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 - Stuck Float?

Car got a full professional service, fuel and electrical tune-up 2 months ago, but has not run since. Today was the day, but the fuel pump didn't stop ticking. Fuel all over shed floor, from rear HS4 overflow pipe. Removed the 3 screw on top of float chambers - front carb float abviously new, rear not new. Never had any probs for 7 years with either.

Is it grit in the float valve, or time for a new float? If so, how do I check, and how do I fit a new float and valve? Are there "settings/clearences" I need to worry about?

Go gentle - I'm not good with carbs yet!

Andrew Middleton

If the float swings freely and is not full of fuel, the needle/seat is the place to look. To get at it just use pliers to pull out the shaft holding the float to the lid. You can then lift the the needle out of its seat. If you want to go further, you can unscrew the brass seat with a suitable socket. Clean the needle and seat with carb cleaner or something similar and non-abrasive. You'll see wear as a ridge on the tip of the needle. I can't remember the float setting gap, but for thoroughness it can be checked whilst you are putting things back together. Having said that, if the car has sat for a while the needle can just get stuck with the varnish and gunk from the evaporated fuel, meaning that a sharp tap on the side of the float chamber is often all that's needed to put things right.
Steve Postins

As long as the float doesn't contain any fuel it is OK. Like Steve says it might simply be the long layup. The first thing I would do is disconnect the fuel pump at the white connectors where the main loom joins the rear loom by the fuse box and run the engine till the carbs empty and the engine stops, then reconnect the pump. Any odd particle in the float valve has a good chance of being flushed out by the fuel rushing through the now wide-open valve. If that fixes it all well and good. If not and it is still overflowing then I'd replace the float valve. If it fixes it initially but it happens again quite soon, then I'd be looking for dirty fuel i.e. particles in the bottom of the float chamber, or again change the float valve. I recently had the same thing on my V8, which still did it after the fuel pump trick, so I changed the float valve. Still did it and it was only then that I noticed fuel in the float, which I changed, leaving the new float valve in, even though the old one was obviously not the cause of the problem. A few weeks later it started doing it again, float OK this time, so I put the 'old' float valve back in, and it has been OK so far, verifying Hunt's Fifth Law: "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals". Think youself lucky you have HSs, mine are HIFs.
Paul Hunt

"To check the float level, hold the float chamber lid and float assembly upside down and place a 1/8" (3.18mm) diammeter bar (like a drill bit) across the diameter of the machined lip of the float chamber lid, parallel to the floatlever hinge pin and under the float lever.

The face of the float lever should just rest on the bar when the float needle is held fully on its seating.

If this is not so, carefully reset the angle made between the straught portion of the float lever and its hinge until the correct position is obtained".

Says a good shop manual. Look to any manual or exploded view.
glg

Thanks guys for the words of advice.

I did the sharp tap with a hammer - that bit was fun! No joy.

I disonnected the fuel pump and let the engine run and die as suggested. Fuel still gushes out once fuel pump is on again.

On closer inspection the old float in the rear carb had it's little tab near the hinge pin broken off. I have now fitted a new float and hinge pin. Easy ain't it?!!Checked clearences as stated above. No joy.

The ball bearing bit of the valve and seat assembly seems happy moving about, and drops when the float drops, and vice versa.

After all this, fuel still gushes out of the overflow pipe!

Does anyone have any further suggestions?

Thanks!
Andrew Middleton

It sounds like the float valve -- or jet -- is not functioning.

Take the lid off the bowl and manually operate the float -- take note where and why the float is not closing the jet -- or float needle valve and seat.

Wild guess, there is an arm on the float not engaging the valve.

Another thought -- are all the screws for the float chamber lid in place -- along with the gasket. AND - are those screws in good shape?

And just one more -- is there any possibility that the float chamber or lid has a small crack -- have you determined exactly where petrol is leaking from?

Best I can do right now.

glg

It sounds as though you've narrowed it down to the valve. As long as you have the right socket it's a simple one to replace as the seat just unscrews. Cleaning often solves things, and it's surprising how tiny a speck of dirt is needed stop it functioning. Seeing movement is positive but you really need to know that the needle is shutting off the flow. As GLG suggests, take the lid off again, leave it all assembled, turn it upside down or even better place it in a bowl of liquid so the float pushes the needle home, and switch the fuel pump on. There should be no flow.

If it is dirt causing the problem, you might want to change/install the fuel filter just before the carbs in case it's coming from the tank.

Otherwise, if the parts are worn it's an idea to do both float chambers at the same time. The other is just waiting for you to drive a long way from home....
Steve Postins

Andrew - had the exact same problem a while ago, but on my MKII Morris Mini. Replaced the needle valve with a slightly more expensive Grosse version (has ball bearing in the bottom instead of a needle so wear is less) and changed the float. The new float came as a one piece plastic moulding rather than with a metal adjusting arm on the top. It didn't fix the problem - eventually traced it to the olive/washer that was supposed to seal the flow to the jet from the bottom of the float chamber. This had come adrift and was slightly blocking the flow thus rendering the float useless. Can't think if it is similar on a B without popping out and having a look at mine in the car park!
Martin

With the lid off manually raise the float while someone briefly energises the pump and you should not get anything flowing out of the valve in the lid. If you do then the valve is not shutting off the flow or is finding a way past it. If it does then the fuel level in the float chamber is not raising the float enough to shut off the fuel, but unless it is punctured and contains fuel and so is a 'sink' instead of a float I can't really think of why this might be unless it is binding on the hinge pin or float chamber side or something similar.
Paul Hunt

Did you perhapse have an aftermarket fuel pump installed with too much pressure for the float valves?
Luigi

Good advice from all.

Ordinarily, those suggestions should have been enough to allow you to fix your car by now. Since
you are still being drenched in fuel - now, look for
the improbable glitches.

Maybe try this one:

There are two hose bibs on the top of each HS4
float chamber. One is a vent. The other is the
fuel inlet. Visually, they look very similar to each
other.

If the chamber lids were oriented wrong - and then
the vent hoses & fuel hoses were installed into the
wrong bibs invites a flood of fuel going into float
chambers and into the charcoal canister.

Refer to a workshop manual (ie: Robert Bentley's
"The Complete MGB" or similar) - and look up the
illustration for the HS4 carb setup. Trace the route
of every fuel and vent hose going into the float lid
that is on your car. In particular...note the exact
installation positioning of the lid(s) themselves.

Hope this works.
Daniel Wong

This thread was discussed between 29/11/2003 and 06/12/2003

MG MGB Technical index

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