RichardS Posted August 12, 2020 Report Share Posted August 12, 2020 Another puzzling issue with my GT6. All advice welcome. 🙂 I don't think that this is actually affecting the running of the car but it just doesn't seem right. The photo below shows the filter on my engine. This is a new one which I fitted a week ago because the original one was always full of air so I thought that it was faulty. However, as you can see the new one behaves exactly the same as the old one! You can see that the fuel only just covers the bottom of the filter, even though the filter outlet is right at the top of the filter and there is no dip--tube inside it. This photo was taken after a run yesterday. Occasionally, after the car has been left unused for a few days, the fuel level does creep up inside the housing and almost reaches the top. However, if I manually operate the fuel pump, then bubbles of air (I assume it's air?) come in at the bottom and the fuel level in the filter rapidly drops down again. Similarly, if the level has crept up, once the engine starts, more bubbles come in and the fuel level always ends up right at the bottom again. This happens whether the tank is full or empty, and when it's full the fuel level is actually above the level of filter. It's probably around the filter level even when tank is low. If I disconnect the line from the top of the filter then fuel, not bubbles, comes out of the top driven by the head of fuel in the tank. I would have expected the air to be driven out first but it isn't. There is a short length (a few inches) of rubber fuel line running from the bottom of the filter to the end of the metal fuel line. This metal line then runs along the chassis back to the bottom of the fuel tank. There is another few inches of rubber hose where the line goes above the rear axle and then it's metal into the tank fitting. There are no fuel leaks anywhere along this line. The car is kept in a garage and there is no smell of petrol. I cannot understand where the air bubbles in the fuel are coming from and why the fuel pump is not removing the air gap at the top of the filter housing and filling the filter with fuel. The more the engine runs, the emptier the filter housing becomes. It only starts to fill when the engine is not running for a few days. Can anyone explain to me what is going on? Many thanks Richard Link to comment Share on other sites More sharing options...
dougbgt6 Posted August 12, 2020 Report Share Posted August 12, 2020 Richard, My fuel filter is never more than a 1/3 full which is quite normal, apparently. I can't figure out why it shouldn't be full but they never are. Doug Link to comment Share on other sites More sharing options...
Badwolf Posted August 12, 2020 Report Share Posted August 12, 2020 Richard - was this photo taken while the car was running or at rest. If, at rest, the fuel will probably have syphoned back into the tank. I have noticed this on my spitfire too. When the tank is full, so is the filter glass. When low the fuel appears to syphon back. Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 12, 2020 Report Share Posted August 12, 2020 bubbles from the supply line when running is the pump sucking in air from hose/joint/leak even from the pump itself it may not leak fuel. air/vaour in the filter is a pretty normal. mounting it completely vertical will solve most air gaps if you didnt have a filter you would not have any thing to worry about ( trump rules) if the carb float valves are shut there is no flow so air gaps dont move or change pete Link to comment Share on other sites More sharing options...
Nigel Clark Posted August 12, 2020 Report Share Posted August 12, 2020 Richard, I've never understood why but the filter mounted just before the fuel pump on my GT6 always has an air bubble like that. Nigel Link to comment Share on other sites More sharing options...
dougbgt6 Posted August 12, 2020 Report Share Posted August 12, 2020 Nigel, me to! Mine is vertical, just before the pump, always 1/3 full. Why?!! Doug Link to comment Share on other sites More sharing options...
johny Posted August 12, 2020 Report Share Posted August 12, 2020 Wonder if its to do with petrol being so volatile that its producing vapour in the filter.... Link to comment Share on other sites More sharing options...
RichardS Posted August 12, 2020 Author Report Share Posted August 12, 2020 3 hours ago, Badwolf said: Richard - was this photo taken while the car was running or at rest. If, at rest, the fuel will probably have syphoned back into the tank. I have noticed this on my spitfire too. When the tank is full, so is the filter glass. When low the fuel appears to syphon back. The photo was taken with the engine off, although it had been running a few seconds earlier and it looked exactly the same when it was running. The fuel tank is actually full at the moment so the level is higher than the filter assembly. Any siphoning would actually serve to fill the filter rather than the other way around, although it can't actually do that because it's a sealed system, of course. Richard Link to comment Share on other sites More sharing options...
Badwolf Posted August 12, 2020 Report Share Posted August 12, 2020 Oh well, another idea in the bin!!! Link to comment Share on other sites More sharing options...
RichardS Posted August 12, 2020 Author Report Share Posted August 12, 2020 3 hours ago, Pete Lewis said: bubbles from the supply line when running is the pump sucking in air from hose/joint/leak even from the pump itself it may not leak fuel. air/vaour in the filter is a pretty normal. mounting it completely vertical will solve most air gaps if you didnt have a filter you would not have any thing to worry about ( trump rules) if the carb float valves are shut there is no flow so air gaps dont move or change pete It is almost vertical but the air gap only vanishes when the car is left standing and then appears again as soon as the engine starts. I did try holding it both vertically and horizontally with the engine running but it made no difference. I can't believe that so much air is being sucked in from one of the four joints hose before the filter (it can't be due to anything after the filter) but that petrol will not leak out. Petrol is famed for finding the tiniest crevice to squeeze through that there would have to be some weeping at least but I checked the whole line and can't find anything at all. I just can't figure it out. Richard Link to comment Share on other sites More sharing options...
RichardS Posted August 12, 2020 Author Report Share Posted August 12, 2020 13 minutes ago, johny said: Wonder if its to do with petrol being so volatile that its producing vapour in the filter.... I did wonder that, but surely the vapour would be sucked through the pump along with the fuel and the air gap would clear? When I disconnect the pipe at the top of the filter, it's fuel that comes out because of the height of fuel in the tank. Why does the vapour not come out first followed by the fuel? There must be an answer to this, surely? Richard Link to comment Share on other sites More sharing options...
RichardS Posted August 12, 2020 Author Report Share Posted August 12, 2020 2 hours ago, Nigel Clark said: Richard, I've never understood why but the filter mounted just before the fuel pump on my GT6 always has an air bubble like that. Nigel It's been driving me mad ever since we bought the car, so much so that I changed the filter in a bid to put an end to it. 😕 We were both trained in chemistry/physics and there must be a scientific answer to this conundrum but I'm damned if I can work it out. Richard Link to comment Share on other sites More sharing options...
NonMember Posted August 12, 2020 Report Share Posted August 12, 2020 Are you absolutely sure the filter outlet is right at the top? If there's an internal pipe, particularly a flexible "fall to the bottom" type, you would not be able to see it inside the paper element, and it would cause exactly what you see. Link to comment Share on other sites More sharing options...
Badwolf Posted August 12, 2020 Report Share Posted August 12, 2020 If you open the float valves in the carbs does the filter fill up, having released any vapour in the filter? Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 12, 2020 Report Share Posted August 12, 2020 ct your old filter open if it has a dip tube as Rob suggests you have an answer for all. that doesnt explain/solve air bubbles when its sucking Pete Link to comment Share on other sites More sharing options...
RichardS Posted August 12, 2020 Author Report Share Posted August 12, 2020 27 minutes ago, Badwolf said: If you open the float valves in the carbs does the filter fill up, having released any vapour in the filter? Is there a way of doing that? You could do it will my old British motorbikes as they had a tickler but I didn't think that Strombergs had such a device unless simply opening the choke valve on the side of the carb will do it? Richard Link to comment Share on other sites More sharing options...
RichardS Posted August 12, 2020 Author Report Share Posted August 12, 2020 17 minutes ago, Pete Lewis said: ct your old filter open if it has a dip tube as Rob suggests you have an answer for all. that doesnt explain/solve air bubbles when its sucking Pete I did look at the old filter and couldn't see a dip pipe. However, I'll go and have another look. Richard Link to comment Share on other sites More sharing options...
Badwolf Posted August 12, 2020 Report Share Posted August 12, 2020 40 minutes ago, RichardS said: Is there a way of doing that? You could do it will my old British motorbikes as they had a tickler but I didn't think that Strombergs had such a device unless simply opening the choke valve on the side of the carb will do it? Richard Ahh... I have SUs so its just a question of taking the float chamber top off. Link to comment Share on other sites More sharing options...
JohnD Posted August 12, 2020 Report Share Posted August 12, 2020 Only do this underwater! Or rather, under petrol! Otherwise the bubbles will get away and come back to plague you. Catch them in a convenient container, an egg cup for instance and dispose of them humanely. Thank you. Link to comment Share on other sites More sharing options...
Badwolf Posted August 12, 2020 Report Share Posted August 12, 2020 Oh dear, John's not taking his meds again...... 1 Link to comment Share on other sites More sharing options...
RichardS Posted August 13, 2020 Author Report Share Posted August 13, 2020 Right .... in the interests of science I've conducted a few experiments with surprising results! I took a new filter exactly the same as the ones I have fitted. There is no dip tube in these simple filters. The entry aperture is in the very bottom of the cylinder and the exit aperture is in the very top. I attached a clear plastic tube to the exit spigot so I could suck, and, with the filter held vertically, dipped the bottom spigot into white spirit. I then began to suck gently and the liquid started to rise in the cylinder. However, the filter acts like a wick and you can see the liquid wicking up in the filter material ahead of the level of the liquid. By the time the level of liquid gets about half way up the filter, the wick level has reached the top of the filter and, as soon as that happens, liquid starts to rise up the exit spigot and into the clear tubing. It seems to defy logic but it is what it is. Inside the cone of filter material, it must be full of liquid even though the actual level of liquid outside the cone is only halfway up the container or lower. It doesn't matter how hard I suck now. The level stays at about half way in the cylinder and liquid continues to exit from the top. A mouthful of the stuff now becomes a clear and present danger! The really surprising thing is that if I stop sucking and just leave the end of the plastic tube in the air and leave the bottom end of the entry spigot in the liquid, the level stays the same in the filter i.e. the liquid does not run out even though the top end is at atmospheric pressure. What is more, if I lift the end of the entry spigot out above the liquid, nothing different happens. The filter just sits there half full of liquid. The filter material acts like a non-return valve which I never expected. I can obviously blow the liquid out quite easily but, now that the filter is wet, when I start to suck again the liquid starts to come up the exit pipe almost as soon as the liquid touches the bottom of the filter material. It is now behaving exactly as in my photo above. I can tip the filter right over on its side or leave it vertical but it makes no difference .... as soon as the liquid reaches the filter material it starts to exit from the top. However, this doesn't work the other way. If I reverse the filter the other way up and such through the bottom end, then the level rises right to the top of the housing before it starts to appear in the plastic tube. If I stop the suction, the liquid simply runs out of the filter as you would expect and it drains completely. The filter material seems to behave differently in each direction. If you want the filter housing to be full, the only way to achieve it appears to be to mount the housing the wrong way round. In theory, I guess in would be possible to fill the housing inverted and then quickly connect it up the right way around whilst it's still full. However, those bubbles which seems to come into my filter from the bottom with the new fuel would presumably just re-establish the air gap again? Anyway, where does that leave us? I suppose with the confirmation that having the petrol near the bottom of the filter housing is not an issue as the inside of the filter cone is full of fuel which is why the engine never seems to suck that air bubble through into the float chambers. I'm still non the wiser about where the bubbles in my incoming fuel are coming from. The research will continue! 😉 Richard 1 Link to comment Share on other sites More sharing options...
NonMember Posted August 13, 2020 Report Share Posted August 13, 2020 Hmm... interesting. So it appears that, once the filter paper has been wetted, the suction will preferentially draw more liquid through than gas. Presumably this is a result of surface tension type effects - pulling the gas through would break the liquid surface but filling the inside of the paper element with liquid doesn't. Not instinctively obvious behaviour! Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 13, 2020 Report Share Posted August 13, 2020 Brilliant , but sorry i got lost when you mentioned sucking from the bottom !!!!!!!!!!! wheres doug when you need him Pete Link to comment Share on other sites More sharing options...
Chris A Posted August 13, 2020 Report Share Posted August 13, 2020 Buy that man a drink, any spirit he likes other than white! My filter also has very little petrol visible but I just shrug. The car starts so I don't worry, interesting effect though. 👏 Link to comment Share on other sites More sharing options...
Nigel Clark Posted August 13, 2020 Report Share Posted August 13, 2020 Richard, I'm glad to find you've retained your scientific curiosity and rigorous approach to research since we worked together more than four decades ago. However, during those decades health and safety have assumed far greater priority. We should certainly not be inhaling petrol these days, it's far more harmful now than in the seventies when it contained lead! Nigel Link to comment Share on other sites More sharing options...
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