'25 DB Vacuum tank and carburettor problems... from a novice...

[invisibleadam]

I was hoping some of you old hands could help this old car newby out with some issues.

I've recently acquired a '25 DB pickup (cut-down tourer) that's been restored some time ago and is reasonably complete and healthy, but previous to me was owned by a very much non-car-person for the purpose of being a promotional piece at a local pub. Before I got it, the car had been sat for 5 years, and I believe, driven regularly before that.

This is my first truly old car, all my other stuff is 60's and 50's, and the vacuum tank fuel system is kind of alien to me.

The car hand-cranks and idles well, and drives around normally if fed from a small jar of gasoline. I disconnected the fuel line from the vac tank inlet (the fuel tank is hopelessly clagged with rust and crap) and ran a hose to a little jar of fuel sat on the running board. The vac tank sucks it all up quick smart and the car runs perfectly till it runs out of fuel. When the temporary fuel hose is put in a 10-litre fuel can, the vac tank seems to overflow. I say this because the engine starts to flood and run terribly rich, and eventually fuel seems to weep out the top of the vac tank around the top plate and slowly runs down the outside of it. Also, the air vent tube intermittently shoots gas out of it.

Additionally, and I'm not sure if this is related or not, the carburettor leaks fuel (quite quickly), APPARENTLY, from the choke valve shaft. Some of the posts I've been reading blame this on a vacuum tank overflow running down the vacuum line BUT this leak continues with the engine stopped, and, left overnight will form a largish puddle under the car. I put a jar under the carby overnight and caught about maybe 2-3oz (about 60-90 mL) of fuel.

I'm looking for some tips on how to start troubleshooting this vacuum tank, and also if anyone thinks the carby fuel leak is or is not related to the vacuum tank. I've bought a late-model japanese car fuel tank which I'm going to install up behind the cab and this has an in-tank electric fuel pump. I was originally going to pull the pump out and just make up a plain pickup tube, but this vac tank issue has me wondering if maybe I should just use the in-tank pump. I'm not too hung up on originality, I just want a fun reliable ole car. But don't worry, I will be carefully removing and storing any original bits I take off so that any mods I do are reversible.

Thanks in advance, Adam, Sydney Australia, 30 years old (and younger than ALL of the cars I own, proud to say)

[Guest DodgeKCL]

Just a quick answer,I'm leaving in a few minutes for a tour with about 50 other pre WWII cars. Hope the weather holds. ---I had a '29 Plymouth which had the vacuum tank setup. The level of gas in the tank is determined in the same fashion as a carb, a float and needle valve. If the gas is 'running over' then the needle valve is not shutting off the vacuum or the incoming gas.I believe it was done both ways depending on make and model. Hole in the brass float? It's drowning?

However as far as the carb over flowing-the downward gravity pressure from the full vacuum tank always seemed to overcome the float and needle valve in the carb and the only way to stop it 'bleeding' overnight is to install a shut off petcock on the bottom of the tank in the outlet line to the carb. You'll see this on just about every one of these vacuum tank setups now. This was a common design fault. You just have to remember to shut it off when you put the car away and turn it on when you are going to use it. The petcock can also be closed slightly to slow the amount of fuel being delivered to the carb and stop that rich mixture. The good news is ,once all the kinks are worked out the system is bullet proof and reliable.

[Guest Ed_Joyce]

We have a 28 Graham Brothers with a fast four motor. Vacum tank and updraft carb. I can leave it set for days with no leaks. Since you talked about the tank being very dirty could there be residue and dirt in vac tank and/or carb. Is the cork gasket decent on the vac tank? I've never had gas run down the outside of my tank. Just a couple thoughts.

Ed

[72caddy]

I just had this same problem with my Dodge. The 'flapper' valve at the bottom of the inner tank has come off - which means that the vacuum tank is constantly feeding the carb gas - instead of 'on-demand'.

Remove the line connections to the tank, remove the 8 or so screws around the perimeter of the tank top and pull the top out of the vaccum tank - like most cars this will just be the float mechanism. Now remove the inner tank and you will see (at the bottom) the outlet for the gas. There should be a metal 'flapper' that is held on but two screws. I bet your flapper is at the bottom of the outer tank (like mine). Just re-install the flapper (make sure the surfaces mate well) and the re-install everything.

Hope this helps.

[DDewey]

Adam,

We are all assuming that the 10L tank is below the lever of the vacume tank, and the hose runs DOWN to it, so you aren't accidently creating a syphon. Otherwise, I'd suspect that you have some dirt in the vacume tank or a rusted out spring on the float valve.

You might consider taking the original gas tank off the car and seeing about cleaning it, if it doesn't have pinholes in it. Might be easier than modifying something else to work--and the electric pump is likely to have too much pressure for the carb.

David D.

PS: Don't worry about the horn working, the radiator plainly says on it, "Dodge, Bro!"

[nearchoclatetown]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 72caddy</div><div class="ubbcode-body">I just had this same problem with my Dodge. The 'flapper' valve at the bottom of the inner tank has come off - which means that the vacuum tank is constantly feeding the carb gas - instead of 'on-demand'.

Remove the line connections to the tank, remove the 8 or so screws around the perimeter of the tank top and pull the top out of the vaccum tank - like most cars this will just be the float mechanism. Now remove the inner tank and you will see (at the bottom) the outlet for the gas. There should be a metal 'flapper' that is held on but two screws. I bet your flapper is at the bottom of the outer tank (like mine). Just re-install the flapper (make sure the surfaces mate well) and the re-install everything.

Hope this helps. </div></div>72CAddy, I think a rust hole in the inner tank would act the same way. Still no big deal, Quick-poly, or solder, or PC-7 can fix it.

[Guest imouttahere]

Couldn't a leaking float also be causing these symptoms?

By all means restore the vacuum tank to stock and use it rather than a fuel pump. It is worth it.

I had a sheet metal shop remove the cylindrical part of my fuel tank and fabricate a new tank using the old ends.

[invisibleadam]

Thanks for all the help. I will endeavour to have a look inside the unit this week. As I've been thinking about it I've come to the conclusion that it's either a clogged up needle valve or leaking/sinking float. I'll see what I can see and report back during the week. Much appreciated for all your suggestions, DDewey yes the tank was sitting on the running board and therefore LOWER than the vac tank but only out of luck, I didn't consider the accidental siphon scenario. I'll also check the flapper valve while I'm at it but I just wanted to get a feel for the system and how it's supposed to work. Incidentally, someone posted a link on an earlier thread about vac tanks so just to repeat:

http://clubs.hemmings.com/frameset.cfm?club=durant

Click on the "Tech" tab and follow the links for some schematics and an overview of a vac tank system. This page specifically refers to Durants but it's pretty much the same system.

[72caddy]

From my very limited experience (6 Months) with these system each of the above posts could be causing the issues (sunken float, hole in the inner tank, flapper valve).

I think I read that if the vaccum line is cold then the issue is with the vacuum tank and if the vacuum line is hot then the issue is with the carb.

I would start with the tank IMO.

[jan arnett (2)]

I provided the article on the Durant web site and I have worked with cars that have vac. tanks for the last 40 years and find them extremely reliable. Just take your time and only do one thing at a time and you will find that they are easy to work on. I am not sure what the temp. of the vac line has to do with the tank.

Have a nice day.

Jan

[72caddy]

The article I read in the DB newletter mentioned that if fuel was being sucked in through the vacuum line it would be cooler than if it was operating correctly.

[jan arnett (2)]

If gas is being sucked through the vac. line then the engine will run very rich and you will have very little vac to the tank. I have seen this happen once in 40 years and when I removed the line from the intake manifold gas ran out. You are correct it can happen.

Have a nice day.

Jan

[brian1925tourer]

Hi, Just reading up on this problem. Will be running very rich with smoke out the exhaust. For fuel to get into the vac pipe means the vac valve is not closing or a hole in the float or broken spring.Either way it means vac valve not closing.

[invisibleadam]

Thanks for all the input everybody, I pulled the vac tank apart today and found about a 1-inch crack top to bottom in the float. So now I'll work out either to repair or replace, but the problem has been found. Thanks!

[invisibleadam]

How cool is this- not 2 hours after I pulled the vac tank apart I have a replacement I paid $20 for. A guy who lives just up the road from me, who sells antique tyres, happened to have a complete vac tank in a box in his garage! I was able to make 1 from 2 and now have a spare. And people think it's hard to get parts for old cars. Pah!

[1929Chrysler]

Let us know if you installed it and it is working properly!!!

Dan