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MG Midget and Sprite Technical - Carbs fettled

Spent yesterday overhauling the carbs ready for the new motor................

New jets replacing the horrid waxstats and also soldered up the anti run-on discs in the throttle butterflys.

They were in a state before all this, hope it runs better as a result!

SR Smith 1

very nice

hope you checked the dash pot to piston match as a couple of recent posted problems have turned out to be this

41 Matching SU Air Pistons -

btw I've been using 90 weight gear oil as recommended by John Twist and it's made mine very smooth with no problems over winter

whichever oil you use it'll soon find its own level and topping up beyond that level will just oil the underside of the bonnet
Nigel Atkins

Hi Nigel,

Pretty original my car, so everything matches, piston rise and fall seem about equal.

I'll experiment with the oil when the engine is back in and running.

SR Smith 1

good, dare I say I was allowing for a possible mix up somewhere in their lives

as it's just a copy & paste have this -

Oil for use in SU carb dashpot (suction chambers)

The three main sources of correct information –

• (owner’s) Driver’s Handbook recommends (clean) engine oil (many owners use 20w/50)

• John Twist recommends “90 weight gear oil” (hold on if you think that’s way too heavy, see link to chart below) –

• SU Damper Oil (SAE 20)

Now before we get too hung up on the numbers bear in mind that the numbers are not a straight comparison to each other, engine oil and gear oil are from different scales and are in ranges

Here’s the chart I found useful –

and link about the ratings -

Other oils I’ve seen suggested for use –
• ATF – seems like it might be on the thinner side according to chart (see link above)

• 3-IN ONE – there might be a bit of confusion here as some may be referring to the ‘3-IN-ONE Multi-Purpose Oil' which the makers (WD40 company) state “is SAE 17, it is quite a fine oil.” whereas others may be referring to the ‘3-in-ONE Motor Oil’ (blue label) which is equivalent to SAE 20

• sewing machine oil – comes in various grades, a surprising wide range of grades

• (suspension) fork oil – a heavy 20 SAE (ISO 68) is detailed here, it may be the same(?) as the ‘Penrite Damper Oil’ which is the equivalent SU Damper oil – (cut & paste this link) -

• mono grade engine oil – 30w seems to be the one that comes up the most

• WD40 (spray) – !? - it is very thin compared to the others here
Nigel Atkins

Hi Nigel,

This car has been off the road for 3 years, but I always used brake fluid in the dashpots.

As I said, we'll experiment with different oils once it's up and running.

Regards Steve
SR Smith 1

Hi Steve

Cautious of the brake fluid stripping paint off things. Probably too thin as well. Most SUs seem to work best on engine oil (20/50) or straight 30w.

Looking at the pics, angle of pic suggests the jets are not both the same number of turns down, may I suggest you set the jets down 65 thou as a good starting point.

Did you slacken the throttle screws right off before you fitted the butterflies? Failure to do this can result in a very high idle which cannot be lowered.

Peter Burgess Tuning

if I'd used brake fluid in mine I'd have a good patch of paint missing on the underside of the bonnet, no soundproofing in my car, I notice you have a lot of soundproofing in your other photos I didn't know those 1500s were so noisy :)
Nigel Atkins

Thanks for those words of advice Peter, I'll make sure the jets are at that setting when we attempt the first start.

There is a fair bit of sound deadening in my car Nigel, one of the first things I did when I got it over 25 years ago. Amazing how much difference the underbonnet sound mat makes.
SR Smith 1

my midget has no soundproofing other than flimsy, cheap carpeting, it means it sounds like it's going faster than it actually is

this helps a bit to keep the points away from my license and saves petrol but it's not so good with the roof up, but it never as good to have the roof up anyway
Nigel Atkins

I second the engine oil option; never had problems using it in dashpots.
Nick Nakorn

I used 90 grade oil as a 'make do' once, it made the throttle action very stiff and jerky. So much so I had gone as far as replacing the throttle cable before realising it was the dash pot oil causing the problem. It might also be a sign of the quality of my carbs of course, as it had been stood for a couple of years before I bought it.
Eamonn Spencer

I think I'd safely put money on it was the quality and/ or cleanliness and lubrication of your carbs, linkage, cable and pedal assembly plus the lack of use rather than whatever oil you used
Nigel Atkins

This thread was discussed between 05/05/2014 and 07/05/2014

MG Midget and Sprite Technical index

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