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 - Problems with carbs

Hi, I have just fitted a set of replacement carbs for my 1975 MGBGT. I purchased them from MGB Hive. The problems are that my mechanic says that when the ignition is turned on both the carbs leak fuel from the over flow. I contacted MGB hive and they have told me that it will be some contaminate in the petrol that has caused the carbs to overflow. They advised me to turn the ignition on and to bang the side of the carbs with a screw driver and that this will shift the blockage. Are they having me on or is this the thing to do. The carbs are guaranteed for a year but postage / removal costs make me want to try simple things first before getting them taken off my car and sent back.

Any ideas?

Eddie
E McGee

Quite possible rubbish in the petrol, but unlikely if you have an in line filter, and unusual to affect both carbs at the same time. Many re-built HIF's don't have the floats adjusted properly, some have them in upside down!! Either way you or your mechanic will need to take the carbs off to check the floats and their valves. Hitting the carbs will only have ant effect if the floats are stuck.
Allan Reeling

Hi Allan, thanks for your reply. I even paid extra to have the jets replaced. The reason being is that the original carbs have caused me a lot of problems in the past and it was my mechanic who confirmed the "AA mans" assessment (after it broke down again) that they were shot. I "thought" that in doing this the new carbs re built and re jetted and guaranteed would sort that side of my engine out. Seems things are not that simple......Eddie

E McGee

Hi, I'm with Allan on this but it depends on the terms of your warranty. Accessing the float chamber may cause a problem if you do need to return it to the MGB Hive.
First thing I would do is blow some air in to each carb to try and shift any crud (the garage should have a decent 90psi supply), then fit a new disposable fuel filter (only a couple of quid) right at the entry to the first carb before running petrol in.
If it's still leaking you need to check MGB Hive are okay with you taking the float chamber apart.

MGmike


M McAndrew

Every-one is giving sound advice. I would be curious to find out the true diagnosis of your old carbs. New carbs, off the shelf as it were, should work well from the start? To have picked up dirt, to hold both carbs valves off the seats seems to me very bad luck. What is the fuel pressure? Remembering from other threads, this would certainly, if high, make both carbs leak,fuel through their overflows.If some-one st Hive opened up to change the jets, yes I suppose there is the chance they did not put it all back together correctly, and if under warranty, send them back. Mike
J.M. Doust

New needle and seat. Give them a bang as they say. A very old and reliable way to re seat the needles. the needles may be stuck because they have been siting on the shelves for a while and the floats have been fully dropped.
A J Ogilvie

What problems did you have before? If it was anything other than overflowing, and you haven't changed anything else i.e. the fuel pump, and seeing as how both are doing it, then it is more than likely a problem with these carbs.

It would be better to disconnect the fuel pump where the white from the rear harness joins the main harness by the fusebox, run the engine to empty the carbs, and then reconnect the pump. This will give a good flow of fuel through the now wide-open valves, which will dislodge anything better than if the floats are up and pressing the valve onto the debris (if that is indeed what it is). How long were the carbs off? Could the hoses be decomposing?
PaulH Solihull

I take it they are rebuilt and not new. You will have to take them off to clear what's causing the overflow, check how many of the parts are new. I had my HIFs rebuilt by a local mg specialist and they left the old floats in despite charging me for new ones. Not that it helped, even when I changed the floats, they still leaked.
The HS4 is much more tolerant and easier to work on, and I eventually fitted a pair and have had little trouble since.
c cummins

HSs are certainly easier to live with, but there is no reason why HIFs should not be as leak free as HSs with good parts assembled correctly. One thing to watch out for with HIFs is that the floats are handed whereas they are not for HSs. Get them the wrong way round and they *will* overflow.
PaulH Solihull

Hi Guys, thank you for all your replies. I'll feed them all back to my mechanic - and see how he gets on.

I have replaced:- the fuel pump, the fuel lines from the tank to the pump, replaced the fuel filter on the bulkhead and the fuel line into the carbs with a new fuel line all from MGOC. They were unable to supply the carbs - but could recondition my old ones.

Replacing all the above, I discovered something odd though - does anyone know why there is a short length of metal pipe attached to the wing with rubber fuel lines attached to each end? Working on the old adage of where there is a joint there is a weakness why did they not run one continous fuel line instead of all the joints - any ideas? Edd
E McGee

New lines and pump *could* have contained muck.

The one rubber hose on the inner-wing pipe should go to the filter and the other to the front carb, both accommodate movement - vibration of the filter and rocking of the engine. All joints must be fuel-tight, so the number doesn't really matter. If anything the very flimsy hoses used with HIFs are more trouble - splitting - than the joints themselves. A pal had one go which I replaced with 'proper' fuel hose and strongly recommended he have the others replaced "I will, I will". A couple of months later another one went, then a third.
PaulH Solihull

Ah thanks for this Paul. I was thinking: would the fact that the bore of the new fuel line which is larger than the diameter if the metal pipe cause too much fuel to go the carbs? What do you think?

I will change the link fuel line as you reccomend.

Regards

Edd
E McGee

New fuel pump is a clue to your problem....Sounds like too much pressure...You should have an in-line fuel-pressure gauge (liquid filled, are more accurate) , that shows 0-15lbs....Your pump should not be putting out more than 2 lbs...(1.5 is plenty)...Much more than 2, can cause fuel to overflow.
Are you using an SU pump, or aftermarket?
Edward
E.B. Wesson

The make and model of the fuel pump is something that should be ascertained before taking the carbs apart. If the pump is the low pressure SU, I would heed Paul's advice and plug the fuel line running to the carbs and run the engine until it stops from lack of fuel. When you reconnect the fuel line and turn the ignition on, any small debris, that may have been holding open your needles and seats, will be flushed to the bottom of the float bowl. RAY
rjm RAY

Hi Guys, I am using an after market pump - supplied by the MGOC. They reccomended it after my SU points one stopped working. The new one is an electronic one -would that cause problems? Edd
E McGee

There's nothing wrong with using an aftermarket pump...BUT, they sometimes have higher pressures, and this is something that you need to know...Get the gauge...they are fairly cheap, and easy to install in the rubber line...A "T" fitting, and two extra clamps,is all you need....
Be very careful, if you decide to use air pressure to blow out debris...If you over-pressurize , you can mess up the float lever adjustment....It can also force debris from the air hose into the carbs. (ask me how I know!).

Edward

E.B. Wesson

Here's a shot of my "B" engine compartment, with the gauge installed. (Ignore the "experimental" breather, later removed).
Edward


E.B. Wesson

Hi Edward, thank you for your photo, I think I will get a guage and fit it just like your picture. Maybe we might have stumbled on the solution to the problem. Regards Edd.
E McGee

Mr. E B Wesson Sir, That is a neat little guage, where can you obtain one? Who did you go to? I have seen guages here, but it is the fuel injection guys that are using them, and register huge pressures ie more that the 2 to 3 pounds per square inch that we wish to measure. Nice engine bay, won't show you mine! Mike
J.M. Doust

Mike,
My Ryco vacuum gauge purchased more than 45 years ago (sobs) is also a fuel pressure gauge reading 0-5 psi.
Something similar should be still around.
David
David Overington

The after-market 'pointless' pump usually supplied by the MGOC is Moprod, identified by it's all-plastic body but other than that the physical shape and size is very similar to the original SU, and the mounting and fittings are the same. This pump *should* deliver the correct pressure for the carbs.
PaulH Solihull

Edd
Sorry it took a while to get back to you....Source of the gauge was AutoZone....Might check their on-line catalogue, as the last time I looked for one, my local store didn't have them....
Also, check JEGs on line, as they have many after-market fuel gauges.
Edward
E.B. Wesson

A hot tip. I was after a threaded adaptor to place a vacuum guage onto the manifold, I did not know what the thread size was so went to the local car shop. No luck, all seem to be metric. ( I did have the blanking plug with me)Anyway the local High pressure lines and fitting shop were very helpful. In fact it was an Ex-pat whom served me, and said 'Oh I've had lots of MG's in the past.' In a broad Manchester accent. He found a way of adapting very quickly. But I shall ask him about an in-line fuel guage. So the tip is this ask, a non-automove specialist, quite often they will have an answer, Oh, and he charged me less that the car shop would. MD
J.M. Doust

This thread was discussed between 20/09/2011 and 30/09/2011

MG MGB Technical index

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