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MG Midget and Sprite Technical - The Cold Start

Hi,

Throughout the winter I’ve been starting and running the engine for about 10 to 15 mins about once a month.
Partly cause I just like the feeling of sitting in the car with the engine running dreaming about driving it again on a warm sunny day.

Sitting there got me thinking…

What technically is going on that makes a cold engine, especially after a few weeks of non running, difficult to start.

If I follow the handbook instructions it does nearly always start on the second turn.

But just wondering why it needs choke for example. Why being “cold” makes a deference?

It is simply the temperature of the fuel in the chambers and the hotter it is the more likely to ignite?

Just interested in the science behind it.

Thanks

James
James Paul

My Car has a DCOE weber and no cold start device so it has to start with several pumps of the throttle to get some fuel into the engine to create a rich mixture to start with.

I like to drive or run the engine until warm with an interval no greater than 3 weeks. Leave it 4 weeks and it less willing to start.

Choke/fuel enrichment to about 1:5 rather than 1:13 (or whatever) is required on a cold engine because the cold metal draws the fuel out of the air so you get a much leaner gaseous mixture to light and it doesn't burn as well or light as well. Petrol doesn't burn as a liquid but as a vapour and in the cold there is less vapour - I believe that's a sound explanation.

A high energy coil (sports coil) does help cold starting more than anything else in my experience.

Reluctance to start when it's not so much cold but been a while since the engine has run is, I guess, due to degradation of fuel in the carb float bowl/s.

I did get the Sprite to start at minus 10 (or thereabouts according to the accuracy of the garage thermometer) and while it fired first time it took several more attempts before it would run - which is unusual but then again it was minus 10.
Daniel

My simple crude answer is, cold air carries more oxygen. To compensate to allow combustion to happen, you have to increase the fuel, by either adding more fuel, or restricting (choking) the air supply to get a combustable ratio.

Plus of course, on a cold day, the battery may not spin the engine as fast because for one, the oil is thicker, more drag, -- less combustable mixture drawn in.
anamnesis

James
When the engine is cold the incoming air, via the carburettors, is too cold to properly vapourise the petrol and that air tends to condense and impede combustion so a greater proportion of petrol is needed to enable initial ignition. Once the combustion chambers have heated up the petrol proportion can be reduced (choke pushed in) to allow for more efficient combustion. At least that's how I think it works!
Bill B

On the topic of the coil, another mod that used to be popular was to fit a 6v coil from a Triumph saloon, together with its ballast resistor.
This also requires a little bit of wiring work but not much.
The idea is that the 6v coil delivers ample spark energy when the engine is running, but a 12v coil may struggle when cranking because of the system voltage drop when the starter is engaged. Triumph saloons (with big 6-cylinder engines) got round this by using a 6v coil and resistor. But the resistor was not in circuit while the starter was cranking, so it was actually getting fed around 10 or 11v momentarily which perked up the resulting spark nicely for initial starting.
Once it fired and you let go of the key the resistor came back into play and dropped the supply voltage back down to 6 or 7v for normal running.
Doesn't answer the Q about choking or over-fuelling, but that's been addressed already.
Only slightly O/T, but it's a quiet Sunday and I'm given to musing.
Greybeard

Daniel,

Is the cold start device an option on a Weber? I have one on my DHLA and use it and it started in the dead of winter no problems. It came from an Alfa Romeo so maybe they thought it was a luxury touch.
David Billington

I have had a number of Webers which have had a cold start lever, although I'm not sure if it was connected to anything.

There is a 'cold start device' elimination kit available here...

https://www.webcon.co.uk/products/15057-dcoe-cold-start-device-elimination-kit/


Description

DCOE cold start device elimination kit


Complete kit which allows the removal and blanking of the internal and external cold start system on the Weber 40 and 45 DCOE.

Essential for motorsport use

Full, comprehensive fitting instructions are included in the kit, and can be downloaded from here

The contents of this kit are all available separately and can be found in the Related products tab


Also, fitting instructions in the attached pdf.
Dave O'Neill 2

Greybeard, From memory of my 15O0 MG that dual 6v - 12v system of coil was used by the later 1500 midgets
GuyW

… also late MGBs.
Dave O'Neill 2

I didn't know that Guy, but it makes sense to me. Thanks.
My '78 1500 (which I sold last week) didn't have it, but it had a non-standard ignition system courtesy of the PO so all bets were off.
The Triumph sixes were much bigger and heavier engines to spin over but had the exact same starter motor which had to work much harder, especially in the colder months. Or at least the Mk3 GT6 engine which is still on my bench has the same Lucas starter as the Midget had.
It also has a 6v coil and ballast resistor.
I should probably have kept the spare motor that went away with the Midget. It was a good 'un.
When I mentioned it being a popular mod I meant it was applied to all sorts, not just Midgets. At least one of my 'Tinas had it and was all the better for it.

Edit: Question for Dave.
Why is the removal of the cold start system on Webers essential for motorsport?
Forgive my ignorance please, I don't get it.
TIA.
Greybeard

In my view, you may also need to consider the actual petrol in the fuel tank as the 'blend' is varied during the year (3 grades - spring/autumn, summer & winter) with different volatiles so always worth ensuring an appropriate blend is in the tank also noting that only the super grade is suitable for use in Spridgets.
S G KEIL

Grey

I don’t know. I was just posting the description from the linked website.
Dave O'Neill 2

Fair enough, Dave.
I bet someone knows though.
Willie?
I'm curious.
Greybeard

28/36 webbers have a cold start device. Effectively it's a mini carb in its own right. It squirts additional petrol in bi passing thr primary jets. I have a great description in the Ford Capri webber manual.

I found this discussion.

" -- "Racing" Webers since the 1960's were delivered with a block-off plate over the cold start system, but that does not mean the system is disabled. The plate covers what was a cable-controlled lever device that lifted the spring-loaded enrichment plungers and allowed some extra fuel to flow to exit holes in the base of the carb under the throttle plate. There is actually a fuel jet, an air jet, and an emulsion tube inside each cold start system.

The trouble is, even sealed off, the cold start system can go bad."

"One or both plungers can lift and begin to leak gas down the manifold, and you'll find you cannot get your idle leaned out, no matter which way you turn your adjustment screws or curse Eduardo Weber, his family, and his dog.

The permanent fix is to remove the plungers, and tap the plunger bore to about half its depth. A 3/8-24 tap works almost perfectly. Drop the plungers back in the bore, and lock them down with a set screw. The fully depressed plunger will seal off fuel leakage at the base."

https://shoptalkforums.com/viewtopic.php?t=147467

anamnesis

I'm curious as well Grey--have had a fair bit to do with webers and never had an issue with chokes coming on , Always just fitted a short stiff wire instead of a choke cable to hold the chokes off.

As far as needing a richer mixture for cold starts, it's not so much the ingoing air temp as that will be almost the same when the engine warms up and starts ok without any need to choke it---It's the actual engine temp. itself, combustion chamber, spark plugs, and specially inlet valve temp. the engine relies on a nice warm inlet valve to help the mixture atomize and same with the chamber temp.plugs etc.
Programming a fuel injected engine you typically need 450-500% additional idle mixture to get a fire happening from dead cold then back around 250% to keep it running cold, tapering off to zero extra fuel by around 30c engine temp.
There's been a lot of work done by manufacturers of factory fuel injected cars to get the cold start/running mixtures sorted--except for RX8 Mazdas, If you start one of them dead cold and turn it off early, before it gets any heat much in it then go to start it again it'll flood for sure, they just didn't get it right.
William Revit

This thread was discussed between 02/02/2025 and 03/02/2025

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