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MG MGB Technical - Fuel tank venting
| Hi, I'm having trouble with fuel starvation on my '71 B. After a long long winter she wouldn't fire up and keep running. The engine would just die out. Somebody suggested removing the fuel cap and she immeadiatly starting running. Replaced the cap, no fuel. The fuel cap does not have a vent hole, but I was told that the tank will have its own venting system. However I cannot see one on the tank and the Haynes Manual refers to a ventilated cap. I am confused, help please. Ken |
| Ken Martin |
| If the vent system hasn't been removed, there will be a round canister mounted inside the trunk on the right wall, with tubing in(from the tank) and out to the fuel collector in the engine compartment (black, with --- I believe --- 3 hose connectors). I have that setup (though it was disconnected) but also have a vented cap. |
| Dan Robinson |
| Afraid I cheated and drilled two 1mm holes in the cap and punched two 2mm holes in the rubber seal ring. make sure they don't line up! M |
| Michael Beswick |
| Thanks for the fast response guys, but there is no canister in the trunk or tube that I can see. I don't have the charcoal filter in the engine compartment either. How did you punch through the rubber seal on the cap Mike? |
| Ken Martin |
| You will probably have an American style filler neck and cap. The cap has an outer chrome bit and an inner sprung rubber seal. On the inside of the cap, in the centre sprung bit, drill a very small hole (your smallest drill bit) inside the circle of the rubber seal. Off centre of course. This will be invisible from the outside and should do the job. Air will find its way around the edges of the outside chrome part of the cap, no problems. Or damage the rubber seal, or the neck. |
| Peter Sherman |
| UK cars were always different to US after 70 (California) or 71 (rest of North America including Canada). UK always had a vented fuel filler cap and they should be easily obtainable from the usual suspects. US cars from those dates had a non-vented, and only if the port in the engine compartment, or the boot, or the tank has been closed off or been replaced with an earlier one should you get a problem. With a Canadian 71 it's quite possible it never had the vent and someone has fitted a non-vented filler cap in error. Simply replace it with an earlier vented type. |
| Paul Hunt |
| One thing I noticed on my '70 North American BGT with the vented tank, has a small threaded port next to where the filler nick goes into the tank. My hoses from tank to vapor canister looked fine but leaked fumes making the inside of the cabin smell like raw gas. I decided to plug the vent from the tank and fit a vented fuel cap BUT on those years the filler neck does not accept the older vented cap so I changed to an earlier filler neck and installed the vented cap and it solved the problem. Easy job! Mike |
| MK Mike |
| Hi Guys, Thanks for the input and I did drill two 2mm holes, but it made no difference. I had this problem last year where the car will idle fine, rev while standing, but once you start accelerating and putting it under load it starts missing as if a plug had gone sour. Last year I ended up changing the whole ignition system including the distributor with the addition of a Petronix unit. Didn't help at all. Then one day I floored the accelerator out of frustration and it cleared itself. So I figured there may be delamination in one of the rubber fuel lines and changed all of those and it ran fine for the rest of the season. Now it's back to square one, but flooring the pedel doesn't help this time. This morning I disconnected the fuel line into the first carb and turned on the ignition and there seems to be plenty of flow. The fuel pump was changed about 4 years ago. Any ideas or suggestions? Ken |
| Ken Martin |
| Ken Rust/dirt in the tank. It is a classic symptom.. I have spent WAY TOO MANY hours dealing with this on B's and midgets. Sometimes it's stuff in the tank, sometimes is stuff moving up and down the pickup tube, sometimes it's a hole in the pickup tube. If you replaced all rubber lines and the metal line is clear than it's probably the tank. I had a '79 Midget that ran perfectly in the driveway, idled fine revved up fine would go down the road a short sistance fine and then sputtering, kicking puffing, ect. It was a clogged screen at the end of the pickup tube in the tank. I have recently battled the same problem in my '70 MGB, dropped tank, had professionaly cleaned and coated still had problems. If this is your problem I would strongly recommend buying a new tank to save yourself ALOT of aggh.and be done with it. Mike |
| MK Mike |
| Hi Mike, I was rapidly coming to the same conclusion. It was missing as I went up the street this afternoon, then cleared itself when idling, enough for about a 1/2 mile run and then back to missing again. It doesn't look too difficult to drop the tank, except mine doesn't have any drain hole. Any quirks that I should know about. Also should I use the etching and slushing compound in the Moss cataloge? Ken |
| Ken Martin |
| What you might try is running a fuel line from a jerry can to the pump. Then see if you have the same symptoms. Of course take all the appropriate precautions, with regard to spillage and no ignition sources near by (ie boot light etc.) You are going to need a large jerry can anyway,I bought a plastic one from Kmart. Somewhere to drain the petrol into before you drop the tank. You unbolt the bolts in the boot first. Don't miss any. Then do the 4 underneath nuts and drop the tank down onto your chest. At least thats how I did it. I paid $300 for a new one from Canada. |
| peter |
| Ken Good idea from Peter to do and make sure . I can run my B on a quart size mason jar fof a good while for testing. Dropping the tank is fairly easy. I put my floor jack under it and then remove the bolts. Unscrew the line into the tank and remove the leads on the sending unit. Once it's empty you can get a better idea of what you're dealing with. If yours has NO drain plug you may have a replacment tank so it may just be accumulated debris in the bottom. Honestly, Next time I need to do this I will buy a new one. You can now get them cheaper than ever on ebay, around $179.The problem with the original MGB tank is that you cant see/or get to the end of the pickup tube where the clog usually is and sometimes that tube rusts out and gets pin holes in it that will drive you crazy. I wanted to stay with my original "baffled" tank. I cleaned and etched it with the POR15 product which did a beautiful job, I followed the instructions to the letter blew air thru the p/up tube and still had problems. After chasing the problem for a long time, dropping tank(s) numerous times I finally fabricated a new pickup tube from coppor tubing and some fittings and soldered it to the tank and now life is beautiful. DO NOT do this uless the tank is completely cleaned out and NO fuel vapors exist inside the tank. Mike |
| MK Mike |
| Does sound like supply restriction, you can test the supply by removing the pipe from a carb, directing it into a container and switching on the ignition. Be careful doing this on a hot engine of if the ignition has just been on as it will spurt fuel when you disconnect. The pump should deliver a minimum of 1 Imperial pint per minute, and in practice more like double that, in a steady stream of pulses with minimal bubbling. If you suspect gunge then this should be evident as a badly blocked filter. If the above is OK the other possiblilty is if you have HS carbs, with these you can get debris in the jet pipe which causes intermittent starvation, and this can be the rubber seal decomposing. They changed to HIF in 71 on export models, so it depends on just when yours was made. |
| Paul Hunt |
| Thanks Guys Good idea Peter, accept the problem only happens with the vehicle is accelerating. I'd be a bit wary of running a pipe down to the pump from the cockpit or the trunk in case it snagged a wheel. I do have a good flow from the inlet pipe to #1 carb (HS4's Paul) with the vehicler standing and I have blown the carb pipes out. I'm moe and more inclined to think that there is muck in the tank which settles when the car is standing, but gets stirred up when the vehicle is moving and blocks the pick up line. Does that make sense. The tank is a replacement which was installed by the previous owner about 6-7 years ago. Ken |
| Ken Martin |
| Through the battery compartment and one or two cable ties should ovoid the wheels/axle. When I was setting up my efi I did one or two experimental drives. I had a spare plastic lid that fit the jerry can which I drilled 2 tight fuel line holes in. However the implication is that the problem very quickly goes away when you stop accelerating, which might not be the case if it was debris blocking the inlet. This problem appeared after winter you say. That implies a long period of inactivity and the same petrol in the tank for a while. It could be that water has settled out. This water might have been sucked up into the fuel filter,How does the filter look, any water down in the bottom corner of it. Change it, Water will absorb into and partially block the paper filter. This would give you your fuel starvation problem. You could still have a big blob of water rolling around the bottom of the tank, although I would have thought it would settle at the pick up point. Also gum will form in old petrol that will block filters. When you are changing the filter, and have the old one off, you might try running the pump and pumping a litre or two of petrol into a clear glass bottle. Any water will settle to the bottom. You might even bounce the rear of the car and mixing up the tank just prior to doing this. Good fuel should be clear and bright, no cloudiness. Most garages will flush your tank for a fee. All they will do is drain the petrol, put in a couple of litres of methylated spirits to soak up and dilute the water, drain that, some metho' again, drain that then a couple of rinses with petrol to get the metho out. Something you can do yourself if you are careful. However usually they won't charge much and there is a convenience factor. |
| Peter |
| Hi Peter The problem also existed last year until I floored the gas pedel in frustration which cleared it for the balance of the season. This was after I had changed the whole ignition system thinking that was the cause. I have checked for water in the gas and added a bottle of methol-hydrate for good luck. The car would not start at all initially, until I removed the gas cap. Then it started right up, which made me think that the tank was not venting. But although I left the cap off for a trial run it still started spluttering after 100 yards. Thanks Ken |
| Ken Martin |
| methylhydrate=methanol. Mentholated spirits =ethanol plus some methanol. Really does sound like water to me. All petrol has water in it, that settles out over time. When it leaves the refinery it must be clear and bright at 4 degrees Celsius. If you cool it some more you can make the petrol cloudy, small droplets of water. Once out of the refinery it goes to distribution points and tankers and petrol station tanks where it is bound to pick up some more water. Try to run the tank right down prior to parking come winter. Your easiest option might be to just use up the tank of petrol, or dilute it out with some fresh. You might put some more methanol or metho' in the tank. Just be aware you might be reducing the octane rating quite a bit. Drain the tank if it is too annoying. Either way change the filter. |
| peter |
| That should read methylated spirits, although I'm sure menthol would give me that ring of confidence :) |
| Peter |
| Can't see it being water contamination, that would affect running at any speeds. It would have to be pretty big to block the strainer, like a piece of polyethylene or similar, but it could only be stirred up when you are moving which is why standing fuel flow test is OK. Possibly the tank has been coated in the past and that has significantly blocked the pickup (it can block it altogether) making it easier for minor debris to block it now, although that should affect static flow tests as well. It's also likely to cause *more* of a problem when flooring it, not less. The only way to prove or eliminate fuel starvation to the carbs is either to rig a very low pressure gauge to the fuel line, or when it starts happening switch off the ignition and coast to a halt, then listen carefully to the pump when you turn the ignition on again. Assuming an SU pump if this chatters a bit when turning the ignition on i.e. the float chambers aren't full it is indeed fuel starvation to the carbs. If you get none or only one click, i.e. the float chambers are already full, it isn't. |
| Paul Hunt |
| I think I'm going syphon off some of the gas from the bottom of the tank now that it has been standing for a couple of days. If I slowly insert a length of tubing until it touches bottom and syphon some fuel into a glass jar, I should be able to see if I have crap or water in the mix. I'm also going to run a line from a seperate gas container to the pump, by-passing the tank and see how she runs then. Only problem is it won't be this week as winter has materialized again with below freezing temperatures and snow! I'm with you Paul, I think it is something fairly large that has got into the tank somehow and is moving around when the car is in motion and being sucked over to the pick-up tube. Anyway I will let you know what I find, if anything. Thanks again Ken |
| Ken Martin |
| I had an intermittent (worn) point problem on my cars SU pump. I did the ride until it gives trouble and switch off immediately test that Paul explained. When I switched on, the pump did not "click". I left the ignition on, gave the pump a tap and it started up again. That night I cleaned/repaired the corroded points and refitted and reset then. The problem had felt like a plastic wrapper floating in the tank. Do the simple test before you attack the tank. Richard. |
| RH Davidson |
| Thanks Richard, but I had a new pump installed a couple of years ago. Non standard but it ticks and they are supposed to be extremely reliable. Can't remember the name right now. |
| Ken Martin |
| Well I have syphoned off some fuel and cannot see any crud or water in it. I also pulled off the filler pipe and looked in with a flashlight. Fuel lookes absolutely clean and there is no sign of a sludge, dirt or rust on the part of the tank that I could see. I'm going to try and get hold of a Dentists inspection mirror. I figure if I angle it right I should be able to see the pick-up tupe further down the tank. I'll let you know. Ken |
| Ken Martin |
| SU carbi pots sticking? |
| peter |
| Ken If the fuel looks clean probably a good idea to do the rolling test as Paul Hunt described. Can't always trust "new" parts. And if the car did not run for the entire winter? and it is a points style pump, they could be glazing over and causing a problem. Mike |
| MK Mike |
| If you do not have the charcoal canister in the engine bay ...then what was done to the fuel tank vent pipe that used to terminate at the canister? Was it sealed off? If so...then maybe there's your problem. |
| Daniel Wong |
| What I did not mention in my previous suggestion was that I initially connected an outboard engine petrol tank with "priming bulb" directly to the carbs (has same size hose). I placed the tank at the passengers feet and he merely squeezed the "priming bulb" to feed the engine while I test drove the car (make sure direction of bulb is correct). Thus I separated possible intermittent fuel feed problems from intermittent carb problems. Rigging a gravity feed could be used to replace the method I used. I have experienced a collapsing inner lining of a rubber fuel hose that acted as a "flap valve", closing on excessive demand. A similar problem has also been experienced where a braided fuel hose slides over a metal pipe. The inner section can be cut by the pipe and create a "flap valve" type restriction. Remember, starting with the longest bypass/isolation and then halving the route is the quickest way to pinpoint an intermittent fault. Richard. |
| RH Davidson |
| Morning Guys, Happy Easter I haven't ben able to do the "rolling test" yet as I couldn't get hold of 4-5 ft of fuel line from anywhere local, only pre-cut 6" lengths. I'll have to try tomorrow as everything is shut today. Good thinking Richard regarding the "flap valve". That's what I thought may have happened last year and I ended up changing all the rubber fuel lines at the carb end. Then I sliced all of the old ones open and found nothing and the problem was still there. Once I have done the rolling test, connecting a small gas can to the pump, I will know if it is the tank or the pump and forward, which will be a major diagnostic improvement. I'm trying not to drop the tank or disturb the Sending Unit locking rings if possible. I have also wondered about the tank venting Daniel, as I cannot see any means of venting. It is possible that there is a vent pipe on top of the tank that I cannot see. However as I did drill the vent cap and the problem is still there I don't think venting is the problem. Thanks guys Ken |
| Ken Martin |
| Tapping the carb wouldn't cure a tank venting problem either. But if it is a float valve sticking closed, and you do it with the ignition switched off and tapping it does make it open again, then when you turn on the ignition again the pump will chatter away while it fills that carb. If you have an SU pump. If you do have an SU pump, and it doesn't chatter when you turn the ignition back on but the engine starts and runs as it should, then the problem isn't a sticking float valve anyway. |
| Paul Hunt |
| OK, just got back in after a short drive with the fuel tank isolated feeding directly from a new portable gas tank. And the answer is............it's still missing. Therefore I guess the tank is innocent and the guilty party could be the fuel pump, although it's only about 4-5 years old. One thing I did notice was that the pump seemed to be pumping - clicking faster than I would have thought when running. I ducted the fuel line up through the battery compartment so I could hear the pump operating quite clearly while driving. I would have thought there would have been a reasonable pause between clicks for some reason, but it was running at about the same speed as the tappet noise from the engine. Is this a clue or would that be normal? For anyone interested I have attached a photo of the beast.  |
| Ken Martin |
| You might check the float bowles and hoses to the jet. There is a small o-ring that has been known (owner error) to come off, go down the tube and restrict the hose. |
| Dan Robinson |
| Thanks Dan, I will check but, everything was running fine at the beginning of last season, for about 2-3 months, then it suddenly started this nonsense, then cleared itself when I floored the pedal and was fine for the rest of the season. Now it's back and everything I have tried and changed hasn't had any effect, so I don't think it's something I've done while working on it, just something that has worn out or broken. Remember it runs fine while standing still, problem only happens when in motion and accelerating, or trying to. |
| Ken Martin |
| Have you done the pump delivery check yet? Have you turned off the ignition while the problem is occuring, come to a halt, then turned the ignition back on and listened carefully to the pump yet? |
| Paul Hunt |
| Ken... I would remove the tops of both float bowls and if you have a regular float needle and seat set and the needle is made with a plastic body and metal or viton tipped I would take a small fine file and clean up the outside edges of the plastic body. It doea not take much. I have had three of my customers have similar problems. It seems to be a slight build up from todays fuels that partialy siezes the needle in the brass body. One of those I had to take pliers and actually pull it out. Sandy Sanders |
| conrad sanders |
| Well after running it around the block on a seperate fuel supply with no change, I decided to replace the rubber fuel lines to and from the pump, seeing as I had one half off already. After installing the hoses, I decided to crank it up not expecting any change...........and off she went like a scalded cat! So, I don't know whether it was the old hoses, or the pulling and tugging that took place on the pump while trying to get the old hoses off. I haven't had chance to try her again and I definately don't trust her yet. I have ordered up a fuel pressure meter which hopefully will be here before the weekend and then I can see if the fuel pump is running at low pressure. I'm thinking of trying to rig it up in the cockpit so that I can see what's happening if the problem is intermittant. Obviously I wouldn't be able to get an instant re-out if the meter was under the hood. I'm thinking maybe to run some hoses up through the hole in the drive shaft tunnel where the gearbox filler is. Any comments. Ken |
| Ken Martin |
| "Any comments" If you had done the delivery check ... Depending on how thick the meter pipe is it would probably be easier to use one of the many holes in the bulkhead, like for choke, heater and speedo cables. |
| Paul Hunt |
| Hi Paul, I assumed the meter pipe would be the same as the rubber fuel line ie 5/16ths, no? Ken |
| Ken Martin |
| Ive had this problem with my TR7. I think the error of your ways was not taking a known good fuel line and running that directly to the SU's from the portable gas (petrol) container. Also not opening up the fuel bowl and looking inside, clearing out the jet etc... This is coming from someone who learned the hard way, but I was already trying to diagnose the fuel line and had taken that into consideration. Also could there be a kink or restricted bend in the new fuel lines you ran ??? That would coincide with not having enough fuel under load. -James P.S. I also have a vented gas cap question that I am starting a new thread for |
| James |
| Well guess what, just went up the road and she's back to her old tricks. Spitting and coughing and eventually dying. Turned off the ignition as per Pauls suggestion, turned it on and one click from the pump. Started right up and drove the 200 yards back to the house without a murmer. However, when I got up the drive I had a look at the clear fuel filter and there was hardly any fuel coming through. James, I have checked the bowls three or four times, changed the jets and run off a brand new fuel line from a seperate tank. It has to be the pump. Unfortunately I'm now away for three weeks so I'll have to figure it out when I get back. Thanks for your help guys. Cheers Ken |
| Ken Martin |
| That was part of my problem. The fuel filter was not filling up. It has to be crap in the Tank and fuel line. Do you have an air hose that you can blow through the lines? Your best bet since you are seeing how much of a headache it is, is to take it to a shop. Like someone mentioned before, they will clear the stuff out of the tank and lines. I dont understand how it could be the fuel pump, you would have different symptoms. Also I apologize if I state the obvious. It tends to be the obvious things we look over as I did till I got good advice about it being my fuel line. It is going to be money well spent and you can go there with full knowledge of the problem and save yourself the notion of being ripped off from not knowing its possible solution as to just blowing out the lines. -James |
| JRB Mr |
| Fuel pressure gauge arrived and fitted in line just before the fuel filter. Shows 4 lbs on the nail. Took her up the road, started missing, hopped out, gauge still reads 4lbs. and the filter is 3/4 full. Now, the only thing I have noticed is that if I lift off on the gas, she will recover slightly. Could be possible that one of the float chambers is flooding or too rich. I have tested the lift pins a few days ago and the engine revs drop slightly and then recovers as it should, so that seems ok. Anybody know of a decent cliff? Ken |
| Ken Martin |
| Ignore what the filter has in it as far as fuel level goes! They can vary from almost empty to almost full and have no effect on running. If it were the filter causing the problem then you would either have had lots of chattering of the pump while it refilled the carbs in order for you to be able to drive off normally again, or it would have still been stumbling when you drove off. It depends on what gauge you get as to the pipe and fitting size, but from all your tests so far including the remote gauge it says to be the problem is nothing to do with tank including venting, pump, pipes or filter but is to do with the carbs themselves. Where do your float chamber vent/overflow pipes go? If you don't have a canister they should go down the side of the engine from the carbs and be clipped to the left-hand front engine mount. If you direct these into a container (which must still be open to atmosphere) while you drive and get the problem, then look in the conatiner for fuel, you will know whether one is overflowing or not. With HSs it is easy to drive until you get the problem, switch off and come to a halt, and remove the float chamber lids. Compare fuel level inside the chambers, and if that's the same (which I'm fully expecting since you get no chatteriung of the pump when restarting) the problem must be further into the carbs i.e. jet pipe, jet, needle, piston movement etc. |
| Paul Hunt |
| Of course Paul is right but it can still be attributed to blocked lines, debris in the tank as well. There is a reason for the fuel bowl to be overflowing if it is in fact the case... Something clogging the jet... Debris. If you solve the problem it can come back if your still pumping crap through your lines. |
| James |
| Rig small gravity tank from the car roof (tie it or get a passenger to hold it) and siphon fuel directly to the carbs. Take the car for a test ride. If the fault is still there, the cause is in the carbs. If it only returns when the original supply line is refitted, then the cause is the fuel supply. Repeat the test to confirm the results. Richard. |
| RH Davidson |
| Debris in the lines causing a float valve to leak and overflow would indeed also cause running problems, but that is something Ken has never mentioned! |
| Paul Hunt |
This thread was discussed between 04/04/2009 and 20/04/2009
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