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MG MGB Technical - MGB major fuel leak when starting engine

H,
after successfully converting 1974 MGB GT from 6 volt to 12 volt, car has been running sweetly. Problem now is a serious petrol leak when the engine is switched on. This happened when I headed to my nearest petrol station, topped up came back, noticed a line along the road all the way back to my car, even from when I headed out of the garage.
Checked under the car, expecting an oil leak, but all I found was a pool soaked into the road.
Switched the engine on to check, looked under the car and liquid spurting out from under the right hand side of the engine were the Carb are! Not good.
Also I noticed, well my better half pointed it out, one of the Carb plastic tops was hanging loose..!
Checked the garage when I started the car, a small pool of petrol has soaked into the floor. This I have covered with cat litter and sand to soak up.
Any help or advice for a fix that won't cost the earth, please get back to me.
Thanks again
Jim
J Giddins

Jim,

If you still have SU carbs you should then have HIF's
if the fuel is leaking out the vent it is most likely you floats or needle sticking If the floats are old they might have fuel in them, you can do them or the car but it's eazy to take the carbs off. remove the 4 bolts from the air cleaners, remove the 4 bolts from the carbs (2 each)pull the carbs off and leave the linkage on the car.
Remove the 4 bolts on the bottom of the carb with the cover off you can see the float the pin that holds it in is on the side of the carb body remove it and the float will come out, shake it and hold it up to the light if it has fuel in it you will see it, if they are you will have to replace them, they are different in each carb.

You can check if it is the needle sticking have someone sit in the car to work the key, gently tap the side of the carbs with a SMALL hammer and have them turn the key on if it still leaks try again if it doesn't stop remove carbs.

Bob

see this link: http://www.chicagolandmgclub.com/photos/hif_carb/
Bob Fisher

Bob,

Thanks for the feedback..
one thing that makes me think it is a leak was the amount that gushing from under the car, maybe from a pipe when the car was ticking over.

Hoping for a quick but cheap fix!

Thanks

Jim
J Giddins

Hi Jim,

What Bob said, above is correct, however, also check your rubber hoses for cracking and leaks.

Herb
Herb Adler

There seems to be more problems with the HIF than the older HS4 with overflow. Removing the carbs and replacing takes about 45mins to an hour. You will need to clean out the float chamber and check the floats for any fuel in them.
Now I found that when I used mine as a daily driver and I used leaded petrol I didn't have a problem. But with occasional use, and additives, I found the carbs reguarly overflowed.
I had the carbs rebuilt at a specialist MG garage, tried Grose jets. Still happened. In the end I fitted a pair of HS4s and have had few problems since, and the side float chambers are a couple of minutes to fix.
c cummins

Jim, take a look at this diagram http://www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=227

You need to check all the petrol feed pipework, that's from number 48 round to number 51. But the first thing to check are the overflow pipes 53 and 54. These hang down from the carbs and take the petrol away from the hot exhaust if the carbs should overflow. Locate them, possible from underneath although you should be able to find them from above, then get your partner to switch on the ignition - no need to start the engine. This will start the fuel pump and you will quickly see where the petrol is coming from. Report back here and we will try to help you.

REMEMBER NO NAKED LIGHTS OR SPARKS. It is very easy to turn a fuel leak into a fire.
Mike Howlett

Could be from any of the hoses, the HIFs should have overflow pipes taking any overflow down past the engine and should only be visible on the floor. If it's visible higher than that HIFs have an interconnecting hose that takes the fuel from the front carb to the back, as well as one between the filter and the front carb, and the metal pipe and the filter. Three pals have had leaks in recent weeks - one on one side of the filter and one on the other with HIFs, as well as an HIF float valve sticking on another, and both float valves suddenly sticking open on HSs.

Look closely with the ignition on and see where it is coming from, *DON'T* crank or start the engine! If from the overflows then it *is* apparently possible to change them in-situ on the 4-cylinder (but not the V8), and if removing the cover you will need a new O-ring. One of several reasons why if faced with a choice of fitting either HSs or HIFs I would *always* go for HSs.
PaulH Solihull

Hi thanks for the feedback.. I have asked a mobile mechanic to check and give me his verdict.
He disconnected the fuel pipe from the carbs, started the engine until fuel was used up in carbs; no leak!
Tapped the carbs! re-attached the fuel pipe, started engine, fuel leaked out from the overflow..
He has suggested taken carbs off and checking the needle and condition, as Bob from US mentioned..
Possible rubbish from tank clogging needle..making it overflow..has been in garage for 18 months before it got through MOT, with welding and water pump and back break caliper!
Any other advice.
Thanks

Jim
J Giddins

Hardly surprising it didn't overflow if the feed pipes were removed form the carbs (and presumably blocked off or the pump disabled).

However disabling the pump and running the engine to empty the float chambers, then reconnecting the pump, *is* a normal step. Once the float chambers are empty the float valves are wide open (normally they are only open a smidgen to let fuel in at the rate it is being used) and this can flush any debris through the valve. But if it happens to the same carb again the valve is suspect, and if it happens to both carbs debris is suspect. You should also check the float carefully, I had intermittent problems on one HIF until I suddenly noticed the float (dark brown so not easily visible) had some fuel in it. Since changing that no further problem.

As well as running the engine with no fuel supply to the carbs you need to direct the front carb feed pipe into a container and switch on the ignition. It should deliver at least one Imperial pint per minute and in practice close to two, in a steady series of pulses, with minimal bubbling. It should also show if there is any debris coming from the feed pipe in the bottom of the container.

A friend had first one carb then the other suddenly start to overflow big-time. The flex hose between the metal pipe and the two carbs (HSs on a 72) appears to be damaged, possibly from engine rocking, possibly from Ethanol as he always uses 95 and that almost certainly contains 5%. It may be that the rubber is decomposing, although there was nothing in the bottom of the float chambers. His garage has recommended a filter, but at the pump, which I'm not convinced is going to help as the pump already has (or should have) a filter on the inlet.

PaulH Solihull

Hi, just an update..it looks as though the float valve had sprung a leak and petrol had got into the plastic container. I take it this has sunk in the chamber, letting petrol flow in without any control, which then spurted out through the overflow pipe. Have ordered new plus gasket from MG Hive. Will let you know if all work after assembly.
Thanks
Jim
J Giddins

This thread was discussed between 01/09/2010 and 06/09/2010

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