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MG MGB Technical - Fuel supply

Hello everyone
I own a 75 MGB. I am having a problem with the engine sputtering and sometimes it will backfire. When I first start out on a drive, the car seems to run fine. After it's well warmed up this problem starts. The fuel filter becomes empty, it is clear plastic so I can see in it. The fuel pump and filter are new. I have checked flow from the pump and through the filter, there is lots. The fuel filter starts out full and becomes empty. HELP!
Thanks
Johnny
J Sauve

Johnny. It sounds like your fuel pump is not capable of delivering the fuel volume you need. There is a good tech article on the SU fuel pump, including pressure and volume requirements, on my website at www.custompistols.com/ under the MG section, then articles. The article was written by David DuBois, who has been rebuilding these pumps for many years and who contacted Burlen, the current builder, to make sure the article was accurate. That would be a good first place to start.

On my daughter's 77B, the line to the charcoal canister was plugged. That did not allow the fuel to flow properly. Removing the gas cap caused the air to "whoosh" into the tank with the engine running. If this line is blocked on your car, the problem would be as you are seeing.

One of these tests should allow you to focus in on the problem. I am assuming you have the single Z-S carb on the car and a fairly stock engine. Les
Les Bengtson

The pump should deliver at least 1 pt per minute at the carbs, remove the pipe from the carb and direct it into a container and time it. My V8 fuel filter got emptier and emptier eventually showing nothing at all with the engine off but still continued to run perfectly. Just for the hell of it I bled the filter so it was nearly full a couple of hundred miles ago, now it is only 2/3rds full and I expect it will gradually empty again. Why I'm not quite sure, possibly a slight suction side air leak, but as I say I don't think that in itself is anything to be concerned about.
Paul Hunt

Also, if there were some debris in your tank that could conceivably block the fuel pickup tube entry, it could starve the pump and your carbs. Usually, tho, this is a pretty random thing and does not happen every time you accelerate. It would depend on how much crap is in the tank, and how the fuel sloshes around inside.
Bob Muenchausen

I have the same problem with filters getting empty but it doesn't seem to affect the way the car runs. FWIW, Clifton
Clifton Gordon

An apparently empty filter is NOT an issue. I know it defies logic but don't let it bother you. A brown filter is another matter...

Mike!
mike!

With only 80 miles on a recently redone 1977 MGB.... my most-of-the-time near empty fuel filter is keeping me from relaxing and enjoying the experience. I've tried bleeding like Paul but it comes back to the same situation.

I will take some comfort from the comments of Paul, Clifton & Mike! because I recognize their experience from other posts.

But I hate not understanding the cause of an issue... or maybe not an issue as Mike! says. I'm so confused....

If someone has a good explanation for this phenomenon I'd really appreciate hearing it!
Brian Denis

Have you checked the condition of the line on the suction side of the pump? You could be pulling in a mix of air and fuel. I recently changed the metal can filter to a see through unit on my last road trip. I have it mounted to the drivers side of the heater box in a vertical condition, and it is full all the time.
R Hill

A small suction side air-leak isn't a problem as long as the pump can still pump enough fuel with the air to meet the maximum requirements of the engine. Any air that is passed through to the float chambers will simply escape through the vent. In practice the pumps are capable of delivering much more than even a V8 requires. How soon a bled filter refills with air can be an indication of how big such a leak might be, but I have no idea how much is too much i.e. close to not delivering enough fuel. You could remove a pipe from a carb and position it at the *bottom* of a container and turn on the ignition, so that once the fuel level rises above the outlet of the pipe you will see if there any air bubbles escaping. It would still be a matter of judgement as to whether it was worth doing anything about it though.
Paul Hunt

PS. Just think, if you didn't have a transparent fuel filter you wouldn't even know about it to worry about it :o)
Paul Hunt

And if you have a filter / regulator that has its outlet at the top you will never see it empty!

My theory is that there is always air dissolved in the fuel and air will get sucked in from the tank when it is nearly empty. Small bubbles will therefore travel down the fuel line even if there is no leak. These collect in the filter which normally lies on its side. The fuel still flows through the filter and eventually it will start to carry excess air through to the float chambers where it escapes via the vent.

If you want to keep the air flowing with the fuel and the filter full, mount the filter vertically so the outlet is uppermost - but it really doesn't matter at all.
Chris at Octarine Services

Well, although it is at an angle and not vertical the base of the outlet pipe on mine is significantly above the base of the inlet pipe, and the fuel level has been below the level of the oulet pipe, infact I have seen the filter completely empty except for a small pulse as the pump ... pumps. It was seeing it so low that led me to investigate it and bleed the filter.
Paul Hunt

I have a similar problem, car behaves well until I lose speed after turning a corner when I accelerate the car splutters misfires and some times a bang. If I accelerate slowly the problem does not seem to occur, I have also reached for the choke but am not sure it made any difference.I have changed points re-checked the gap, changed the condenser, changed to SU electronic pump and replaced the in line fuel filter which also never shows very much fuel. I am told that this is normal. The plugs are all the same colour grey. In this part of France temperatures can get pretty high and I have also suspected vapourisation leading to air lock and have lagged the fuel pipes with reflective material also the deflector plate. However last year Idrove the car across France in very high temperatures and did not experience any problem.
EJ Chapman

As ever, and assuming it has the electric tach, is the tach steady or flickering when this happens? If so check the grounding wire inside the distributor, and check the conductors haven't broken inside the insulation of the points wire which goes to the coil, or for other bad connections in the ignition LT circuit. A bang in the exhaust does imply an ignition failure that allows unburnt fuel to accumulate, which explodes when the ignition gets going again. This can also happen if the points gap is very wide, so do all the usual checks on plugs, points and timing.

Vapour lock is a highly overated phenomenon in my view, and is more likely to happen on a hot start in very high ambients, or possibly when idling, not normal running and not only when cornering and accelerating. It really can only affect HS carbs in the exposed pipes between the float bowl and the jet. Any vapourisation that *did* occur in the feed pipes to the carbs would simply be pumped out of the float chamber vent to be replaced by fresh fuel. To create vapourisation faster than the fuel pump could pump would require massive heat, and although France gets some nice weather in the summer it doesn't get anywhere near as hot as American desert states and MGs run there just fine.

Air in fuel line filters does seem normal, mine were recently completely empty but the car, a V8, runs just fine in all weathers.
Paul Hunt

I have to agree with Paul regarding vapor lock. I have only experienced it in our TD and then only after the car has been run in very hot climates and after it has been shut down and left for 20 minutes or so allowing the heat to soak into everything in the engine compartment due to no air flow through the engine compartment. It has occurred then in the channel between the float bowl and the carburetor. It builds up enough pressure that it keeps the needle valve from opening under normal pump pressure. Installation of a heat shield has minimized the problem. I have never suffered the problem in the MGB, even though we have driven it in the Southern California heat and having run the temperature gauge up to around 220° F on occasion. I would only consider vapor lock as causing a problem after having exhausted all other possibilities and certainly not as a cause of stumbling when the car is in motion. Cheers - Dave
David DuBois

Interesting thread. The pressure on the gas vapor you see in the filter is the same as on the fuel sooo as long as there is fuel touching the outlet...you get fuel. If it gets low enough for the vapor to escape thru the outlet then more fuel will enter to take it's place if the pump is working properly. The vapor will then escape thru the carb vents. A pressure & flow test will still check "good".
71 MGB Question: On the way to a car show I ran out of fuel with a little over 1/4 tank showing on the gauge. 2 gallons got me to a station with a little over 1/4 tamk showing. 6-1/2 gallons filled it up (in a 12 gal tank). What does the pick-up tube look like? How can it be accessed? Is the design such that it could only go 1/2 way to the bottom if something was wrong? Is there a rag or pc. of paper floating to the pick-up at 1/2 tank level? What will make this problem go away? (Note: I'm using a vented cap)
Thanks, Ron
Ron Stuckey

Any of the things mentioned above could be causing your problem. I would like to add another suggestion.

Both a freind of mine in the local MG club and I had a similar problem with our respective cars (both '72MGBs). Ours turned out to be caused by holes rusted through the pick-up tube inside the fuel tank. Until sometime in the 1977 model year, the fuel pickup went from the side of the tank into its middle. When I removed my old tank and looked inside, I found that the pickup tube ran along the top of the tank until it reached the middle, and then dipped down. This would increase its exposure to moisture (and the possibility of rusting) unless you always kept the tank full. The solution for us, was to each buy a new tank.

You can determine whether or not you have a similar problem by doing the following. Attach a line to the output side of your fuel pump that contains six or twelve inches of clear plastic tubing. Tun the end of this into a gas can. Turn on the ignition powering up the fuel pump, and then examine the clear plastic tubing. When I did this, most of what I saw (40-50%) was air bubbles. My car would idle fine, but would not run under load. Just to check the pump operation, I used this same process, running the tubing down the fuel filler pipe. I had 100% fuel with no air.

If you try these tests with the same results, you can either buy a new fuel tank ($200 +/-) or buy the fuel sending unit for the 1977 and later cars which contains a new fuel pickup. The Moss catalog that I just checked shows it at $25.95 (may not be current price). Your fuel guage would need to be recalibrated (adjusted). Others on this BBS will know for sure whether this sending unit would be electrically compatible with your particular car.

Good luck!

glq Greg

I believe Matt Kulka installed a later sending unit in his 74 fuel tank. I don't remember the details or if he had to calibrate the gauge. Clifton
Clifton Gordon

Both UK and US cars changed to the sender with integral pick-up pipe in August 76 but there is no corresponding change in fuel gauges shown in the Parts Catalogue, so I wouldn't expect to have to recalibrate the gauge for that change. However unless you have recently changed the sender, and recalibrated, you probably *would* have to recalibrate as the senders that have been available for at least 15 years in the UK do not have the same characteristics as the originals, as I found out when replacing them on both my cars, the hard way on the first car. Both have needed subsequent replacements but have not needed further recalibrating.
Paul Hunt

The fuel filter issue seems counter-intuitive but consider:
Fuel vaporises at a low temperature and with increased heat it attains much higher vapor pressures. This can lead to so-called vapor-lock but usually only in vehicles with a long suction run in the fuel line (mechanical pumps) and/or a pump that sits in a hot location. Apparently, this can be an issue with the fuel "tee" between carbs but I have not experienced it.

Point is, even if you have a no air in the system at all, you will eventually get a fuel vapor pocket in ANY "wide spot" in the fuel line. The vapor will take up all available space above the "level-line" of fuel and, if no fuel is pumping, will force fuel to go below the level-line when it is warm out, especially if the pressure can bleed off somewhere (leaky valves in pump or evaporation of fuel in float bowls).

Making the thing even harder to see is that you also have a liquid (fuel) operating under a slight "skin effect" so the actual fuel flow is a nearly invisible thin film or skin along the inner surface of the filter.

Since the fuel system operates under positive pressure from the pump forward and the float bowls are vented (either to atmosphere or to the carbon cannister) any bubbles will that find their way into the tubing will be delt with and vented.

So, "empty" fuel filter is no problem.

The problem with the sputtering and backfiring seems to be ignition related to me. I'd start with new ignition components and see how I stood after replacement.

Mike!
mike!

I installed a new tank in my '74 and had the problem glq did. Except my pickup tube wasn't rusted through, it was just badly manufactured. If my car was at 1/4 tank or less, and I got stopped at an uphill, the pump would no longer pick up fuel.

As Clifton noted, I resolved it by installing the fuel sender with the integral pickup tube. It was a drop-in replacement. My fuel guage did not change reading with the new sender. (It was equally inaccurate as with the previous replacement sender, but that's another story.)

Ron Stuckey, if you pull out your fuel sender and shine a flashlight into the gas tank towards the "roof" of the tank, you'll see the pickup tube. It sounds like you have the same problem glq and I had. You can resolve it as I did mine. However, if your tank is rusting out, deal with it now. The rust in my old tank devoured my piston rings. Yes, I had a fuel filter installed. Two, in fact.

(Clifton, kudos for your memory.)
Matt Kulka

Hello All,
I've been away from the computer for the last few days and was hoping that the thread was still going.

Thanks for the informative responses on the tank issue. Sence this is a "new" tank (2 years) there is a strong possibility that there is a manufacturing defect in the pick-up tube (or rust?). I will pull the sending unit to see what I can find out. My experience, the odds & Mr. Murphy tell me that I will have to pull the tank before it is all over.

Some mentioned that a late model sender has the pick-up tube built in and offers a possibility. Has anyone tried to modify an early sender by installing a pick-up tube in it? Just a thought. We all get creative when the price goes up.

Thanks Again,
Ron
Ron Stuckey

This thread was discussed between 12/07/2004 and 23/07/2004

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