Vacuum Fuel Systems

[fossit]

Could anyone please explain to me how these systems work?

Thanks in advance,

Fossit.

[Ivan Saxton]

A vacuum tank is a fairly simple device. They generally work very well if in proper order. In a word picture, there are two chambers. The top chamber contains float, lever, and valves. Connected to the top cover are the fuel line from the tank, a vacuum line from the intake manifold, and a vent to atmosphere. It can be convenient if there is also a removeable plug sealing a hole that can be used to fill the tank initially if it has been empty after the car has been sitting for a while. In practice I find with the Roamer Duesenberg if there is fuel in the tank, a few seconds spinning the engine on the starter with the choke closed is enough to start and drive.

When the top chamber empties the float falls, which closes the vent to atmosphere while allowing air into the outer and lower supply chamber, and opens the vacuum valve drwing fuel from the main tank. There is a bakelite flapper valve beneath the vacuum chamber which seals automatically. When the vacuum chamber is filled to the correct amount the float rises, shuts the vacuum and opens the vent, then the bottom chamber is replenished from the vacuum chamber.

For a vac tank to work properly the float must not leak, the valves must seal and open as intended, there should be no extraneous leaks, and it is preferable that the linkages and not badly worn. Repair is simple and self-evident. You just consider the basics. If the float leaks so it drowns, the tank may feed raw fuel direct into the inlet manifold, which ruins the economy and the black smoke makes you look a bad neighbour. I understand that some people have experienced annoyance due to fuel starving on a long hill running with the throttle wide open and therefore low manifold vacuum, but I have never encountered this. Vacuum tanks were made in different sizes, and obviously a large vac tank will better feed a larger engine. Best reference is Dykes Automobile Encyclopedia. Everyone has one.

Regards, Ivan Saxton

[fossit]

Thanks Ivan, really hard to get info on web. Stacks on vac

wipers, brakes, etc. Again, appreciate time and effort for

reply. And good job AACA.

[jrbartlett]

Send me a personal e-mail with your snail-mail address and I'll send you a copy of a Stewart Warner technical writeup on their vacuum tanks. jbartlett1@houston.rr.com

[carbking]

If you drive the vehicle, consider:

A) vacuum systems did not last long in production after a reliable pressure pump was introduced.

as new fuels become more volatile, vacuum systems give more and more trouble.

Jon.

[Mark Shaw]

Vacuum tank systems work great as long as you drain all the fuel from the vacuum tank and carburator as part of your winterizing process. I just close the valve at the top of the tank with the engine running until it runs out of gas.

[ply33]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If you drive the vehicle, consider:

A) vacuum systems did not last long in production after a reliable pressure pump was introduced.

as new fuels become more volatile, vacuum systems give more and more trouble.

Jon. </div></div>

I have no experience on point B. However on point A: Carl Breer wrote in his autobiographic book on Chrysler Corporation's early years that the issue of vacuum fuel tanks was not reliability. It was cost. Stewart-Warner had all the patents sewed up and all manufacturers had to buy from them at whatever Stewart-Warner decided to sell at.

Ford got around it by using gravity feed. Chrysler and others got around it by getting a a totally different pump, the mechanical one, developed by AC. Apparently one big stumbling block on the introduction of the mechanical pump was the redesign needed on the engine. But once that was done the manufactures could play AC off against Stewart-Warner and get much better prices.

Once the down draft carburetor came into use it was all over for the vacuum tank (and gravity feed) fuel systems.

[Jan Arnett]

I have vac tanks on four early cars (Star, Moon, Dodge) and they all work well when taken care of. Don't shy away from them but vac wipers are another thing.

[Guest Silverghost]

Ivan has a super post here as he usually does !

Vacuum leaks are a big problem with these old units. The old cork gaskets dry out and crumble up...thus you get vacuum leaks.

I have foud that the two cork gaskets are usually at fault. One gasket that seals the top control of the unit to the outer Autovac tank; and the second that seals the inner to the main outer tank. Replace these with new gaskets.You may have to make your own. seal all screws on top lid also! Make sure there are no vacuum leaks! This usually solves the problems. You can tolerate no vacuum leaks! Also make sure that the fuel valves seal properly including the vent valve to the top vent!(see Ivan's post!)

Make sure that vent on main gas tank or cap or tank is open and not plugged with dirt or rust! Fuel lines & fittings from main gas tank to vacuum tank should not leak. Also carb float valve should be in good order.

These Stewart Auto-Vac systems work well if you understand them.

Also be sure that all fuel lines are away fron any sorces of heat so you do not boil the fuel and get into a vapor-lock situation!

The engine intake basically produces a small vacuum in Autvac tank (negative pressure)Atmospheric pressure through rear fuel-tank vent that then pushes or syphens fuel from main gas tank to the Stewart Autovac tank...gravity then lets it flow to carb float chamber through float valve!

Simple process...

If you wish I can Xerox a copy of the Stewart Autovac cut-away & system description & instructions...

Send me a PM if you would like to have his info!