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MG Midget and Sprite Technical - Erratic idle

My 1969 midget idle is not steady. it is smooth for 20-30 seconds then it slows down by an estimated 50-100 rpm's for a couple of seconds the again increases to 1,000 rpm's. I recently noticed that when I pushed the dipstick tightly into the dipstick tube the idle smooths out for a much longer period of time, with less frequent instances of a blip of lower idle speed.
Does this sound like a vacuum problem, or am I off base. I'm interested in your thoughts.

Thanks,

Keith
KTM Moore

Earlier vac systems used regulator between crankcase and inlet manifold; maybe your regulator isn't working properly?

I don't know when the changeover occurred - certainly my '73 used to be ported.

A
Anthony Cutler

Hi,

Your problem is called "hunting". Sounds like an air leak. But not sure if this is caused by your sump breather.

Easy way to find out, is to temporarily block off the breather pipe from the sump to the inlet manifold, and see if the tickover steadies.

If it does, then likely you have a problem in the system somewhere between the timing cover and the manifold.

If not, then it's another air leak somewhere.
Lawrence Slater

only thing to add to A and L's comment is the obvious check all pipes, valve and filter are clear, not blocked and clean

could just be wear or lack of lubbing in dissy, carbs ect. or grit and muck in somewhere

possibly a photo of your engine bay would help by comfirming what you have for those not familar with US cars and to allow for variations made bt previous owners
Nigel Atkins

I am at about the 98% point in the restoration to original of a 1971 MG Midget Mark III. I have what I believe to be an oil separator welded to the front of my timing chain cover. The top of the separator appears to be setup for a hose connection to somewhere. Some articles and pictures lead me to believe the connection is to the carbs in some manner. Another maintenance article leads me to believe it is connected to the crankcase. Any advice, articles, pictures would be greatly appreciated.
JRS John

From the top of the oil seperator, a pipe goes into a y piece. From the y piece two pipes go into the carb bodies.

This picture is courtey of Dean Smith ('73 RWA), Suffolk, United Kingdom, who just posted it in "Midget 1275 carburettor springs"

Handy that. :)

You can see the pipes and connections you need quite clearly.



Lawrence Slater

Keith has (should) the early Smith's PCV valve. This needs to be cleaned and possibly a new diaphragm fitted. Also the oil filler cap needs to be correct type, and clean.

John's car should be as Lawrence stated, assuming it has the correct carbs.

FRM
FR Millmore

Thanks so much for the quick response. The picture is exactly what I needed. This should put me at about 99% restored. I learned 2 MG's ago that the person that says he has completed a restoration is quickly in for a big surprise.
JRS John

the pipe into the Y piece I had dealings with recently

over here the standard replace pipe for separator to Y piece is a 1 metre length of expensive vacuum pipe that stands very little chance of fitting the Y piece without crushing it because the internal size is too small

IIRC the pipe is ½” and the two short pipes are ¼” the pressure is very low so as long as the pipe is oil resistant no problem with it

you’ll also want a clean or new vented oil filler cap, the standard one is black plastic with built in filter (rolled length of wire mesh
Nigel Atkins

Erratic Idle?



...."always look on the bright side of life"


Coat

Gone!
Matt1275Bucks

Changed the coil and the problem disappeared. The coil sounded as if the was an internal spark which resulted in a simultaneous drop in idle speed for a few seconds. Thanks to all who replied to my question, and most of all to my son Shannon who finally, accurately, diagnosed the problem. He is becoming quite the mechanic. Attached is a picture of his restoration currently in process.

Keith


KTM Moore

well done Shannon

I can't remember hearing of a coil presenting that problem before - so I've learnt something new





or I've forgotten and relearnt, which amounts to the same anyway until I forget it again





oh, and, how did he account for the pushing the dipstick lightly :)
Nigel Atkins

I was the person that found when the dipstick was pushed into its tube slightly the idle smoothed out. Neither Shannon nor I understand why this was the case. That was part of my initial question.

Keith
KTM Moore

Keith,
I was pulling your leg, I saw that, doesn't tie in with the coil

it does however perhaps raise a question about the venting/breathing

on the threads before I've seen cars plumbed, er differently to original, some right dissasters

the Driver's Handbook shows the various arrangements and additional arrangements the Cal cars have might be worth checking yours is as it should perhaps be

have a look at the John Twist videos too as the general subject is covered on I'm sure (IIRC) more tham one model of MG
Nigel Atkins

Keith,
You should get a similar effect if you remove the oil filler cap on the rocker cover.

When the engine is running, if you take off the oil filler cap, and place your palm over the opening on the rocker cover, you will (should ideally) notice a slight suction.

Unwanted fumes are drawn from the sump via the breather pipe on the timing cover, and vented to the inlet manifold. The oil filler cap is vented to allow air to be drawn in, and the carbs are tuned to accomodate this. Other than that, the breather should be a ( more or less) sealed system.

Removing the oil dipstick when the engine is running, in effect introduces an air leak, which weakens the mixture, which alters the idle.
Lawrence Slater

I didn't think it was that sensitive but it might be another reason for the grommet (seal) on the dipstick which is usually missing
Nigel Atkins

Thanks Lawrence...That confirms my suspicions, even though I didn't fully understand the actual consequences I could tell immediately, when I pushed in the dipstick, that there was an increase in engine speed and a somewhat smoother idle, even though it wasn't perfectly steady. With the additional change of the coil the idle, I believe, is as good as can be.

Nigel...I didn't mean to come across as lacking a sense of humor, I was merely trying to clarify the situation. Being an ex pat. I like to think I understand the British sense of humor. My dipstick is missing a seal so I have inserted my dipstick into a small piece of rubber hose and inserted that into a piece of 1/2"ID vacuum hose which fits perfectly over the dipstick tube, eliminating any air leakage. Not original but functional, until I can come up with something else, if necessary. Thanks again, to you both, for your input.

Keith
KTM Moore

Keith,
no problem I was trying to avoid putting anything about dipstick double good job I did :)

as I put a couple of John twist videos covered this

one time I actually got the rubber washer for the dip stick and it must have been a substitute part as it was a waste of time instead I use a blanking grommet with a hole I put in it

it's not that tight so I'll have to try and see if it makes any difference to the idle

of course as I put earlier the Cal cars have additional systems that were the Driver's Handbook has good details and drawings as I know from posting on another thread if I can find it I'll try to post a link here or the thread title
Nigel Atkins

found the thread but it was about a Canada car!

your '69 car by the look of things could be just like the photo Lawrence put up in which case you could also look at cleaning the rolled up wire filter inside the plastic vented oil filler cap

and probably(?) an evaporative system for the fuel and air pump(?)

or for an early car you may also have a control valve that needs taking apart and cleaning
Nigel Atkins

Keith, I can't quite visualise the rubber seal you've used, but if it lifts the dipstick too high, you should be aware that you will get false readings for the oil level in the sump.

I'm not sure how thick the original seal is, or how critical it is, as there is a fairly wide margin between max and min, but it might be worth finding out.

I have PTFE wrapped around mine in a taper, so that when pushed down, it seals.
Lawrence Slater

When I got my car it had no dipstick. One of a number of minor issues that made it a not-quite concours example. I got one from a scrapyard - I think from a mini. I then had to re-calibrate it for my car by filling with the specified amount and marking the dipstick to match. Of course I had to run the engine to circulate/ fill the filter and galleries and check to make sure I had the correct amount added anyway.
Guy

That's interesting Guy.
I have what I believe to be the original 1275 dip stick. ---- Actually I only have one, but 2 engines, I lost one somehow. --

Anyway, when I fill up from a rebuild with the specified amount, the dipstick registers it wrongly, even after running the engine. It reads low, so I just ignore it, as I know I have the right amount in there, and only top up to a llitle below the max level.

Does anyone know the exact length of the correct 1275 stick, and the measurements for max and min along the blade of the stick?

Also, the plastic pipe I believe varies in length among the different versions of the A series engine. -- different part mumbers.

Does anyone know the measurements for that?

Lawrence Slater

This would be useful information. I have run my engine now for 10 years based on my DIY level calibration, so I presume I got it right, or maybe it isn't that critical. But it would be nice to have some factual data to confirm. There are certainly a number of variables for the relevant parts that could very easily get mixed up over the years.
Guy

Lawrence,

I just measured the dipstick of my 1969 midget mark III of which I am the original owner and it has the original dipstick. The length of the stick from the bottom to the underside of the flange is 9 9/16". The distance to the bottom of the stick to the center of the "min" notch is 7/8". The "max" notch from the bottom of the stick to the middle of the notch is 1 11/16".
My dipstick sits 5/16" high due to the way it sits in the tube with the rubber hose fitting over the dipstick tube to eliminate air leakage from the sump.
Knowing this it is very easy to determine the depth of oil on the dipstick.

Keith
KTM Moore

Keith,
Brilliant, thanks very much for that. I'll measure my stick asap. Can you also tell me how high the plastic tube is from the block.

I'm not even sure if I have the correct plastic tube.

Lucky you being the first owner. I've never bought a new car, and it's never bothered me. But now, having owned my '66 Sprite since 1977, I wish I had been the sole owner too.
Lawrence Slater

Lawrence,
as best I can see the dipstick tube stands 1¼” up from the block

if you need to get the tube out don't do as I done (breaking my own rules) don't try and pull it out with small pliers until you have cleaned and lubricted it as the tube will be brittle and break at its collar level just about level with the block

Keith,
I'm glad to say my dipstick matches your measurements, though the dipstick isn't as easy to measure accurately as you'd expect, also highlighted the different fractions of an inch used on different measures

would it please be OK for me to put these measurements up as a seperate thread so that they might be found when in the Archives as I'm sure it'd be of use and reassurance to others
Nigel Atkins

Nigel,
Here are my measurements, if you want them for your database table:
But, remember, my dipstick is not an original item. And I calibrated it myself by filling with a measured quantity of oil and then marking the stick

Height of block tube: 1.252" (1 1/4") This should be as standard. Although the tube is metal, not plastic

Dipstick measurements taken from underside of sealing washer (because height from bottom of the stick is the least critical dimension): Overall length to bottom of stick 9"; Down to Max mark 7 5/8"; Down to Min mark 8 1/2"

Comparing to Keith's dimensions is not quite definitive as he doesn't give the height of his dipstick tube, but assuming it is the same as yours and mine, which we agree at 1 1/4", then my Max mark is actually 1/4" higher and my Min mark is 3/16" higher.

I am usually pretty careful to keep the oil up to the max mark, which suggests I operate with slightly more oil than Keith would have. I guess 1/4" higher might be around 1/3rd pint?
Guy

Guy you devil I've just finished working to base 16 and now you introduce tenths of an inch - you have to admit metric makes much more sense

if I take it as your washer being about ¼” thick then this ties in with Keith’s figures, see what you think

I think excluding the washer is probably best as it would be difficult for most owners to know if the washer on their particular dipstick is of the correct type and size

I converted measurements to flange without washer to end of dipstick, thus –

the length of the dipstick from the underside of its flange (without washer) to;

the end is 9 and 9/16" – (243 mm)

the ‘max’ line is 7 and 7/8” – (200 mm)

the ‘min’ line is 8 and 11/16” – (221 mm)
Nigel Atkins

I'll also put -

. millimetre measurements are rounded to whole figure

allowing for use of only a metal tape measure or plastic rule (or is it ruler) +/- fractions of a millimetre are out
Nigel Atkins

Nigel, you've stolen my thunder. :)

I was on the brink of starting another thread for the very purpose you describe. Hence as a punishment, you can do it, and encourage others to measure their 998s and 1098s, -- as long as they are the original part --- and send them to you for recording. lol.

Here's mine I found the plastic tube for my spare engine, (amazingly, but not the dip stick), and compared it to the one on my current engine. (It twists out relatively easily, with only finger grip). It is the same as the one in my current engine, so I conclude it is a 1275 plastic dip stick tube. It measures the same as yours 1 1/4 inch from block.

My dip stick is the same as Keiths. As his is definately original, so is mine.

From collar to the end of the stick is 9.5 inch maybe 9/16th.

The distance from the collar to the max mark is about 7 7/8th inch which coincides with Keiths measurement from the bottom of 1 11/16ths.

I'll have to investigate next time I completely drain and fill the engine, why my oil level doesn't read true according to the dip stick.


Lawrence Slater

perhaps my tube might also just have lifted out I didn't try after last time which ended with me renewing the sump gaskets (don't ask why as it very embarassing) and introducing a leak I never had until then

998s and 1098s would need another two sperate threads to save confusion but if anyone has the info we can put it in a uniform er, form

as for quanity of oil it's quite suprising how much remain in and on the engine - even when I took the sump off and cleaned it dry

that's why I never initially try to put in more than three quarters of the specified quanity
Nigel Atkins

Yes, I suppose there must be some still in the galleries, and the pump too, and probably in the crank and ends. I could well have fooled myself all these years that my stick was wrong. But I first noticed it from a dry rebuild after filling the engine, from a measuring jug.

I guess I must have made a boo boo, and as a consequence have been under filling ever since. I'll measure by how much at the next oil change, which will probably not be until I re-build my spare engine. --- See other thread on engine breathing started by Bob B.
Lawrence Slater

I always try to get the oil as hot as possible when draining and to leave it to drain whilst I'm off for a mug of tea but even then you could get a little more out if you left it much longer

refill quantities seem to vary but I don't measure exactly so don't know for sure

mine, last oil change and last top-up were to 'max' line as I wanted to check consumption whilst in Wales

I've found with other engines that they seem to have levels of oils and coolants that may be below 'max' levels but once at these levels they remain steady at those levels
Nigel Atkins

Nigel, I really don't know what you are grumbling about!!
""Guy you devil I've just finished working to base 16 and now you introduce tenths of an inch""

I took one measurement with a dial gauge and immediately provided the conversion to fractions ( 1 1/4") All of my other measurements and calculations are similarly to 1/16 of an inch, or multiples of. So, what is there to complain about?

My calculations from a measured quantity of oil added was to a completely dry engine. i.e. not after a normal drain at oil change, but on a newly rebuilt engine. So no issue of residual oil in the galleries etc. Even so, my levels seem to come out about 1/4" higher than the factory dipstick from the evidence now provided. So maybe I have been running with slightly too much oil, or maybe it isn't that critical. Obviously too low an oil level would be more of an issue.
Guy

Guy as if I'd moan, a mere light comment

Lawrence's experience seems to agreed with yours so it could be that yours is more accurate as I've no idea about the accurancy of the factory dipstick, I get the feeling if the cars originally when tested from newish used a pint of oil every 500, 600 or a 1,000 miles that they would expect the average owner to get the oil spot on the max line

as I put I think with length of ownership you get to know where on the dipstick the oil remains for the longest

is your washer about 1/4" thick? as the one I was given was about that size any looked too thick to me ans just a stock grommet but perhaps I was wrong, the washer sat on rather than in the dipstick tube which I thought must be wrong
Nigel Atkins

I've just used an o ring and a wrapping of PTFE into a taper to seal it, as I don't recall ever having a correct rubber grommet for it.

I wonder by how much, raising the height of the dip stick by a grommet or other device, translates to quantity of oil in the sump?

How much extra per 1/8th inch say, if you then filled to the max mark, would you add in quantity of oil?
Lawrence Slater

Nigel, the washer on mine is a sort of semi-hard rubber. A bit like a tap washer, which may indeed be what it is! It fits flush within the recessed metal "washer" which is welded to the dipstick itself. This metal washer is flanged - rather like the penny-sized washers that go over the rocker box stud rubber seals. So in taking my measurements from the underside of this washer this would be how far down into the liquid gold, the dipstick extends. I therefore don't think one should be adding or subtracting a nominal 1/4" for the washer.

Lawrence, not sure if you saw that earlier I was guessing that my 1/4" greater depth of oil might represent about 1/3rd pint. Maybe. Approximately!
Guy

Guy I'm not adding or subtracting for a washer

Keith put >>The length of the stick from the bottom to the underside of the flange<<

which I thought was a good idea as it eliminates variances in 'washer' thicknesses

you put >>Dipstick measurements taken from underside of sealing washer<< so to compare the two the thickness of the of the washer needs to be take into account, like me measuring my height in high heels one day and flatties the next :)

also I wanted to know the size of the washer you got to compare with the one I bought using the original part number but it was probably just a bog standard general stock item

as it happens the very end of my dipstick has been cut off but as it's below the minimum mark as you rightly put it doesn't matter

the inverted grommet I used was soft rubber but is now hard wonder how long before it goes brittle

I was just going to put you'd already put 1/3 pint for 1/4" but yo've already done so
Nigel Atkins

Ah thanks Guy, I missed that in all the discussion. :)

So I could have been under by around a 3rd pint. As the difference between min and max, is about 3/4inch, I doubt it was that critical, and I haven't noticed anything untoward. But I'll top up to the line tomorrow just to be correct.
Lawrence Slater

Lawrence,
I'm surprised at you, why bother

anyway unless you have the correct thickness washer and the factory dipstick is very accurate you might still be wrong with too much or too little - or it might make it spot on

a third of a pint is what percentage of your intial fill up from engine rebuild and with say about 1 pint difference between min and max
Nigel Atkins

I have just had a closer look at this dipstick of mine. What I thought was an inverted flanged washer turns out to be a flat washer after all. At some time I had painted the dipstick and receiving tube yellow to make it easier to see to reinsert in the gloom down beside the block. And I had run paint around the edge of the rubber washer so it looked like the metal bit! Looking at it now, I think the rubber washer is indeed a tap washer that I have used. It is 1/8" thick
Guy

yeah my inverted blanking grommet is I guess a lot nearer 1/8" than 1/4" so that's a 1/6 of a pint less then

mine keeps on the max mark so it must be around correct but normally I have it at a bit under the max mark

good idea to highlight the dipstick and tube

my HT lead also sits in the way of the dipstick which is a bit of a pain
Nigel Atkins

Matt. I got it, if no one else did. Say no more!
Jonathan M

Have I missed Matts post? Ah yeah, sorry. say no more indeed, :)
Lawrence Slater

Lawrence,

The dipstick tube appears to be metal and it protrudes 1 3/8" above the block.

Keith
KTM Moore

Thanks Keith.

Have you seen Nigels new thread on the subject? He's documenting the information for the future record.
Lawrence Slater

This thread was discussed between 27/01/2012 and 20/02/2012

MG Midget and Sprite Technical index

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