22touring Posted October 4, 2015 Share Posted October 4, 2015 When I first bought my DB, the vacuum tank didn't work so I installed an electric fuel pump with a regulator to drop the pressure down to the 1-1/2 psi that the Stewart carburetor likes.However, when I'd go to DB get-togethers, my fellow Brothers subtly let me know that "real" Dodge Brothers get their vacuum tanks working. And since I wanted to be a real Dodge Brother, I set out to fix mine.Don't misunderstand; even at this late date I can't claim my vacuum tank works all the time. Something always seems to be going wrong with it! However, I will say that I've had to fix it so often that at least I have figured out its theory of operation pretty well.It's actually fairly simple. Inside the vacuum tank are really 2 tanks, the inner one and the outer one, and they are basically sealed from each other except for the flapper valve. The inner tank is shaped like a cylinder and the outer one is shaped like a tube because it surrounds the inner tank. The inner tank also contains a float and, again, has a one-way flapper valve connecting it to the outer tank.The inner tank sucks fuel up through the fuel line from the gas tank, and when the inner tank is full, it delivers the fuel into the outer tank which is connected to the carburetor. (So obviously your fuel line must hold a good vacuum in order for the vacuum tank to be able to suck fuel from the gas tank in the first place.)The inner tank can operate in two modes, which I'll call filling and draining. When the float in the inner tank is low, it operates in filling mode wherein brass, spring-loaded over-center levers near the top of the vacuum tank close the inner tank's atmospheric valve in order to hold a vacuum, and open the vacuum line from the carburetor, thereby creating a partial vaccum in the inner tank. As vacuum is created in the inner tank relative to the outer tank, and since the fuel in the outer tank is now at greater pressure than the fuel in the inner tank, fuel will try to flow from the outer tank and into the inner tank. At this point, though, the flapper valve is supposed to close so the fuel in the outer tank cannot do so.So the inner tank fills and the float rises. When it gets to the top of its travel, the spring-loaded over-center levers close the vacuum line to the carb and open the atmospheric valve, so the vacuum tank goes into draining mode with authority. The over-center lever mechanism is designed so as to prevent any back-driving of the mechanism by fuel pressure, in order to foreclose any possibility of the tank overflowing onto the engine, were the float to fail to close the vacuum line completely.Now the inner tank goes into draining mode. It has had its vacuum removed because the atmospheric valve is open, and it contains fuel at a higher level than in the outer tank. Both inner and outer tanks are at atmospheric pressure, so the higher level in the inner tank pushes the flapper valve open and the fuel in the inner tank drains into the outer tank until the float reaches the bottom of its travel. Then the cycle repeats.Problem areas:*leaking or inoperative vacuum and atmospheric valves*inoperative, leaking, broken flapper valve*failed seal between inner and outer tanks*broken or missing lever springs and pins*broken, leaking top*in a car that has been sitting a long time, carefully check for spider webs and bugs' nests in all of the lines and orifices. Push as large a gauge wire as possible through all the fuel and vacuum lines.*use a Mity-Vac or the like to check whether the inner tank is pulling a vacuum, and whether the line from your gas tank will hold a vacuum. I hope this post has helped to prevent the chronic insecurity that can result from not knowing whether you are a real Dodge Brother or not. 1 Link to comment Share on other sites More sharing options...
Spinneyhill Posted October 5, 2015 Share Posted October 5, 2015 After reading that I almost wish I had a vacuum tank to have a relationship with. Almost... Link to comment Share on other sites More sharing options...
MikeC5 Posted October 6, 2015 Share Posted October 6, 2015 Excellent description of what goes on in there. Link to comment Share on other sites More sharing options...
stakeside Posted October 7, 2015 Share Posted October 7, 2015 When I first bought my DB, the vacuum tank didn't work so I installed an electric fuel pump with a regulator to drop the pressure down to the 1-1/2 psi that the Stewart carburetor likes.However, when I'd go to DB get-togethers, my fellow Brothers subtly let me know that "real" Dodge Brothers get their vacuum tanks working. And since I wanted to be a real Dodge Brother, I set out to fix mine.Don't misunderstand; even at this late date I can't claim my vacuum tank works all the time. Something always seems to be going wrong with it! However, I will say that I've had to fix it so often that at least I have figured out its theory of operation pretty well.It's actually fairly simple. Inside the vacuum tank are really 2 tanks, the inner one and the outer one, and they are basically sealed from each other except for the flapper valve. The inner tank is shaped like a cylinder and the outer one is shaped like a tube because it surrounds the inner tank. The inner tank also contains a float and, again, has a one-way flapper valve connecting it to the outer tank.The inner tank sucks fuel up through the fuel line from the gas tank, and when the inner tank is full, it delivers the fuel into the outer tank which is connected to the carburetor. (So obviously your fuel line must hold a good vacuum in order for the vacuum tank to be able to suck fuel from the gas tank in the first place.)The inner tank can operate in two modes, which I'll call filling and draining. When the float in the inner tank is low, it operates in filling mode wherein brass, spring-loaded over-center levers near the top of the vacuum tank close the inner tank's atmospheric valve in order to hold a vacuum, and open the vacuum line from the carburetor, thereby creating a partial vaccum in the inner tank. As vacuum is created in the inner tank relative to the outer tank, and since the fuel in the outer tank is now at greater pressure than the fuel in the inner tank, fuel will try to flow from the outer tank and into the inner tank. At this point, though, the flapper valve is supposed to close so the fuel in the outer tank cannot do so.So the inner tank fills and the float rises. When it gets to the top of its travel, the spring-loaded over-center levers close the vacuum line to the carb and open the atmospheric valve, so the vacuum tank goes into draining mode with authority. The over-center lever mechanism is designed so as to prevent any back-driving of the mechanism by fuel pressure, in order to foreclose any possibility of the tank overflowing onto the engine, were the float to fail to close the vacuum line completely.Now the inner tank goes into draining mode. It has had its vacuum removed because the atmospheric valve is open, and it contains fuel at a higher level than in the outer tank. Both inner and outer tanks are at atmospheric pressure, so the higher level in the inner tank pushes the flapper valve open and the fuel in the inner tank drains into the outer tank until the float reaches the bottom of its travel. Then the cycle repeats.Problem areas:*leaking or inoperative vacuum and atmospheric valves*inoperative, leaking, broken flapper valve*failed seal between inner and outer tanks*broken or missing lever springs and pins*broken, leaking top*in a car that has been sitting a long time, carefully check for spider webs and bugs' nests in all of the lines and orifices. Push as large a gauge wire as possible through all the fuel and vacuum lines.*use a Mity-Vac or the like to check whether the inner tank is pulling a vacuum, and whether the line from your gas tank will hold a vacuum. I hope this post has helped to prevent the chronic insecurity that can result from not knowing whether you are a real Dodge Brother or not.The motor may need a valve job. Check the vacuum draw at the manifold. Not sure what minimun requirement is. 1 Link to comment Share on other sites More sharing options...
22touring Posted October 7, 2015 Author Share Posted October 7, 2015 Good point, stakeside! I hadn't thought of that possibility. Link to comment Share on other sites More sharing options...
Pete K. Posted October 9, 2015 Share Posted October 9, 2015 If you stop and think about it, you are not using or losing much engine vacuum at all "IF" your system is working as it should. Vacuum is sealed by the valving in the vacuum tank and when used to siphon fuel, the "extra" engine vacuum needed is only in the little fuel line to the bottom of the fuel tank. I've run a few early cars with tired engines and never had a problem getting the fuel up to the vacuum tank. If you can start the engine and get an inspection sticker on it, you've got plenty of vacuum to operate your system. 1 Link to comment Share on other sites More sharing options...
DavidMc Posted October 9, 2015 Share Posted October 9, 2015 broken, leaking top This is a real problem due to aging of the pot metal top which can not only warp or crack the top but it can expand around the small brass valve seats preventing them from sealing , all causing loss of vacuum. I always replace the original top with a reproduction aluminum top even if it appears to be OK. Failure on tour is a real PITA. Once fixed a vacuum tank is very reliable. Link to comment Share on other sites More sharing options...
stakeside Posted October 9, 2015 Share Posted October 9, 2015 If you stop and think about it, you are not using or losing much engine vacuum at all "IF" your system is working as it should. Vacuum is sealed by the valving in the vacuum tank and when used to siphon fuel, the "extra" engine vacuum needed is only in the little fuel line to the bottom of the fuel tank. I've run a few early cars with tired engines and never had a problem getting the fuel up to the vacuum tank. If you can start the engine and get an inspection sticker on it, you've got plenty of vacuum to operate your system.There should be some vacuum requirements for the tanks to operate correctly. Is there any value noted anywhere? Has any one checked this with a vacuum gauge?Curious of the vacuum readings on these motors. Link to comment Share on other sites More sharing options...
MikeC5 Posted October 10, 2015 Share Posted October 10, 2015 With engine timing set properly, at idle, I believe you should get a steady reading between 17 and 21 inches HG. Link to comment Share on other sites More sharing options...
22touring Posted October 11, 2015 Author Share Posted October 11, 2015 I seem to be getting 3 to 5 psi of vacuum from the vacuum tank, but I have no idea if this is within spec or not. It's less than MikeC5 says it should produce, but it does seem to work OK. I took the car for a drive today, and it seemed to run fine. When you're testing the vacuum tank to see how much vacuum it produces, don't be fooled if you get no vacuum because the vacuum tank is in its "drain" mode. In that case you will have to wait until it cycles through to the "fill" mode again before you can test it for vacuum. Link to comment Share on other sites More sharing options...
stakeside Posted October 11, 2015 Share Posted October 11, 2015 Thanks for information you all and congrats on getting the touring going down the road. Link to comment Share on other sites More sharing options...
MikeC5 Posted October 11, 2015 Share Posted October 11, 2015 I'm not sure what the conversion is for psi to inches HG... Automotive vacuum gages are usually in inches HG (in the U.S.). Link to comment Share on other sites More sharing options...
JACK M Posted October 11, 2015 Share Posted October 11, 2015 I seem to be getting 3 to 5 psi of vacuum from the vacuum tank, but I have no idea if this is within spec or not. It's less than MikeC5 says it should produce, but it does seem to work OK. I took the car for a drive today, and it seemed to run fine. When you're testing the vacuum tank to see how much vacuum it produces, don't be fooled if you get no vacuum because the vacuum tank is in its "drain" mode. In that case you will have to wait until it cycles through to the "fill" mode again before you can test it for vacuum. PSI is a pressure reading. Totally different.Curious where you got this reading. Link to comment Share on other sites More sharing options...
Spinneyhill Posted October 12, 2015 Share Posted October 12, 2015 30 inches of Hg is one atmosphere, as is 14 psi, so 3 to 5 psi = 30 times (3 to 5)/14 = 6.4 to 10.7 inches Hg, say 6 to 11" Hg. Just different methods of measuring it. One atmosphere is also about 100 kPa so 3 to 5 psi = 21 to 36 kPa. kPa is a little less than psi x 7. We are having to learn this to pump up our tyres when we buy some brands of car! Link to comment Share on other sites More sharing options...
22touring Posted October 12, 2015 Author Share Posted October 12, 2015 Yes, I think both psi and inches of mercury are pressure readings, but they just use different units. I got the psi reading from my "Mity Vac" vacuum gauge and pump. Link to comment Share on other sites More sharing options...
JACK M Posted October 12, 2015 Share Posted October 12, 2015 Well, I am totally confused.I have both pressure and a vacuum gauges and use them for different tasks.I have seen a gauge the rests at zero and will measure in both directions.But there is no way I could use a pressure gauge for testing manifold vacuum or a vacuum gauge for checking say tire pressures. Link to comment Share on other sites More sharing options...
Spinneyhill Posted October 12, 2015 Share Posted October 12, 2015 OK, so what is a vacuum? Most pressure gauges are set with zero at 1 atmosphere because we operate under one atmosphere. A "vacuum" is created when we reduce the pressure below atmospheric pressure. So on a gauge with zero at 14 psi = 30" Hg = 100 kpa, a vacuum is a negative number. This is Jack's gauge that goes both ways. The convention has been adopted that we just call it a positive amount of vacuum. If we set the gauge zero at 0 psi = full vacuum, it would read 14 psi = 30" Hg = 100 kPa at the dining table. A "vacuum" would then be a pressure below 14 psi = 30" Hg = 100 kPa. 20" Hg of vacuum would still be 10" Hg pressure. 5 psi of vacuum would be 9 psi pressure. A "vacuum" cannot be more than one atmosphere and it is very difficult to get near a pressure of zero on Earth. It is easy enough to get to three quarters of 1 atmosphere "vacuum" (11 psi = 22.5" Hg = 75 kPa) but hard to get much further. For one thing, maintaining the seals necessary is not easy. Remember the astronaut's outdoor suit? It is a pressure suit because space is a vacuum and we exist in a pressure of 100 kPa. The suit maintains that compression. Link to comment Share on other sites More sharing options...
R.White Posted October 13, 2015 Share Posted October 13, 2015 (edited) bent valve stem in top I have noticed that one of the little valve stems seems to be bent which I believe to be the problem with mine. I have seen the reproduction castings for sale but I don't know if they come complete with valves and I am not sure how to get to the valves or I would try and mend mine. Any ideas? Ray. Edited October 13, 2015 by R.White (see edit history) Link to comment Share on other sites More sharing options...
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