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MG Midget and Sprite Technical - SU carb leaking from float chamber

I've spent the last 3 days building a working pair of HS2 carbs and finally met with success except for one small issue. When under load fuel is leaking from the float chamber vents.

Everything has been thoroughly cleaned, new valves have been used and when on tick over there are no leaks. The fuel pump is a SU one and hasn't there weren't problems previously. Any ideas greatly appreciated.

Gus
Gus

welcome gus

congratulations, your the proud owner of an MG....

the easy answer is the float needle is sticking inside the float chamber lid

but being this is only happening on the hard accelration ... im leaning towards the floats maybe sticking/ binding somewhere

another therory that is expressed here from time to time ... but I cant explain it very well but has feet and can run is...

the seat adjustment at the bridge of the carb venturi maybe a little low



after rebuilding a couple of Su carbs im guessing you may be a bit fried in the head... a couple days off and freash eyes solves alot of problems

prop
Prop and the Blackhole Midget

Thanks Prop. It's all part of the fun.

Tried needle valves, grosse valves and Minispares own version and same issue. Blowing down the fuel line with the float chambers full and the valves seal fine. Used both plastic unadjustable floats and those with a metal arm to adjust the height. Even tried different pivot pins.

Going to change the fuel pump tomorrow invade by some miracle it is outputting at too high a pressure.

Will also take a look at the seat/bridge. Car is running great apart from a risk if fire!
Gus

you could check for cleaning debris or a wear ridge from previous use

if you've got the old needles and seats you could try putting those back in, of course you might need swap front for rear to get the correct one

I solved one of my carb overflows by dropping in a s/h needle from a spare set of carbs, worked where the new one wouldn't
Nigel Atkins

Is this happening with standard viton needle valves? The usual advice is to bin the Grosse valves. Although the fact they are cr*p is a widely held view, I personally think they are upset by vibration, which is why, in my experience they don't leak on tickover, but do when the engine is working.

Could you have rust particles coming through from the tank? A lot of people fit fuel filters for that reason.

I have know leaks caused by a small indentation wearing in the metal lever section on the float valve, where the valve sits. This can cause the valve to lock against the float and not shut off properly.
G Williams (Graeme)

Nigel, when you say "wear ridge" where do you mean?

The jets are new but I've switched back to the old needles as was having trouble with the new ones. The carb chamber itself doesn't seem to be flooding and the car is basically running fine.
Gus

Graeme, it's happening with all types of valve.I've currently got one valves from minispares which are similar to the grosse valve but have a peg rather than a ball.

Fuel filter is fitted, tank is about 6 years old and pipes are the same age.

The floats I've ended up in are the all in one plastic type so no metal arm. Did try those type as well.

I have been thinking vibrations might have something to do with it so wool try switching back to the needle valves
Gus

hey gus,

what nigel means and makes good scence... is the little sleve that screws into the float lid that the float needle slides into... it maybe damaged were the needle valves seats into it...esp if its rubber viliton.

try switching / reversing the float lids between the 2 carbs...it it now leaks on the other carb... then its a issue with in the float lid, if its leaking still at the same carb its most likely where the jet meets the bridge

I seriously doulbt its the fuel pump... it worked before the carb rebuild without issue..so im sur its working as per spec now...if it where bad, then it would be under pressurizing, aka... as engine bearings wear you get lower oil pressure, not higher :-)

just a thought....you do have the hoses on correctly ..right?

if you did accidently put the hose on the wrong brass tube connection on top of the float lid...well you know we are going to laugh at you... but then agian we have all done it im sure... the smart ones just wont admit it....hahaha

prop
Prop and the Blackhole Midget

Solved!

Turns out the problem seems to be with the float chamber lids. Swapped the current ones for a slightly different design - same floats and same valves - and touch wood have yet to see a leak.

Of course there is now a problem with the spare fuel pump so the Sprite still isn't running so swapping back to the original tomorrow.

Was meant to be spending the week wallpapering though, so every cloud and all that.

Thanks for all the advice
Gus

well done

perhaps you mixed and matched parts from different sets of carbs

you now need to get shot or dismantle that spare pump as unreliable parts add to the confusion and are no good for use
Nigel Atkins

That's exactly what I did Nigel.

Bought a replacement pair from ebay, turned out they were off a 1100. Further examination showed that they weren't even a matched pair. One of mine had a damaged casing and both throttle shafts were worn. Managed to get two good carbs from 5. Bit of adventure but got there in the end.

Pump will be overhauled, it's been sat for a couple of years at least so probably nothing a good clean will sort.
Gus

you have to be a bit careful because a lot of these carbs look the same but they're not so you can get mismatches as well as matches

I trust you matched the air pistons and gave them a wipe of light oil before you install them

there's a download of a SU book on the net and you can go to the SU web site

plus there are links to relevant John Twist videos I can post up for you if you've not seen them and about set up tuning

as for the pump I usually put that most servicing work involves mainly cleaning and lubing

one of the first things I done with my car was to swap to a Q&H electronic replacement pump I've got no time or patience with getting mucky slapping a stick under the wheel arch
Nigel Atkins

Everything is matched. The carbs themselves were working fine, it was the float chambers flooding.

The chambers look identical and the lids appeared interchangeable but there is obviously some subtle difference that meant the float was catching in the chamber.

I'm in no rush to change the fuel pump. It is working fine, was only changed today as when you seek to have tried everything else you may as well try everything. It'll be upgraded to solid state one day but plenty of other jobs to do first.

Thanks for the offer of the links but the carbs are nicely tuned now they're not drowning!
Gus

great stuff

wasn't just grommet height was it
Nigel Atkins

I'm thinking that there is a slight difference in the height of the inlet seat (where the valve screws in) used the same floats and valves and same washers. Other possibility is the orientation of the float within the chamber.

Gus

oooohhhhhh


####Other possibility is the orientation of the float within the chamber.####

that will do it...it sounds like you got 2 fronts float chamber lids for both the front and rear carbs

or you got 2 lids from 2 differsnt lids... 1 vented and 1 non vented

prop
Prop and the Blackhole Midget

just seen this might be a couple of points here to pick up from - http://www.youtube.com/watch?v=82YNx-RkGNI&list=UU40j4KqUJPMVv4FQ29ro-xQ&index=1
Nigel Atkins

This thread was discussed between 19/03/2013 and 25/03/2013

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