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MG Midget and Sprite Technical - Terminal Carb Problems - please please help

Hi everyone,

I'm having trouble with the carbs - they keep overflowing from the (wait for it) overflow holes in the casting on top of the float chambers. It's an intermittent problem, and has happened from both carbs. Also, the rear dash pot is stone cold to the touch.

In terms of what I've tried:
- new valves (TWICE), even tried grose jets but couldn't get them set up properly.

- new float in the rear carb only as the front one is fine

- new fuel pump a some months ago (standard SU) (the fuel supply stops (as it should) if you put a finger over the end of the delivery pipe, though it does still provide a good pressure.

I mean logically it can only be the float valves or the float calibration - which have both been replaced and set up/checked - or a fuel supply at too high a pressure that's forcing the fuel through the float chamber and out of the overflow (but it CAN'T be this as the fuel seems to stop if the delivery pipe is covered by a finger....!)

I'm completely stumped. Any help would be so hugely appreciated I can't tell you!

Josh
Josh Spooner

Josh,

A quick comment is that blocking the fuel pipe as you have will indicate if the pump is working as it should but not give much information about the pressure, for that you would have to use some sort of gauge to tell what the delivery pressure actually is. Was the problem there before the new pump was fitted and are you sure the pump is the correct one for the car.
David Billington

Times two on are you sure it's the correct fuel pump.

Having said that, if it's intermittent, it suggests something else.

Also, as has been said here before, sometimes new parts are crap, so your new seats and valves might not be any good. But again, as intermittent, seems unlikely.

Are the fuel bowls sitting at the correct angle? are they free to rotate? This will upset the valve seating.

Are the floats free to pivot correctly?

Remove float chamber covers, turn upside down and blow through the fuel inlet. Does it resist your breath, or can you overcome the weight of the float and blow air in?





Lawrence Slater

Daju vue ....hahaha

X 3 on lawerance thoughts

And good call on the wrong fuel pump...these carbs dont do well eith any thing stonger then 3 or 4 psi

I recently found that my float bowl lids where clogged up in the passage ways
Prop

When you replaced the needle valves did you ensure they sat at the same depth as the originals? Some have thick alloy washers under them, the replacements may have or need different thickness washers to maintain the proper float height, any differences may have your carbs flooding.

And do you have moulded floats or brass ones? Brass ones can have their hinge pivots misaligned and need resetting

AND

Grose Jets have a definitely lousy reputation in here from those who have used them

I prefer to use the viton tipped Speedwell "type" valves myself rather than the steel ones. I remember when every car spares place had a card of Speedwell S.U Needle valves hanging behind the counter. I know that some like them were available from Rover dealerships (Burlen make?) for Metros just before Rover died. Lovely rubbery tips. Nice.

Just a few thoughts, hope you get sorted.
Bill 1

I had a similar problem when I got my sprite started up after a very long lay off. Intermitent leaks from the float bowls whenever I started it. Tried various fixes but eventually wondered if having the fuel lines open for many years whilst the pump was off in a dusty garage had encouraged cack into the lines. I tried pumping a pint or so of petrol through into a container and then fitted an in line filter. Since then it has been run several times, up to 15 mins at a go with no problems.

Mark
M Adams

Thats right Mark, crap in old fuel systems can exist for months on end. Josh can you perhaps get a decent filter installed before the carbs?
Robert (Bob) Midget Turbo

I have a theory that a faulty head gasket can cause the float chambers to overflow.............
S G Macfarlane

"have a theory that a faulty head gasket can cause the float chambers to overflow............. "


And, ------------------ ? :)
Lawrence Slater

You have me at theory....

Im guessing part 2 is going to cost us $19.95
..lol

Prop
Prop

Josh,
I'm another vote for a filter. When recommissioning my 1300 the carbs would spit fuel. I did all the checks you've done and in the end I fitted a disposable filter (about £3!) problem solved. You'll be very surprised how small the bits of crud are! I needed to replace the filter after about a 1000 miles as it was well mucky! The second one showed little sign of contamination after 3k miles and I've been running without one again for another 3k.

If it's a pressure problem it will still flood with the filter fitted. Then you should fit the correct pump or a regulator.

MGmike
M McAndrew


I had the same problem with my 1275 Frogeye and Facel electrical pump. Tried everything and did not work. Occasionally it overflowed. In the end I installed a King filter and since no problems any more.

http://product-images.qedmotorsport.co.uk/lotus/twin-cam/fuelling-ancillaries/filter-king-glass-bowl.jpg
J.W. Vlaanderen

Sorry - didn't mean to leave you hanging. I was merely waiting to be shot down in flames by someone more knowledgeable, or for someone to agree so that I didn't have to explain myself. Its not actually a theory anyway, more of a suspicion, which hopefully someone else can expand on.
When the HG is leaking between 2 and 3 gases get forced back through the carbs (can sound like the exhaust manifold is blowing)which somehow "froths" (?!) the fuel, causing the float chamber to overflow - I cant prove it and it may be total nonsense, hence the dots.
S G Macfarlane

Im trying to visualise how that would happen

Certianly an interesting idea

Prop
Prop

True, I've certainly experienced spit back thorugh the carbs on quite a few cars.

Not sure on the SU though, if it would actually blow all the way back to the float chamber. I think it would disipate through the air filters/intake.

But anyway, does this happen when the ignition is on, but engine not running?

What we need now is more input from Josh?
Lawrence Slater

By "does this happen when the ignition is on, but engine not running? ", I mean fuel overflow btw. :)
Lawrence Slater

Hi again,

Firstly, apologies for the slow response - undiagnosable computer trouble and a lack of free time to spend fixing it, I'm afraid!
Secondly, thank you all so much for your advice - I've has a go at thinking about all of it:

- I think it's the right pump, although can't be sure - how would I find out (serial number, online records, etc)?
I've taken the carbs off and put them back on again, blown through the inlet valves and they are working perfectly. (Got the viton tipped ones by the way).
Adjusted the float heights again and now it seems to be working (we set it to the extreme so that as little fuel as possible would be entering the bowl and that seems to have done it (although it still seems too rich)).
Got a fuel filter fitted before the carbs, catching loads of crud - doing it's job, changed regularly.

Happens even with only the ignition on and no engine.

Been running OK the last few days - intermittent faults I suppose!

Thanks so much for all the help, really don't know what I'd do without it!

Josh
Josh Spooner

Oh, and they are still popping A LOT - tend to more or less stop when the water reaches about the 190 mark, even then a bit rough.
Under acceleration only though does it happen - even very low down.

Any ideas?

P.S - by popping, I mean it sounds like a mini-explosion in the carbs when under load, all power drops as if de-clutching and feels very spluttery...) Sorry I can't speak with more technical accuracy!
Josh Spooner

Josh,
I'm a bit concerned about a fuel filter at the front catching a lot of crud

if your pump is at the back it means the crud may also be going through your pump, or is it crud from a pipe breaking down after the pump

could it be crud from the fuel tank

remember originally Spridgets wren't fitted with an engine bay filter (good idea to have one though) and that were they are fitted in a B say they're changed once a year as a service item and may not be that dirty then

ETA: taking off your air filter(s) and then taking a good quality photo of the carbs from the side initially might help
N Atkins

Hi Josh yep the filters will start and solve your leaking problem but I am now concerned about your other issues.

You say that you have set the fuel levels low and have suggested that you still have a rich mixture?

Reducing the fuel level will lean the mixture somewhat as I think you know. However your popping and spluttering is TYPICAL of a weak mixture. So what makes you believe you have a rich mixture?

I would ask you to do one other thing and that is to empty both carb dashpots of oil and refil with engine oil. This will ensure both carbs ar dampened to the same extent very important to smooth acceleration.
Robert (Bob) Midget Turbo

This thread was discussed between 16/10/2011 and 25/10/2011

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