the non ported side for my brake booster. i will plumb the distributor to both ported sides of both carbs like you did for the neat look. Once i get all my pipes constructed i will have them powder coated with the cermachrome like i did the fan and the by pass oil canister. I am in it too far now and its almost as i visioned.

Btw what size line should i use for the distributor?

I used the original size/length line in stainless steel (Jim Carter or Classic Parts or Chevs of the 40s - whichever was lowest price). I think it is a 1/8" line, but that really does not matter. You simply have to get the line to attach at one end to the fitting on the vacuum advance unit on the carburetor, and cut off the fitting on the other end of the line and adapt the line to whatever fitting you need at the carburetor(s).

Jamie:
The line to the distributer (vac advance) MUST go the the carb ABOVE the throttle plate. You were talking I think about Teeing the line to the brake booster. That won't work as it would be constant vacuum from the intake, causing full advance all of the time..The advance line needs to be on the carb ABOVE the throttle butterfly, doesn't matter which carb

Thanks guys Yes I agree now that the line must go above the carb plate. I will mock up one tomorrow and get one ready to bend. I will run one to both carbs like Tim did on his. Oh thanks Tim I just saw all the pics of yours and gave me some good ideas. I like what you did on yours. after seeing yours I think I can complete my engine with all the ideas I now have.

Thanks guys