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MG Midget and Sprite Technical - Rear Float Chamber Again

In June I reported that we had to be recovered from North Yorkshire because of a fuel leak. It was entirely down to the rear float chamber overflowing, and the fuel had somehow found its way into the passenger footwell.

Since then, I have a new pump and all new flexible plumbing, and have sent for new jet and needle.
These were no use, being too big to allow the float lever to be fitted, so I've tried to find a combination of jets and needles (I had a spare set) that works.
It was OK for a while, but now the problem's returned.

Areas I've considered:-

1. Intermittent failure suggests crud in the fuel line.
2. Float appears not to be leaking, but . . .
3. Lever is carefully set to 7/16" from lip of chamber to apex of curve.

I believe the last item is important, to get the fuel in the float chamber at the same level as the top of the carburettor jets.

What tests would you try?
Nick and Cherry Scoop

The fuel level in the float defines the fuel level in the carb, so if you change the float level, it will change the height you set the jet to, but provided you have adjusted the mixture after messing with the float- that IMO is non issue.

Have you had the top off the float chamber and checked for the white crud that builds up in the bottom ?

As it's the rear float chamber - I am assuming here you have fed fuel to the front and then to the rear float - so it's acting as the pressure regulator...

So what fuel pressure do you have / std fuel pump ? and is the top of the float chamber horizontal- has some fitted the wrong angle adapter ?

Malc Gilliver

1. Intermittent failure suggests crud in the fuel line.

Blow out the fuel line between pump and carbs - both ways

Might also be worth removing the jets and the pipes on the float bowls, check nothing is obstructing in there.


2. Float appears not to be leaking, but . . .

But what ?

3. Lever is carefully set to 7/16" from lip of chamber to apex of curve.

This sounds correct for HS1 carbs, HS2 is 5/16" and with conical floats 1/8"

Tim C

Yes, done most of that.
When we got home, I dismantled the entire fuelling setup: new pump, new filter, new soft pipes, carbs cleaned up and all linkages taken apart. But I broke one of the little pedestals in the rear float chamber lid, that the spindle goes through.
Ebay found me a lid, in good nick, and I put it in. I had a spare needle and jet, so I've been trying the combinations. Thought I'd cracked it - got everything right - but then today after another brake test down to the road and back, it was dripping again.

Perhaps Burlen can help.
Nick and Cherry Scoop

What fuel pump are you using, Nick?
GuyW

Mechanical, Guy. Just like the old original, except that it doesn't have the useful manual lever (though fuel doesn't flow back through the pump, so I haven't missed it yet).

The overflowing on the way back from Northumberland was with the old pump, in fact the whole fuelling setup we'd had for twenty years.
Nick and Cherry Scoop

Sorry I've lost track of things. Goldfish memory! Have you tried a new float chamber needle valve?
GuyW

Yes, I got a new one. But it doesn't fit. The new one - on the left - has a longer thread, and the jet bore is too shallow, making the needle stick out too far. There was no chance of fitting the springy lever thing.
So I tried combinations of the two jets and the three needles I had, and thought I'd got it. But it failed again yesterday, after going through the whole of the carb tuning process without overflowing once.

Nick and Cherry Scoop

Nick,

I had to search for a very very long time before I discovered the problem of the accidental overflow: In my case, the float needle would vibrate and resonate at certain RPMs. A needle with a spring (like the one on the right in your photo) with a Viton tip was not sensitive to vibration. The solid needle with a metal tip was.
Years later, I discovered a report in the Healey museum stating that Eddie Maher (the engine and gearbox guru for Riley, BMC, BLMC, and the racing department) had already discovered and remedied this problem in 1958. It occurred mainly in the HS2. That was the reason he recommended Geoffry Healey H1 SUs on the Frogeye. In my case, however, the H1 was not perfect regarding vibration either.

the left one on my photo is OK

Flip


Flip Brühl

here is the report

Flip Brühl

Thanks Flip - that's very interesting. I've been on the phone to a chap at Burlen this morning, and he was pretty confident the problem could be overcome with a new valve and Viton needle. The vibration thing never occurred to me.
I've spent a happy half hour this morning with grinding paste on one of the metal tipped needles, trying to make the fit between cone and entry hole perfect.
Nick and Cherry Scoop

I used Viton tipped ones as that was recommended to me at the time,though many years ago. But I never knew why!
But that is why I asked Nick if he had tried new needle valves.
GuyW

Viton tip, a spring and aluminum. They all give an other natural frequency
Flip Brühl

I had the same problem with both front and rear hs2. But the rear definitely worse.

The sprung viton tips did *NOT* cure it.

I figured it was vibration, but I thought it was the old ruber connector/buffer, between the bodies and the bowls. But because my spindles are worn too, I didn't want to keep wasting money and time.

That's one of the reasons I now have the single Hif44 instead.


anamnesis

Hi,
I had what sounds like a very similar problem after my first attempt at driving following my restoration. Had fuel leak at the float chambers. My mistake was not replace the very worn valves.

Once I’d replaced it with new and correctly adjusted the float it was sorted.

What’s bugging me is that I have a memory that the new valve was different in design to the old one. I’m sure I spoke to a guy at SU Burlen who explained the reasoning for the difference was that they had updated the design to make it more efficient and reliable.

Could be wrong but I think they had removed the piston on the valve. I’d have to check mine. If I have a spare gasket I’ll have a look as it’s annoying me I can’t remember exactly what it looked like and I can’t find a photo of it on my blog.

I can’t find a picture of the valve so might be worth giving them a call.


James Paul

Actually it could be the middle valve in Flip’s photo above. That does look familiar.
James Paul

I think that's the one, James. I did speak to Burlen, got the reassurance that I was ordering the right one (of three) for my H1 carbs, and the new valve and needle have just arrived - the needle with no damping piston.

Fingers crossed.
Nick and Cherry Scoop

All back together and ready for a test.But I came in for lunch, to prolong my optimism.

Here's a picture to remind all HIF-types (me included) how many bits and how many beastly connections there are.

Nick and Cherry Scoop

This is off-topic!

@ Nick

From your photos (in this and other topics) you appear to have an oil catch can coming off the timing case cover? Where does it breathe to ?

@ Others

Just thinking out loud, and happy to be "flamed" (!?), I have 1275 with HS2 carbs, no diaphragm thingy, just a Y piece to connect to the carbs. I have an oil catch can off the timing case that breathes to the Y piece and into the carbs. Do I need to do this? Could I just breathe to the air with a filter on the end, still using the oil catch can, and cap off the two connectors on the carbs? Nick doesn't have these two connectors on the HS1 carbs.

I do remember a while back I tried just connecting the two connectors together with a tube, but engine died after about three minutes of driving...don't remember what i did with the timing case tube though...

Or should I get a diaphragm?
Tim C

I will start a new topic, don't answer here...
Tim C

This thread was discussed between 23/08/2026 and 03/09/2026

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