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MG Midget and Sprite Technical - HS4 SU carb jets

Hi folks.

I have a vague recollection of, many years ago, changing the front carb jet with both carbs in situ. I'm struggling to do it now - is it possible to change the jets with the carbs remaining on the inlet manifold? The jet is leaking around/underneath the floatchamber area.

Also, as I get older I'm losing my sense of smell - is it just me or does petrol smell totally different today, compared to say 20 years ago? Is the cause of the change in smell the ethanol that's put in it? Has the ethanol damaged my floatchamber/jet grommet seal? Thanks.


J Thomson

"is it possible to change the jets with the carbs remaining on the inlet manifold?"

That should be possible. You just need to disconnect the choke lever from the bottom of the jet and undo the nut securing the jet tube to the bottom of the float chamber. The seal on the tube may come out, but it may get stuck in the bottom of the chamber. Obviously, any fuel in the chamber will leak out.

" is it just me or does petrol smell totally different today, compared to say 20 years ago?"

Definitely does smell different.

Dave O'Neill 2

"Has the ethanol damaged my floatchamber/jet grommet seal?"

Very possibly that's why I use Esso Supreme+ which has no ethanol added to it unless you live in Devon or Cornwall, Teeside or Scotland which it seems you don't :)
Jeremy MkIII

Thank you for your replies. Interesting about Esso Supreme+ not containing ethanol. I'll try some - if it smells of proper petrol, of old, I'll switch to that.

I realise now why I'm struggling (nigh on an impossible job) to replace my jet(s) with the carbs in situ. About 30 years ago, I fabricated a heat shield from heat resistant cloth/aluminium fabric-mabob. The heatshield that I made is folded and held underneath the carbs, as well as having a vertical 'wall', in the same fashion/plane of the original factory heatshield. Because my home-made shield is folded underneath the carbs, I have almost zero access to the jets' fuel pipe brass nuts. What a mistakka-to-makka! LOL
J Thomson

If your car sits static for very long periods of time then perhaps the ethanol might have some effect but I think it's too readily and too often blamed for other causes. I don't think I've ever had any problems with the ethanol that might or might not be in some petrols (up to 5% over the range for large suppliers) but I've had many, many problems with piss-poor rubber in parts and other parts and SU Berlen now seem to might have joined the ranks of suppliers who's quality of products has dropped from previously.
Nigel Atkins

Having all the replacement bits in stock so readily to hand, I decided to be an ethanol guinea pig in 2012 when kit local outlet changed to 10% and have used that as my main fuel since to see what happened.
I finally had a possible negative effect earlier this year when the diaphragm of the Delivery pump on my MGB stopped working due to being too stiff.
We did have a positive, more power on RR after switching from race fuel to set up for a new owner's only handy 98 octane. Optimised timing and fuelling on both fuels. Now that was a mystery, not quite sure why. But an interesting mystery.
But it's worth remembering that hs jet seals leaked, fuel pump diaphragms got stiff, etc, on the non-ethanol fuels.
Paul Walbran

And as for the smell, UK petrol has always smelled different to ours since I first visited in the 90s. It also seems much stronger, but that could be because it is unfamiliar.
No idea which is "right" or "wrong" or if at all. Just different.
Paul Walbran

I've never had any trouble with ethanol yet in car engines, but plenty of trouble with other small engines. Outboards, garden machinery etc. Two stroke and four stroke. Subjectively it seems to affect Japanese and American carbs (tillotson) the worst. Doesn't seem to worry Amals or SUs so far. Perhaps they are made of sterner stuff.
As I understand it the ethanol reacts with atmospheric moisture and forms acetic acid or something very like it. The resulting corrosion blocks jets etc with white crud which I assume is aluminium oxide. It can be a bastard to clear. Now I shut off the fuel on the grass cutters, chain saw etc and run the carb dry.
Recently I've heard recommendations for fuel stabilising additive. I don't know anything about it, but Briggs and Stratton (for example) recommend using it.
Greybeard

I use Aspen fuel in my 2 stroke garden machinery. It's not cheap but great for occasional use. It does not go off over the winter, starting seems easier and they run well and very clean.

Part of it's sales pitch is it's lack of ethanol.

Mike

Mike Dixon

I've had problems with the fuel pick-up pipe in my strimmer disintegrating, the diaphragm in my lawnmower carb failing, amongst other things.

I now use Esso Super unleaded in everything.

Dave O'Neill 2

I've heard many mention more recent petrol being a problem but for my 4 stroke lawn mower I just buy standard pump fuel and put it away at the end of the season with what ever fuel is in it and next season fill it up with fuel, likely bought the previous year or the one before that, and it normally starts on the 2nd or 3rd pull after sitting for about 6 months. I'll drain a bit of fuel from the float chamber and give it a minor service ie oil the air cleaner and check the oil level but that is it. It seems to run fine and is a 20+ year old Tecumseh engine.
David Billington

20+ years ?
I think my dad had at least 5 in this period, but basically do nothing as maintenance... Interesting to see that even (quality) lawn mower can last if maintained :)

Makes me wonder...why they till use carbs, and not injector ?
Is carb cheaper than an injector, despite the higher amount of parts ?
I would also guess that Emission Regulation doesn't care about those engines, so no need of "more controlled" technology....
CH Hamon

You're exactly right Cedric. Injectors and injection pumps are high precision, high pressure devices which are expensive to make.
Carbs, especially simple ones designed for engines running at fixed speed, are uncomplicated and cheap to make from cheap materials. Also there are fewer parts in a simple carb than in an injection system.
Greybeard

You would also need the electronics to run the fuel injection, as well as a power source.
Dave O'Neill 2

Almost all modern injection systems are electronically controlled which has been found to be much more accurate, but there are exceptions. Some are still purely mechanical and require no external power supply.
Greybeard

Make sense !

Greybeard, do you mean this ? What made TDI so famous and popular, at least in France. Never drive any though, was only a teenager at that time :) (and my parents were having only French cars :P)


CH Hamon

I know diesels often have mechanical injection. Even the Triumph TR6 has mechanical fuel injection, although it still had a wiper motor - errm, I mean electric fuel pump 😀
Dave O'Neill 2

Yep, I dumped the injection system on my 2.5PI and used a head and carbs off a GT6. Suddenly I was able to pass fuel stations lol!
Greybeard

Thinking about mechanical injection takes me back. My Dad had Peugeot 504s with the Kugelfischer mechanical system. Sequential injection which still seems good to me. We had a BMW with this system on the rollers a couple of years ago, took me back to 1973 era :)

With regard to fuel problems we have seen fuel line eaten at times.

Peter Burgess Tuning

This thread was discussed between 05/08/2019 and 09/08/2019

MG Midget and Sprite Technical index

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