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MG TD TF 1500 - Fuel tank venting

During last weeks hot weather (88 degrees) my '53 TD engine began running roughly due to fuel starvation, verified by the front carb tickler pin down low. Happened 4 times, but would seemingly clear after a few moments rest. Each time this happened, I was able to chug along on two cylinders fed by the rear carb! I have two fuel pumps in series, one SU, one "electronic", switchable from inside the car. The problem was not helped by switching to either pump.
After an ice cream break, I opened the gas filler cap, left it ajar, and had no problems for the last hour of the trip. I have a tubular, glass inline filter between the tank and the first pump, and when I stopped for the problem, there was very little fuel visable in the filter. After a few minutes with the engine off, the filter filled with fuel when the ignition was turned on.
The cork gasket on my gas cap is intact, having been replaced several years ago, and I wonder if there could be some kind of vent in the cap that is clogged, generating a partial vacuum in the tank causing the problem. Since home, I have cleaned the inline filter, but the small amount of sediment found could not have been responsible. Any thoughts?
Bill
Bill Cole

Bill, according to the archive (fuel tank vent?) it's in the cap.

Mike
Mike Hart

Bill,

... a common problem. Usually a result of a rebuilt fuel tank, in which the fresh cork insert is too tight and creates a vacuuum.

Best is to cut away some of the cork. It will beargued that this subjects the tank to leaking when sloshing/driving hard. True, but the only true solution is the MMM way! See att/pic.

... from my PA - a pigtail! (sorry for the quality).

Gord Clark


Gordon A Clark

I can't believe the engineers anticipated a tight cork seal would serve as a vent.

My wife jump on me to put a new o-ring under the gas cap since it was sloshing out. So yesterday, I dug out old cork, thinking it was an old o-ring (the o-ring is in a different tank), but that's another story. Just interesting it was yesterday... so many bugs arise in spring when different cars get put back on the road.

Anyways, while rooting around, I reinspected the cap to convince myself it is adequately vented. You'll note the cap is spring loaded where it seals to the cork, (or whatever material). I have to assume there's a slight space between that and the pin it rides up and down on, that serves as the vent. Also note the rigid cup that slips down INTO the filler hole... looks like they intended for that to deflect the sloshing gas somewhat, keeping it away from the vent up in the center.

Try shoving the springloaded steel cover up and down to make sure it moves freely. Maybe your cork has compressed to the point it doesn't lift that cap and open the vent. You might rejuvenate the cork with a touch gasoline (or maybe water) and leave the cap loose to let it swell.

You'd be surprised at how many hydraulic problems are due to tank vents.

Reminds me that there's some character fabricating new aluminum fuel tanks to convert diesels to waste veggie oil (yeah I burn that in some of my toys). Interesting on how many of them run a mile and quit, over an over; and some of those tanks implode for some unknown reason. I did suggest he vent his tanks, but he's the expert.
Jim Northrup

In studying my cap, the inside metal dome that rests on the cork seal is spring loaded down. When shut the dome rises against he spring putting pressure on the cork. The only vent I can figure is out thru the center of the dome around the rivet. The spring seems pretty heavy duty.
Mike
Mike Hart

Bill,

Not that you asked, but according to Dave DuBois, our resident SU Fuel pump expert, a filter between the tank and the pump can lead to pump failure. The pump doesn't mind a filter after the pump, but prior may cause it to run too much trying to pump what it can't get at. Also, the pump is able to pass fairly large chunks of debris without problem, so it is unnecessary. You do, however want to keep the debris out of the float bowl needles and seats, and the filters in the pump bottom and inlet banjos may not be up to the task. A filter after the pump can do that job admirably and is easier to see in the engine compartment... or just clean the other internal filters once in a while.

So keep a close eye on that filter before the pump!

warmly,
dave
Dave Braun

In my efforts to find an answer, I also came across the DuBois article, and it exactly describes my setup, as well as those of many others in our club. Seems a natural place for an inline filter, between the fuel line and the pump. I don't remember ever getting enough "stuff" in the filter to cause a clog, although if I am right about the fuel tank vacuum as the source of my fuel supply problem, it would likely have the same effect on the pump. I have an old pump on my workbench, and one of these days I will try to verify where the points stop with blocked inlet.
Re the tank vent, there seems to be some sort of valve in the center of the bottom most portion of the cap. Could that be the vacuum relief? Offhand, if it is a valve, it looks to be such as to vent pressure, rather than relieve vacuum.
I've tried the archives and can't find reference to Fuel Tank Vent. Anyone know of one?
Bill
Bill Cole

"I have an old pump on my workbench, and one of these days I will try to verify where the points stop with blocked inlet."

No need to fuss around verifying that situation, I have verified it on my test stand. Think about the action of the pump. When the pump is at rest, the points are closed. When power is applied, current goes through the points and the coil to ground, pulling the diaphragm up against the bottom of the coil and producing a vacuum in the inlet chamber of the pump. Just as the diaphragm reaches the full travel up, the carrier of the lower set of contacts toggles over and the points open, interrupting the current flowing through the coil. When this happens, there is no longer any magnetism being produced by the coil and the diaphragm is pushed back down to it's rest position, and in the process pushes fuel out through the outlet of the pump (there is a set of check valves in the pump that keeps the fuel from being pushed back into the tank). If the float bowls are full and the needle valved are shut, the diaphragm may take awhile to reach the bottom rest position and during this time, the current through the coil remains off. Once the diaphragm reaches its rest position, the lower contact carrier toggles to its other position, causing the upper and lower contacts to close, again allowing current to flow through the coil and repeating the inlet cycle.

Now, consider what happens if the is a clog on the inlet side of the pump (or a vacuum is pulled in the tank due to a lack of venting). If the vacuum produced on the inlet side of the pump by either a tank not venting or a clog exceeds the vacuum produced by the rising diaphragm in the pump, the diaphragm will not be able to rise all the way to the point where the lower contact carrier can toggle over and open the points. When this situation, the pump stalls with the points closed and current continues to flow through the coil. The older pumps draw somewhere around 3.5 amps (later pumps draw around 5.5 amps, but in normal operation the points are only closed for about 10% of the on/off cycle, so the average current flow during normal operation is only about 10% of full current and the pump remains relatively cool. But, if the pump stalls with the points closed, full current flows all of the time that the points are closed and with full current through the coil, heat builds up rapidly, until the internal swamping resistor (part or all of the arc suppression circuit) burns out.

In the SU pumps, unlike the after market pumps, such as the Facet pumps, there is no bypass valve (used to set output pressure), if there is a clog on the inlet side of the pump, the pump will always stall in a current on condition as described above. On the earlier pumps, like the ones in the T series cars, MGAs and very early MGBs, the inlet and outlet valves consist of a brass disk that seat on either brass or aluminum seats and over time have a grove worn in them. The result is that they don't seal as well as when new so that even if a clog develops on the inlet side of the pump (or a vacuum is pulled in the tank), a worn pump will continue to click, but at a much slower rate than normal, the extra time between ticks being the time that the points are closed, current is flowing and the pump is heating up. On the after market pumps like the Facet, the pump will actually cycle faster when the inlet side is clogged and no damage occurs to the pump. This is why I recommend that a modern, high efficiency filter not be used on the inlet side of a SU pump. In the T series car, there is a filter over the pickup tube inside the tank and filter inside the pump. Both of these filters are a relatively coarsely woven mesh that will pass any fine rust particles, stopping only rocks and small birds. The SU pumps and even the SU carburetors are relatively tolerant of the fine rust particles, where as the modern filters will soon clog with these particles and cause damage to the SU pumps. Cheers - Dave
David DuBois

I just finished my TF restoration. The car stopped on me after 10 miles. I would let it rest and it would go another 1/4 mile. Flatbedded it home. My mech came and after much inspection found the problem - inside of the brand new pump - looks like the old ones, but has electronic innards - directly ordered from Burling (sp?) in England - in the inlet line there is a 1/4 inch plastic tube - like aquarium whitish tubing - and it is maybe 1 1/2 inches long that goes toward the center of the pump. Mine was bent and crimped from the factory, allowing only a bit of gas through. When it got just a tiny bit of particles, it stopped the car - it would settle, and I would get a bit further down the road.

Because of this, we installed a clear pencil shaped filter about 3 inches long, right before the pump in the rear. Now do I have to worry about it hurting the pump? If I watch it to keep it clean, I won't have a problem will I - or will it cause an issue right out of the box? This filter would have prevented the earlier problem, albiet the plastic tube needed corrected.

I see the talk about the venting. I have an aluminum gas gauge that covers the entire top of the tank filler hole, and it has a tiny vent hole about the size of pin head or I guess slightly larger - yet I have never experienced a vent problem.

So let me know if I am ok with the filter if I check it to make sure it is clean.

Keith
K E MURPHY

Keith.
Please supply a photo of the gas gauge. Do you mean that you do not have a normal gas cap?
Sandy
SANDY SANDERS

This thread was discussed between 27/05/2010 and 31/05/2010

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