MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fuel leak from carbs - help please!

Hi Guys. I recently fittes some K&N Fileters together with new needles. Everything was fine, but the other day i noticed fuel was leaking as engine was cutting out.

I noticed that the long copper pipe that link the 2 carb lids was cracked. This is a long copper pipe. I decided to replace the pipe with a rubber flexi pipe that i picked up from the hardware store. I have to state here, that the copper pipe was very long, going down into the engine bay and curving to come back up again. This made no sense to me, so the hose that i used to replace is a lot shorter.

I managed to get the car started again, but then it cut out and i heard the fuel pump working away which suggested a leak somewhere. I traced the leak to coming from where the gasket meets the airfilter. Is this merely a worn gasket that needs replacing, or is fuel supposed to come out the jet into the airfilter (where the gasket meets the filter)? This doesnt make any sense to me and i would appreciate if someone could offer come advice?

Thanks,
Rudi.
Rudi

Sounds like you have replaced what should be two independant vent pipes open to atmosphere at the bottom with a single linking pipe. As such neither float chamber is vented properly and it is not surprising you have fuel leaking, probably out of the jets and a grossly rich mixture. The vent pipes were originally steel or alloy (although the material itself isn't important) like clutch hydraulic pipe, with a short piece of rubber hose about 1" and two worm clips connecting each to the float chamber lid on HSs or body port on HIFs. Both are routed such they come together and are held by a clip attached to the left-hand front engine mount, and end shortly below that.

If there really was a continuous copper pipe connecting the two then the PO that fitted it was pretty D(umb) and the fact that it was split was acting as the vent.
Paul Hunt 2

Hi Paul,

I have to say, that i never knew if the two pipes were connected or not, but when i pulled them out, fuel was spewing from one of them. As such i assumed they were connected and thus replaced them with a pipe?

I they are vented pipes, why is it then that they need such long steel hoses? Why need hoses at all?
I'm just trying to get my head around this. Also, why was fuel spewing from them?

Thanks,
Rudi
Rudi

Rudi. There are, as Paul Notes, two vents associated with the twin SU setup--one on each carb. Normally, their operation is to allow air, and sometimes fuel fumes, to be vented out of the carb fuel bowl so that you do not get a "vapor lock", meaning that no fuel can flow into the fuel bowl, hence to the jets and up into the "throat", or "venturi" of the cab.

If there is some form of problem, such as a float inside the fuel bowl, which means that it cannot shut off the needle valve which controls fuel coming into the fuel bowl; a bad needle valve which does not shut off properly and allows the fuel to keep flowing; or a speck of "crud" which accomplishes the same thing, fuel will flow out the vent hole. Hence, we need an overflow hose and, since the hot exhaust manifold is directly below the carbs, we want that overflow hose to direct the fuel away from the exhaust system. This takes a fairly long hose and a rigid hose is not affected by the drafts within the engine compartment and under the car while a flex hose is.

If you join the vents of the two carbs together, the system does not vent properly and the engine may not run. It certainly will not run well. If you have a bad float or needle valve the overflow from one carb is directed to the other carb and has no place to go. In that situation, the excess fuel is forced up through the jet and into the carb throat. Massively rich mixture and, sometimes, fuel escaping from the front of the carbs where the air filters attach.

Remove the current "bridging" line between the two vents, hook up long flex hoses to each and run each hose to a container to catch any fuel delivered by the vent line. Disconnect the line from the coil to the distributor to prevent overheating of the points and coil if the points happen to be in the closed condition. THIS IS IMPORTANT. Fire danger if you do not.

Turn on the ignition switch and see if one of the carbs is overflowing through the vent hole. If so, take the carb apart and determine if it is a float problem, a needle valve problem, or crud inside the bowl. Reassemble, retest, if it works, hook up the proper vent hose system.

REMEMBER TO RE-INSTALL THE LINE FROM THE COIL TO THE DISTRIBUTOR BEFORE TAKING A TEST DRIVE.

Les
Les Bengtson

And they need to be as long as they are to get any vented fuel and fumes down past the exhaust and not dripped onto it.
Paul Hunt 2

As fuel flows in air needs to flow out, also if the fuel level control needle valve fails the fuel needs to be lead safely to a level lower than the exhaust manifold. There is a lot on incontinent SUs in the archives.
Stan Best

I concur with what Les and Paul have said.

I had the same problem, my BGT wasn't running smoothly at all on one particularly hot summers day this year (a rare occurence!).

Sat a a set of lights it kept stalling and when I set off I looked in the mirror to see a pool of liquid on the tarmac where I was stationary.

A quick hasten back to home and onto the drive (big mistake!) I peered down underneath and saw fuel being pumped out of the vents. Engine off very quickly and then parked up to swap for the midget. But thank goodness the bents end beneath the exhaust... perish the thought about that amount of fuel hitting a hot exhaust!.

That evening I stripped down the offending carb and found a sticking float, once duly free'd up and cleaned off it was all reassembled and no leaks found.

Amazing how much better the fuel economy became after that!

Andrew
A I McGee

Hi Guys. Thanks for all the helpful suggestions.

I did all that was suggested, but now i cannot get the car to start. Engine turns, but its not 'taking'.

any suggestions?
Rudi

Check the spark plugs. They have been soaked with gas. I usually replace then when that happens.

Steve
Steve

It's also possible that now the fuel is at the proper level the car has been set to run too lean to start. A quick look at the jets relative to the bridge will give you clue.
Stan Best

"A quick hasten back to home "

Had a similar thing on the V8, but I cross-connected the overdrive switch to the fuel pump and manually switched the pump on for a few seconds every now and again as I didn't wan't fuel pumping out all the time I was driving!

For the non-starting after cranking a bit take out a couple of plugs and inspect/sniff them. Normally you would get a strong fuel smell but they should not be wet.

If they are black and cruddy then they probably fouled in the short time they were running rich, and because it was only a short time cleaning them should do fine. If they are wet then it is flooded, which could be because they are fouled, but you would normally get a strong fuel smell with this unless you are in the open air and it is windy. With the (clean) plugs back in crank with the throttle wide open and the choke closed and it should clear, get ready to release the throttle and half-pull the choke when it catches.

If there is no fuel smell on the plugs fuel isn't getting through, so you will have to remove a fuel feed pipe carefully (it should spurt if the ignition has been on recently) from a carb, direct it into a container and check you have a good flow with the ignition on.

If that's OK and you have HSs then lift the float chamber lids a bit and check you do have fuel in both. If HIFs then if you remove the vent pipe and air cleaners, put a bit of hose on the vents and blow gently, you should see fuel bubbling up out of the jets.

If all that is OK then you need to start from first principles and check you have a spark. This is where I'd normally begin with a non-starter but given the history it is worth jumping straight to fuel. The easiest way to check spark is to clip a timing light onto the coil lead and each plug lead and watch for flashes when cranking. When on 1 and 4 you can also check the timing is about right. Without a light you will have to lay a plug on the block, conenct each HT lead to it in turn (don't get them jumbled up) and watch for the spark while cranking.

Paul Hunt 2

Paul

You are obviously very clever.... if I was to try something as ingenious as that the car would have definitely set on fire...

and as for my big mistake driving on to the drive.... it's tarmac... and now has a big hole in it where the petrol flooded onto it!

Mind you there is a large oil slick under where I park the Midget anyhow!

Andrew
A I McGee

Rudi

A quick test for the mixture,if you have a manual choke, would be to pull the choke all the way out. This will enrichen the mixture hopefully allowing it to start.

Stan's thinking is right on. With the vents blocked, any fuel entering the floatbowl was pressurizing it, and therefore forcing extra fuel out the jets. Someone might have then leaned the mixture to get it to run. Now that the fuel has to be sucked out rather than forced out it is probably too lean.
Steven J. Korotky

"now has a big hole in it"

A neighbour spilt some oil on her drive, and tried to clean it up with petrol, with the same result!

As far as the mixture goes don't forget this car was running OK until the owner replaced the split i.e. 'ventilated' pipe with a non-split pipe, and the flooding started immediately after that. By going back to ventilated pipes again the situation should be as before, unless someone grossly maladjusted the carbs in the meantime. You could *never* lean out the carbs enough to compensate for fuel flooding up out of the jet because the vents were blocked.
Paul Hunt 2

Rudi. The first thing to do is to replace the spark plugs as Steve has suggested. Always a good thing to do whenever there has been any form of carb disorder.

Next "quick test" is to remove the air filters and pour about one table spoon of petrol down the throat of each carb and try to start the engine. If it starts, you know the ignition system electrics are good and can concentrate on the fuel system. David DuBois has written a fuel system troubleshooting guide and I have one on the ignition system. Both are on my website at www.custompistols.com/ and may be of use.

Paul seems to think that the problem began when you replaced the old overflow tubes. I am not sure about this myself. You could have had the problem before but the overflow tube was allowing the bad carb to vent the fuel to the outside. When you cross connected the tubes, you simply allowed any problem to affect both carbs. Thus, neither we, nor you, know when the problem started, only when you first noticed it. Start with the basic "fuel into the carb" test, then go from there. If that test does not get the engine to fire, with new spark plugs installed, start checking out the fuel system and ignition system using the tech articles and correcting any identified problems as you go. Also a good idea to put a fresh charge on the battery before testing again.

Paul Hunt has a good article on setting up the carbs on his website and it might be of use to you to read it. Should help you determine if the carbs are set up so they will be working properly. Not only idle mixture, but the functioning of the choke feature should be examined.

Les
Les Bengtson

Cheers guys, thanks for all the help. Will let you know how i get on.
Rudi

Rudi. Please do let us know. Both something of interest to those of us who have responded and worthwhile information to those who may, some day, be in a similar position as yourself. Thanks.

Les
Les Bengtson

Success!! The sweet sound of an engine taking.

Took out the plugs and they were soaked. Replaced them and engine started. Mixture smells a bit rich now i have to say? Adjust the jets too fix it?
Rudi

Don't tweak it and hope for the best, set them up properly from scratch. Takes time and care, but well worth it, and not fiddled with afterwards they will keep their tune for many tens of thousands of miles. But first you need to make sure all the following are good: valve clearances, plug gaps/condition, points condition/dwell/gap, condenser function, timing including vacuum and centrifugal advance function, HT lead, rotor and cap condition, and not least all connections in the ignition system. http://www.mgb-stuff.org.uk should help, click in 'Spanners' then 'Valve Clearances, 'Ignition' and 'Distributor Adjustments', and 'Fuel' then 'SU Carbs' and 'Setting-up'.
Paul Hunt 2

This thread was discussed between 18/11/2007 and 26/11/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.