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MG Midget and Sprite Technical - jet and bridge gap

afternoon, Chaps. So, during lockdown the midget has been locked down in the garage but today I thought I'd start it up and drive up and down the drive BUT the Gremlins had been at work and it wouldn't run and idled like a bear. It seems that it is overfuelling...but nothing has moved since I put is away several weeks ago. With standard SUs and AN needles does anyone know the drop of the jet from the bridge to get me started? Currently it is 50 thou..but that seems too much. Thanks all. Stay safe. Dave
David Cox

I think she is just telling you that you didn't pay enough attention to her, she is jealous :)
On 1275cc, with twin HS2, it is "11 flats", so almost two turns down from when the jet is flush with the bridge. Don't know if the same for 1500cc..

Are you piston falling freely too ?

If you can get the feeling, you can also try the "lift the piston" technique at idle: you lift about one milimeter the piston, and see how the engine reacts :)

I don't know if you have this already: http://www.spritespot.com/Downloads/Workshop%20Manual/04%20Carburetters.pdf
CH Hamon

50 thou is about right.
Chris at Octarine Services

thanks, both. A tricky one...I measured the needles this afternoon and they seem cock-on....there does seem to be too much fuel getting through...the mixture seems uber-rich. Oistons rise and fall correctly. The only thing left is the jet size...anyone know what that should be at the very outlet? I seem to remember a figure of 90 thou from somewhere??.....Dave
David Cox

Float height? Needle valve stuck open?

You need to take the carbs off and give them a good clean inside!
Chris at Octarine Services

If it was running ok before, then it surely isn't something that has mysteriously changed when you weren't looking. Carb settings don't alter on their own accord. If it's overfilling then are you sure the floats or needle valves aren't just sticking, giving you too high a fuel level?
Or maybe mice are self isolating in the air filter. That would make it run rich.

See! Chris knows about these things 😁
GuyW

thanks again...super place to pose questions here! I will take the float bowls apart tomorrow...I agree that carbs don't alter themselves but after 45 years with this car she's allowed to pull a stroke or two!...
David Cox

I'm with Guy, why do all this carb fiddling when it probably isn't necessary - start with the basic stuff first. Is it just choke cable/linkages not fully closing or simple stuff like that.

I'd had the engine running until it was fully warmed then give it a good few bursts of 2,500 for 20 or 30 seconds and see what it does, looks like, sounds like, smells like.

Normally I'd just take it out for a blow out run - carb fiddling would be last on the list.
Nigel Atkins

Chris,
I thought you start was 60 thou, how many layers of skin or thumbnail is that(?) but then I thought it was 12 flats - all only from what I've read.
Nigel Atkins

Also- if you're going to check in the float bowls , check to make sure one of the floats hasn't sprung a leak and has got some fuel in it, that will make the fuel level in the bowl higher and it will run rich
William Revit

Nigel,

60 thou, or 12 flats is the STARTING point, I find the optimum setting is usually slightly weaker than that, hence 50 thou would be in the right ballpark.

When starting the engine, I always listen to the fuel pump as I turn on the ignition and wait for it to stop before firing up. If there is any issue with the needle valves or floats then it will continue to tick and a puddle of fuel will appear from the overflow pipes.

Pretty simple routine, eh Nigel 8-)
Chris at Octarine Services

Ah, I see now, thanks, you mean the 60 thou 12 flats is your pre-starting point for your usual starting point of 50 thou.

You say it's simple but I can't get my feeler gauge in there to measure and I can't get an accurate measure of the indent on the skin of my finger.

Do I take 50 thou as 10 flats then.

I always let the pump stop ticking and if it doesn't I start sniffing, after turning the ignition off.

Red Arrows just flown over, Sywell is very nearby to us.
Nigel Atkins

Most calipers have a depth gauge ....

No fingers need to be harmed in taking the measurement.

In the absence of calipers use finger (gently) to feel when the jet is level and then wind down 12 flats.

Since 12 flats = 60 thou, it is 5 thou per flat, 50 thou would be 10 flats... but, no, I have no pre starting point, the starting point is 60 thou. Then the correct running adjustment is usually weaker than that whether you use a lift the pin or an analyser to measure it.

I think, young Nigel, that you are attempting to poke fun.
Chris at Octarine Services

Chris, I can't I've hurt my finger on the jet and bridge. ;)

On a serious note, all this "you only need a few tools to service these cars" goes out of the window when more than the most basic work is required, calipers and the like aren't things you're average person has in their tool kit, I've never had any, and of course you need more than one set for various requirements.

There's an endless list of tools and equipment you chaps have in your garages and villages of tool sheds and workshops that most wouldn't have, lathes, pillar drills, spectrometers, 10-ton press and so on.

And you can't even get the substitute tools like wire coat hangers now . . .
Nigel Atkins

Nigel, the point about using a feeler gauge isn't to attempt to measure the 60, or 50 thou! It's simply that the feelers are small enough to reach into the carb mouth across the top of the jet and the bridge, using the edge of a blade as a straight edge to get them flush. From that precisely checked position you can then lower by the 12 flats, or 10 if you prefer.

A feeler gauge is hardly specialist equipment. And after all, you will already have a set for adjusting your dizzy points gap.
GuyW

Guy, I was joking about the feeler gauge - I don't have fingernails as I'm not a guitarist, if I ever (but why would I) needed to set CB points I'd ask you to kindly send me one of your thumbnail cuttings.

It was the calipers with a depth gauge, I only have a Stanley 10ft steel tape and I can't see much of the fractions on that.
Nigel Atkins

Nigel, You don't need a depth gauge. Refer to the Drivers Handbook. It sets out how to adjust your carbs, including a reference on use of a feeler gauge. No thumbnail cuttings needed.
GuyW

Thank you Guy, an excellent recommendation.

Shhh, just to let you know, the reference to feeler gauge isn't in relation to reaching into the carb mouth across the top of the jet and the bridge.

I'm not the one carb fiddling though, not my type of thing. I've barely room to start my car on my drive (hardstanding) let alone drive up and down enough to find out it's not running right.

It just amuses me that only the very slightest of excuse is needed for carb fiddling to be indulged in, sure sometimes it's required but nowhere near as often as it's carried out and then the repeats become necessary because of the first fiddle.

I'm going to start selling oversized boxing-gloves with embossed MG logo for the addicted.
Nigel Atkins

Lol Nigel - and there's you suggesting that instruments are tested and calibrated before each use - carbs are precision instruments too and easily upset by being left alone too long or by being given crap fuel to deal with.
Chris at Octarine Services

<<the reference to feeler gauge isn't in relation to reaching into the carb mouth>>
indeed Nigel, but you had to check to be sure didn't you! ;-)
GuyW

Precision but not that delicate surely, fully warm engine and other car system then an Italian tune-up, top up the tank with fresh fuel and/or bung in a fuel line cleaner if you have a delicate disposition.

Actually, by caliper I was thinking of brake type arc ones rather than the venier type, I've often wondered if those DIY electronic ones are very accurate and if they'd stand up to the average abuse of being rattled around with a load of other tools in a tool box or drawer?

Or do you need to treat them like an expensive certificated calibration torque wrench kept and lightly returned to it's own secure case?

(I'd line mine with pages from the good book).

BTW - the boxing gloves will have their own storage box with intricate fasting mechanism to quelsh the owners' needs a little - but of course also with emergency quick release opening for the desperate. I care about my patients, I mean customers.
Nigel Atkins

No Guy, I know that page particular well as my carb linkage isn't set up that way!! (must have been done by someone without the benefit of the good book) and I can't be ar*ed to change it.

Anyway I know every page of the good book as I read at least a few sections every night - as well you know.
Nigel Atkins

Good grief - all my calipers, dial gauges, micrometers, etc live in their padded / fitted boxes in the top of my tool cabinets.
Chris at Octarine Services

Yes but I bet they're not cheap DIY electronic ones.

I seen others open their tool cabinet drawers and there might be one or even two, three or more sets of cheap DIY electronic ones lying loose with other tools, and they use them (when they can find one working) and confidently declare a measurement to x-decimal spaces.
Nigel Atkins

Nigel
I have a cheap electronic one from Lidl. Fine for DIY work, I don't want to be faffing with a proper micrometer! But I keep mine in its case as it would be easily damaged.
Bill Bretherton

Bill,
do you think it's good enough for use on the car, say with measuring 50 or 60 thou on the carbs? (for if you keep miscounting to 10 or 12).

With my eyes they'd be so useful for measuring a few millimetres or fractions of an inch (I can see half and quarters) but I wonder how accurate they'd be and for how long. I've been bought a couple of cheap digital multimeters, one lasted 13 months the other is still going but plays up so is unreliable.
Nigel Atkins

Nigel
Easily good enough to measure 50 or 60 thou. I haven't checked it against a known good reference but I bet it's close enough.
Bill Bretherton

Thanks Bill.
Nigel Atkins

I was upset and hurt by some of the remarks made so I've decided to show you how to set up and balance carbs without the need of caliper or feeler gauge, just by eye, ear and delicate adjustments.

I reckon it'll take me less than 10-15 minutes.
Nigel Atkins

There, preparation, setting up and balancing carbs and they are spot on, and tidying up, all in 16 minutes!
Nigel Atkins

How long to do these Nigel and what engine is it?


L Langley

I couldn't begin to guess, I'm only at very basic level.
Nigel Atkins

I expect some of you will doubt my word of my abilities so I'm working on the proof.
Nigel Atkins

There, proof, read it and weep - like I did yesterday - set up and balanced !!



Nigel Atkins

Now I'm not exactly sure how they go back on the car, I might need help.
Nigel Atkins

Be sure to check the floats. One may be cracked or have taken on some fuel through a pinhole. I have observed both conditions on the Midget and the B.
Glenn Mallory

This thread was discussed between 07/05/2020 and 14/05/2020

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