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MG Midget and Sprite Technical - Adjusting Idle Speed

I thought this would be simple however ......

The car starts and runs OK but won't idle on its own. With no accelerator input it goes down to 500 on the tacho and stalls. This is in an outside air temperature garage and if I leave the choke out at all after a minute or so it chokes to a stop. So using up my old petrol from the tank is proving troublesome as I can't get out of the car and let it burn fuel, warm up, and test all the other mainly cooling stuff when hot. (How long ish should it take to warm up to idle speed temp at this time of the year?)

I have looked at the Haynes manual and the archive and understand there is a choke (enrichment) a fast idle and a throttle adjusting screw on each carb. I think I understand the relationship between these from my previous reading.

So assuming the carbs are in tune etc. to adjust the ordinary non enhanced idle speed I think I have to adjust the throttle adjusting screw on each carb by the same amount each? or the fast idle adjusting screw by the same amount on each? or should I simply be adjusting the throttle cable at the engine end to pull the throttle open more when at rest position?

A head scratchin:

Dave
Dave Squire - Notts

Idle stop screws on carb body equally for normal idle.

Fast idle cam screws for fast idle, first 5/8" of choke travel.

ALL cables have slight freeplay at rest - DO NOT use cable adjusters for speed changes!!!

The carbs are by definition NOT "in tune etc" as the throttle stops basic settings are the FIRST ones made in tuning, and the fast idle are the last.

Was it not you I told to look up some videos or books? A couple of days ago on this board I believe.

FRM
FR Millmore

very detailed about this and other stuff in the Driver's Handbook - at least a few errors in the Haynes for 1500 and I don't look at it for 1500s
Nigel Atkins

Maybe my problem is I like to understand the underlying principles so I can tweek things in my mind instead of simply following a set of steps.

After sleeping on it I now get this from the Archive in 2007.

'Chris. The hot idle screws are adjusted to set your basic "tick over" or idle. The choke mechanism has two functions--lowering the jet to create a rich mixture and a cam to hit the "fast idle" screws. As the choke cable is pulled out, it should first engage the fast idle screws, causing the throttle shafts to move off the hot idle position and open more than the hot idle setting allows. This should happen before the jets are lowered. As the choke comes out more, the jets are lowered. Both are required for starting, but, depending on several factors, only the fast idle may be needed for running. Hence, we normally let the choke move inwards somewhat immediately after starting and during the warm up. The idea is to allow a somewhat larger throttle opening so the engine will run when cold. As the revs begin to go higher, reflecting that the engine is warming up and rotating more easily, you release the choke, move the fast idle screws off the fast idle cam and the basic idle speed is governed by the warm idle screws. Les'

Maybe it was a long day yesterday and it didn't go in.

Cheers Guys.
Dave Squire - Notts

Dave, My 1275 reaches its normal running temperature fairly quickly, between 5 & 10 mins max.

You should follow FRM's advice & have a look on UTUBE, there are good videos that show you how carbs work & how to adjust them...the ones by john twist are good.

If you want to drain the petrol tank you could use a siphon or disconent the fuel line after the pump & pump the fuel into suitable containers.

To get the engine to idle correctly I would start with checking that the spark plugs are clean & gapped correctly, then set the ignition timing...There are good videos on UTUBE that will help....Then start adjusting the carbs.

Alan.
Alan Cotterill (1972 standard 1275 )

Principle 1) both carbs to be the same.
Principle 2) throttle open amount = engine speed amount
Principle 3)' Idle means you are doing nothing except looking at it - it holds itself there. (throttle stop screws)
Principle 4) fast idle = more than usual, but something else holds it there (choke fast idle cam)

Principle 5) Cold start = choke = rich mixture, by pulling jets down.

So:
Loosen all cables and interconnecting linkages.
Back off fast idle and throttle stop screws until they are not touching anything.
Screw throttle stops in one turn from throttle full hard closed,
Set interconnect throttle linkage with slight play but same opening of throttles, tighten,
Set throttle cable to faint slack - numbers/pics in book.

There is more, but it all goes the same way.

I like that you want to learn from principle - that is how you really understand things, and can eventually fix anything - BUT - it takes a lifetime and ALL your time and in the end you are smart but old and walking because you never have time to fix your own cars.
YOU have to put the work in, and study, instead of asking me to write more of what I know.

If you start with a book or video from somebody good, like John Twist, then you first get your car fixed, and second learn something if that is your tilt and you pay attention. And by all means, send Daniel or Vizard, or Twist or me a few shekels in some way.

12 step programs and religion make it easy for beginners, the lazy, or brain addled to get some desired effect without trouble, and they keep the experienced, intelligent, interested on track to the task at hand. That is why we have ritual - you can do it on automatic pilot. You can then progress from a kid in Sunday school to a Theologian, or stop anywhere in between.

Nigel has a ritual, and a little bible to suit, and everybody can and will benefit from paying attention to what he says - from the guy who just opened the bonnet the first time in his life, to people like me who could build a whole car out of scrap metal.

FRM
FR Millmore

er, well, er, thanks for that :)

I just think none of us are born knowing we all must learn (and relearn) and that it's best to learn from a good reliable basic source, to begin with at least

we all have things we can do or learn and can't do or learn or don't want to do or learn

somethings are easy but not all of us will do these things well for ourselves - baking a lovely sponge cake is very simple but some can do this very well whereas most can't - brewing a good quality real ale is very simple but few can actually do it well

another of my favourite methods of car repair that I don't often mention here is to have the work done by professionals (at least in the first instant) then don't fiddle with it unless you absolutely have to

unfortunately now I can't, as much as in the past, pay professionals to take away the hassle from me or provided the expertise - yes I've had a few bad experiences with some 'experts' but on the whole most have done compitent jobs and a few have done very good jobs for me

I learnt a long time ago that somethings are beyond me because they are, or I'm too lazy, or I don't want to spend my time on them but there's no shame to pay someone else to do these things it's comerce
Nigel Atkins

Its OK Guys, I took the hint.

I restarted with the S.U carbretters book. (Tuning S.U. Carbutetters including full needle charts) Pages 24 to 29. Section III Tuning Multi Carburetters. Gives all the relevent diagrams with carburetter, float chamber and linkage parts shown clearly for S.U. Carburetters (unlike other publications I have looked at) and precisely related descriptions and explainations in a clear and concise manor.

I believe I downloaded it from this or a similar site a couple of years ago when I started work on the car first before the lack of an economy affected my life and I therefore forgot about it.

The S.U. Company also have a web site with help and services available together with an outline of the company products and their development, application and set up for anyone who is further interested at www.sucarb.co.uk.

Thanks for the nudge in the right direction.

Cheers, Dave
Dave Squire - Notts

Good man there!
Soon you will be answering the Q's.
More than glad to help with any odd details, once you learn the basics.

FRM
FR Millmore

also always a good idea to cross reference and check any info you get, even manufacturer's web sites and certainly most books and other web sites can have mistakes, errors or misprints
Nigel Atkins

<<brewing a good quality real ale is very simple but few can actually do it well>>

Marstons usually get it right, :)
Pete Ottewell

when I was a lad Pedi used to be one of my favourite brews but not now, can't argue about the fact thaey do good if not outstanding beer

what about Marston's Bass, Marston's Fourtyniner, Marston's Hobgobblin though

good that you support your local brewery but that would leave me supporting Carlsburp! - even four out of the last five new micro-breweries in the county have been poor
Nigel Atkins

Oi you lot

This is supposed to be technical not theology

:-)

(says he whilst drinking CarlsbUrp 'cos its a cold drink)


I would still rather drink Adnams---in Suffolk

(in fact that sounds like a good idea...)

Dave (for 'twas you who this thread is dedicated to) well done taking the hint

FRM and Nigel

How could their peerless advice be bettered?

Bill1

This thread was discussed between 31/07/2012 and 09/08/2012

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