MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - dodgy idle speed

Good Afternoon All

I currently have a 1978 1500 midget which I have only had for the past month so as yet I have touched nothing under the bonnet. Took her for a run out this last weekend and while the engine starts up fine, runs and pulls fine the idle speed varies the warmer the engine gets. When fully warmed through she idles at about 1500 rpm and I am sure it should be no more than 650/700 rpm?
She may just need a good service (I have no service history with her) but any thoughts or help you have would be greatly appreciated

Blaggard
SR Blagden

Have you tried slowing the idle down using the idle screws?

Naturally a good service and by that I mean setting up the carbs correctly and also setting the timing should be done first before anything else is questioned.
Robert (Bob) Midget Turbo

I'd be tempted to lower the idle speed to say 800 rpm when hot, and use the choke cam to raise the idle speed when cold (and choke pulled approx 1/3 on).

[This is normal operation, BTW.]

A
Anthony Cutler

Thanks guys

I am not an expert on twin carbs so will there be two screws to adjust or does one do both carbs ?

Also why do you think problem might have occured? it does not make any sense to me to set so far forward.

I plan to service very soon but will certainly do this fix first.

Thank you

Blaggard
SR Blagden

SR welcome

You're ideal for my usual advice to new owners, don't ignore it or put it off as it would quite propably have prevented/cure your present problem (but there's no 100% guarantee to that)

Simple stuff here but it took me years and many £000,s to fully learn - yours for free

Buy an owners Handbook as it tells you so much you need to know as to own, drive, service and maintain your car - (Ref: 0058) http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html
which will show you all about adjusting the carbs and lots, lots more useful stuff

As soon as possible do a full and proper 36,000 miles service only miss out items that you have 100% proof have already been done very recently, including;
g/box and back axle oils
brake and clutch fluids
coolant
dissy cap, rotor arm and leads (buy good quality ones)
http://www.distributordoctor.com/rotor_arms.html
fan belt

Always have the battery in good condition and all battery connections and leads clean, secure and protected, same for all electrical wiring and connections

Check the age of your tyres if they are 6 years old or more replace ASAP regardless of tread depth, this will improve, the braking, steering, ride, handling and possibly noise, of the car greatly

Use the car regularly - to get used to it, sort out any wrinkles and prevent others, and enjoy it

Every few weeks check your horn and wipers work (no problem if you use the car regularly)

Follow regular checks, servicing and maintenance as per owners Handbook

Drive in all weathers, the Midget hood should be very watertight if fitted correctly and the heater is more than adequate, if working correctly, for the small cabin

Do not do any cosmetic or improvement work for at least 12 months of regular use, unless you need to replace parts or components, as you may need to use the money elsewhere on the car

Also very useful, you can get suppliers catalogues for free to get you started to see where things go but in my personal opinion they’re not as good paper copy like - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Parts_Lists_3.html

Cheers, enjoy your car
Nigel Atkins

Just a thought before you adjust the idle, check the cable is not sticking in any way and preventing the linkage from returning to the correct idle position.

When it is warmed up and idling too quickly try to push on the linkage with a finger to return it to its "correct" idle position.
If the revs drop then your cable/linkage is sticking.

If they don't then the idle speed is set too high or you may have a slight air leak in the induction, in-board of the carbs, which will cause an increased idle speed.
JB Anderson

Thank you all very much

I am certainly new to Midget ownership so the help here is gratefully accepted.

Will keep you updated

Regards Blaggard
SR Blagden

SR

a few points from a me a recognised non-expert on here

I hope you find them helpful

1500 rpm does sound a bit too fast but bear in mind the rev counter might not be 100% accurate so it might not be quite as high as that, how fast does it sound

cars of this age don't always run exactly "to the book" (if they ever did)

your engine or other bits may not be as standard

my car (1275) runs at a faster idle sometimes but has brand new carbs and many other new bits and has been set up on a rolling road by a marque expert but still does it

(that's not to say that despite that there might still be a fault with my car)

there are many more important items on the car than just what's under the bonnet and how the engine runs (hence the need for a full and proper 36,000 service)

a thought - have you any kind of electronic ignition fitted?
Nigel Atkins

Nigel thank you for your comments.

The rev counter may be little out but it certainly revving way too high for normal idle speed. I had the dizzy cap off on sunday to check the points which appeared to be gapped correctly and not dirty or pitted so I have left them alone at present.
I have pondered over electronic ignition conversions but never got around to doing it on previous classics and have no immediate plans on the midget unless you think they are worth it.

The 36000 mile service makes sense and will be done asap.

Regards, Blaggard
SR Blagden

SR, as an addendum to my post above about sticking cables etc, as you are new to Midgets and the 1500 in particular, they (and not the 1275, I believe) are prone to having accelerator cables that can give a jerky ride and can be a pain in the neck, almost literally, to drive smoothly with.
You may be lucky and have a "good" one but if you find your driving is a bit jerky then it could be the cable.
New Midget cables are not the answer, in my experience, and I use a teflon lined cycle brake cable instead which works well.
Others have other solutions which you may find if you search the forum, but again you may not have the problem at all, if you don't then please disregard this post!
JB Anderson

JB which carb/s do you have on your car?
Robert (Bob) Midget Turbo

SR, mine does exactly the same. Go for a run and when the engine is hot the engine idles about 1500. I have messed with the settings got it running lovely at 750-800. Go for a run and bingo running at 1500 again ! Its real weird ! Are you running a standard air filter box or pancakes ?
S Langston

Messed with the settings? are you sure that is a good ida S.L.?
Robert (Bob) Midget Turbo

SL Hello

its a standard BL airbox filter which houses two 'pancake type' filters. I have got two replacements on order with Moss Car parts.


Have you not bothered to re-set again then if the problem has re-occurred

Blaggard
SR Blagden

Robert(Bob), they are standard SU HS4s.
I bought a new pair about 8 years ago.
The carbs are fine.
Over the last 15 years I have been getting the car just as I want it, which left the slightly jerky throttle to fix, which I have now done and all is well.
I expect most people wouldn't notice it but I'm afraid it wasn't good enough for me to live with.
So after trying a variety of cables including some Escort Mk1 and 2 ones I had on my spares rack I tried a bike cable, with suitably manufactured ends, and that did it.
JB Anderson

Blaggard, if that's the only problem with your engine/carbs I'd suggest leaving them alone until you get the Handbook and do a full and proper 36,000 service as that only could resolve the issue, you'll see what I mean by full and proper service when the Handbook arrives

If in the time being you don't like the fast idle then simple drive on roads where you rarely need to stop - use the car everyday you can (sse previous post)

Electronic ignition - better still full electronic dissy is what I'd go for but only after you run the car regularly for a full 12 months - otherwise Sod's Law whatever you speed unecessarily you'll need and and not have for something very necessary

On ther other hand a freind insists on points but he's doesn't use a classic as a daily and is an engineer - plus modern made points can be more faulty than those they replace, same as condensors, rotor arms and many other electrical bits

JB
now I should know as I used to be a CTC cyclist but I don't - are the actual brake cable and end nipple both exactly the same as the 1275 accelerator cable ?
Nigel Atkins

Hi,
Before you start fiddling with the Carbs, check this out, it could save a lot of time and possibly money.
My 1500 used to do exactly the same thing, when it was cool, the engine would idle normaly, around 800 rpm, the warmer the weather got the more erratic the idle speed became, sometimes up to nearly 2000 rpm, i checked all the usual things plugs, timing, tappets, stripped and cleaned the carbs, replaced the throttle butterflies with plain discs, changed the damper springs and all this did not make the slightest difference. the answer came to me when I was not looking at this problem, but changing the gearbox oil, I noticed that the fuel pipe runs from the tank up the drivers side and into the g/box opening and across the top of the g/box (as a metal pipe) clamped to the top on the g/box with a metal P clip and then onto the fuel pump. I rerouted this pipe using rubber pipe across the back of the g/box and up the left hand side of the g/box, to the fuel pump. this almost completely cured the problem the final cure was to reoute the fuel pipe from the pump to the first carb, again using rubber, behind the battery to the carbs.
In short the problem is almost certainly fuel vapourisation, and doing these mods reomves some of the heat soak into the fuel lines.

HTH
Alan
A Pritchett

Nigel, I can't answer for the 1275 cable as I haven't seen one as my car is a 1500, but to fit the cycle cable (it was some years ago so I hope I remember correctly!) I cut the inner and outer cable to suitable lengths, then I filed the "nipple" thing on one end of the inner cable to fit through the hole in the bulkhead/firewall and fit the end of the accelerator pedal rod.
The other end is fitted to the accelerator linkage in a similar way to the original cable but I used a "solderless nipple"(2 actually for security) that I ordered from a website that caters for trials motorcycles, whose riders are forever making up such cables.
I just used Google to find it, but can't remember what it was called.

I don't know if that answers you question, but I tried!

Alan, you are spot on with heat causing all sorts of problems.Routing fuel pipework away from hot spots and making sure the heatshield is intact are good ideas if people are having problems after the car has been held up for a time in traffic, say.
With the bonnet open, as when adjusting things with the engine running, it is not a such a problem, but as soon as you shut the bonnet there is nowhere for the heat to go.
JB Anderson

Hi JB just wondered why you thought that the 1500 would have a differen throttle response (sticking?) than the 1275 which has the same carb set up?

What was the method of connection of your old cable to the carbs? was there a cam arrangement on the carb lever? Was the actuating lever on the throttle shafts long or too short?
Robert (Bob) Midget Turbo

Robert, I am only going by what I have read in the forums over the years.

I know the "sticky throttle" syndrome is definitely applicable to the 1500 engine because that is what I am interested in, and in my travels I have not picked up the "vibe" of the same problem being apparent on the 1275s, but I have not analysed it in any way.

Someone could come on now and say "of course all 1275 have sticky throttles" but I wouldn't be aware of that, I have just not heard of it being a problem as it can be on the 1500.

The actuation system is absolutely standard, and nothing odd about actuating levers.

For some reason the mix of concentric rather than linear return springs on the throttle shafts (which is probably different to the 1275 setup, but I wouldn't know as it is irrelevant to me), and possibly more than specification as regards friction within the replacement throttle cables that one has to buy now, cause an arrangement and situation that is more prone to jerkiness.

The teflon lined cable solves the problem for me.
JB Anderson

JB,
thank you, it might answer, I'll have to stop being lazy and look for myself

I know my mate used an outer nylon cable but not sure about what he used for inner - I must try to remember to ask
Nigel Atkins

Don't know if it helps anyone, but this site sells a variety of cables and the components needed to make your own throttle cables... http://www.venhill.co.uk/Cable_Kits_and_Components

It was recommended to me when I wanted to make my new throttle cable for my 1500 Midget. Very helpful and knowledgeable guys on the end of the phone too.
Jue Williams

Two things:

Set the idle at just around 1000rpm, then you don't get the huge swings that you're finding.

The throttle cable does snag, and give you "all or nothing" at the point - either find that point and live with it, or get another one - either a brake cable, or, infact, the Spitfire one fits without having to change anything! The other thing is that they tend to come in different lengths, the shorter one is the better one!

As for cooling the engine, well, don't forget that the hot air has to come out, so, one option is to put escape holes in the sides of the wings. I've never had a problem on road journeys, but, the new front is going to have them :)
rachmacb

I remember having the jerky cable syndrome on my 1968 Mini Cooper way back which I solved with a cable sold for the purpose that had a smooth green (?) plastic outer (probably nylon, but I can't be sure) and I think it was marketed under the Paddy Hopkirk name. Now that brings back happy memories of night rallies and autotests and the odd hairy moment.......

<http://www.youtube.com/watch?v=j9lr9uKDlss>

Anyway, I had the car for about 10 years and the cable was still working fine when I sold it.

Peter
Peter B

I had my Midget engine running nice with a colour tune on Sunday. Went for a run and the revvs still picked up to 1500 after a short while. This is where a forum like this is brilliant, I now have to check the throttle cable as I have never heard of the cable sticking before. Especially as the previous owner has fitted new return springs in all the carb levers. So perhaps this is a problem that has been on my car for a while now.
When you think about it slowly releasing the revvs as you drive gives a slow movement on the cable, but a blip of the throttle has a fast movement so the return springs work better.
Just got to get five mins to investigate now !
S Langston

S, do you have an aftermarket rocker cover with a chrome oil filler cap?

If not then you will probably have the standard pressed metal cover with a filler cap that looks a bit like a radiator cap.This cap should be unvented as the 1500 has a closed circuit breathing system.

If you do have an aftermarket rocker cover, probably cast metal, they usually come with a chrome filler cap that has a breather hole in it that needs to be blocked (by means of a self tapper for example)to maintain the integrity of the closed breather system for the 1500 engine.

Extra air on the induction side, like through this breather hole, can mean an increase in tickover, so it is worth checking anyway.
JB Anderson

Yes i do have a chrome rocker cover. I will have to check the cap. Also the breather hoses could do with replacing, to ensure the system does not draw in any unnecessary air.
S Langston

When I first got my 1500 many years ago it had a sticky throttle cable, I replaced it with a new original type which was no better. I replaced that with a Teflon coated bike brake cable from Halfords it cost about £8 from memory. The nipple on one end is ok but you have to fit a solderless one at the other end. It isn't as heavy duty as the original cable but it is very smooth. I wouldn't use the original type again but I do keep it in the boot as a spare.

Bob
R.A Davis

The reason for the 1500 sticky cable is because of the path it has to travel. The A series cable came out of the footwell and curved to the left straight into the linkages. The 1500 comes out of the footwell has to curve left and then do a U turn right back to the linkages as the 1500 motor is in a turned around position. The double turn creates friction within the cable such that the cable wears the inner casing of the cable and thus becomes sticky by having metal cable friction against the outer metal housing. I solved this by having an extra long accelarator cable hence smoothing the sweep of the cable. By the way my 1500 is identical to rachmacb but in a South Africa.
MA Wilson

This thread was discussed between 11/04/2011 and 16/04/2011

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.