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MG Midget and Sprite Technical - AUD502 Spring colour

I have a AUD502 Carb Set on my MKIII 1275 from 1971. The haynes manual says use blue spring. The SU site says use red spring. Wich is the right one and what is the difference in carb/drive handeling?
Niek Lammerts

Hi Niek,

I am not surprised that the Haynes got this one wrong, as the red spring was only used for that one year. All of the other years of 1275 midgets used the blue spring.

The difference is that the red spring is stiffer, leaning out the acceleration and the over-run.

Red = 4.5 oz
Blue = 2.5 oz

Since the spring was then changed back to blue for the following years, I have suspected that it was an attempt at reducing emissions that did not work as well as intended. so my suggestion would be to go with the blue spring, unless your engine is running too rich on acceleration and you want to lean it out with the red one.

Norm Kerr

Norm, Thanks very much for the info, this is very clear!!.
At this moment i have the Blue spring installed, when i open the trottle and rev it up, i get very black smoke from the exhaust, seems that it is running very rich at that point.
Now i know why Red or Blue a can "play" with it and see what the best setup/performance is.
Niek Lammerts

Very clear, yes, but completely backwards.

From "Tuning SU Carburetters" (and others)
"It should be noted that the change from a medium spring (say the red) to a weaker one (blue), will have the effect of weakening the mixture throughout the range. The effect of going to a stronger one will be to enrich the mixture throughout the range."

At least for US cars, the same ABC needle is shown for 502 and 549 carbs, with the same (unspecified) spring, which is listed for all HS2 Spridget, so that would be the blue one. This per Moss, which I do not necessarily trust. However, since the spring does have a full range calibration effect, it is clear that if the spring changes so must the needle. I'm inclined to think that any listing that gives a different spring for one year is an error.

Cooper S engines and modified engines generally take red springs, since they produce more power by virtue of using more air (more compression, cam, valves). The ultimate point of the spring is to ensure that the piston does not top out before the engine reaches max power; the needle is then matched to the requirements.

FR Millmore


yes, exactly correct, Mr. Millmore, I looked up the specs and then, as they say, "grabbed the wrong end of the stick and got it all wrong".

The slower rising piston enrichens the mixture, because the air must flow through the smaller opening (the engine will draw the full amount of air regardless), creates greater vacuum at the venturi, sucking in more fuel = rich.

Norm "was backwards, but I pulled a 360 on that!" Kerr

Norm Kerr

And then there's different dampers and the thickness of oil in the dashpot to play with.
Alex G Matla

I use 20W50, as sugested by the manual. wil try the blue spring soon, because there is a lot of black smoke when revving up and keeping it there fore a little while. Seems the mixture is to rich
Niek Lammerts

20W50 is way too thick IMHO. Try a synthetic 10W40.
Alex G Matla

SU Zenith damper oil SAE 20

or you could try 90 weight gear oil as itís a bit thicker
Nigel Atkins

Nigel, 90 gear oil in a dashpot? It would drown the cylinders in fuel, creating an inflammable mixture!

Well maybe not, buts it's quite the opposite of SAE20.

BTW in the SU's on my volvo B20 engine I use ATF.
Alex G Matla

FRM may be able to confirm, IIRC 90 gear oil is about 30 engine oil(?)

when I look at 75-90 gear oil it's quite thin

IIRC John Twist mentioned using it in one of his videos

I'd have also thought you could possibly use one of the alternative lever arm damper oils(?)

shocker oil 1 or 2 ('light' and 'heavy') - (you'll have to cut and paste this in)

this video might also help (can't find the one about oil weight) -
Nigel Atkins

Gear lubes and engine oils use completely different rating scales. 90 gear is usually taken as about the same as 40 engine, but the ranges are so wide that this can vary not only by manufacturer but even in the same oil from the same company. An oil rated at "Y" may meet the "X" and "Z" specs, and that's before you start playing with pour point depressants and viscosity index improvers to make a "Multigrade".
The "W" rating is done at zero F, and the main rating at 210 F, so if you have a multgrade rated xxW-yy, it does not get thicker as it gets hotter. Rather, it stays pretty much the same, but the ratings are different.

Once, before internet, I researched and compiled a table of viscosity in all known rating scales. Then got manufacturer actual test numbers for oils we were interested in to compare. Was a lot of work, but you can probably find such pretty quickly now. I can tell you that one 20-50 or whatever is not the same as another! We used to use the thickest ones for clapped out engines and Jaguar XK in summer, but they wouldn't start when it was cold, so we changed brands and used the same nominal grade - then they started fine but drank oil if the weather changed!

For carb damper purposes, it makes sense to use a straight grade instead of a multi. When the engine and weather are cold, you need more enrichment, so you want thickish damper fluid. After everything heats up, it does not need additional enrichment, so thinner oil is better. Those are the basic characteristics of straight grades, which multis negate.

My personal solution is to use ATF, ever since Volvo told us to in 1967. Very rarely I find it better to use something else, but it is unusual. The big advantage is that the viscosity appears to be much more even across brands, it is always available, it is identifiable by colour, and you can use it in any SU or ZS carb (and lever arm dampers!).

FR Millmore

at last my memory hasn't let me now

thanks for that FRM, when my SU oil runs out I'll try ATF if I can get a small bottle

I find you need to keep the SU damper oil level quite low down the neck of the chamber or it quicky ends up on the underside of the bonnet
Nigel Atkins


sorry I've just realised I put up the wrong John Twist video for you, I clicked on the wrong link, this one I meant might help you -

it might not but worth a look
Nigel Atkins

Too much oil. Regardless of what various books, even
"Official" books, say, the oil level only needs to be above the damper piston, when the vac piston is all the way down = not running. If you can drop the damper in and it stops 1/2" before the threads contact the chamber neck, then sinks slowly, that is sufficient oil.

A big problem with too much oil is that it can crawl down into the vac chamber and screw up the nice clearances. Really excessive amounts cause hydraulic lock above the piston rod, especially if the wrong (non vented) dampers are fitted.

Vented dampers tend to adjust the oil level to about what is needed, by ejaculating all over the bonnet, as you have seen. SU carbs really have no way for the oil to get out, other than turning the car upside down, so required additions should be few and far between - multi year for most usage. ZS carbs can develop a leak which lets the oil run out the bottom.

All of which leads to the idea the Niek may have far too much oil or wrong dampers causing his problem. There have been numerous discussions re dampers lately, but I cannot recall if any were Niek's, or on this board. (was big recent thread(s) on MGA board)

(PS: you have inadvertently (mis)typed one of the best Prop-ish sentences ever: "at last my memory hasn't let me now". Could take years to analyze the possible meanings!

FR Millmore

I didn't notice that 'Propism' - made me laugh, of course it looked fine to me when I read it back

it's the times I forget to type the word 'not' that are worse

I do apologise to Neik though for spelling his name wrong even after checking

yes I must admit to having been an over oiler of the dampers - I'd forgotten that in the 70's when I had a VP 1100, with single SU, I just used to pull the cap up and if it had suction I knew it was OK
Nigel Atkins

This thread was discussed between 10/06/2012 and 11/06/2012

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