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MG Midget and Sprite Technical - Cylinders 1 and 2 not running properly

Hello everyone, newbie to the forum here.
Our early Mk1 midget broke down at the weekend and had to be recovered.
It has run perfectly fine for many years, but started to feel a little lumpy and then progressively got worse until it seemed to be only firing on two cylinders and then stopped altogether.
The plugs, points, leads, coil, cap were all changed with no effect.
The issue is that cylinders one and two don't seem to be doing much (there is some but very little effect when the plug cap is removed and replaced on either) - three and four are OK it seems as the same removal of cap on either causes the engine to nearly stop altogether.
I've removed and cleaned the front SU (HS2) float chamber in case it was fuel starvation, but it's not that.
Without the air filters fitted, and lifting the centre pivot of the accelerator mechanism by hand it can be seen that both carbs pistons lift, but there is a significant blowback through the front carb.
Has anyone any thoughts where to look next....?
Thanks
Rod
R Pearce

Rod. You seem to have been changing parts prior to gathering any information that would help you decide what parts might need changing. The first thing I would do would be to run a compression check on all the cylinders and see what that might have to tell me. All four of the cylinders should be within ten percent of each other. If not, further investigation is in order.

With the engine warmed up and running, have you sprayed carb cleaner (or used an unlit propane torch) around the intake manifold to carb and to the cylinder head joints? If the engine starts to work better, there is an air leak in the system near the front carb.

Do you have one of the fine German tools for adjusting and checking out the SU carbs? This is the kit that has two hollow tubes that fit into the tops of the pistons when the damper nut is removed, then have two wires that go into the top end of the hollow tubes, sticking out at 90 degrees, so you can observe if the pistons are rising in a synchronized manner. If you have such a kit, you might check out the balance of the carbs. You might also, if you have a dial caliper, measure the depth of the jet below the bridge of each carb and see if they are very close to being the same depth.

If you have a strobe light having the inductive pick up, you can see if each plug is firing properly by simply hooking the light up to each spark plug lead in turn and observe if the light is flashing regularly.

All of these tests will help you to focus on where the problem might lie. Changing parts out, at random, gives you little real insight as to what the problem might be. Right now, you only know that the four most common causes of your complaint are blown head gasket, air leaks at the intake manifold, carbie problems, and distributor problems. You need to do some basic tests to determine where to focus your trouble shooting efforts. (Always remembering that it may be a combination of more than one problem.) Time spent gathering information, in an intelligent manner, is seldom time wasted.

Les
Les Bengtson

Thanks for letting me post here.
I would just add that as well as the blowback through the front carb, I'm getting a loud popping noise from the exhaust that we didn't before....
R Pearce

Thanks for your very prompt and thorough response Les.
I changed the plugs, leads and distributor cap first as these are relatively simple and quick to do. This had no effect.
When the car completely stopped I then changed the points and the coil - again no effect.
With the breakdown mechanic we restored the original cap and leads and coil and did some checks on the sparking which all seemed OK.
It might have sounded random, but there was some logic to my approach (honest there was) as in my experience most issues of poor running are electrical especially when the car was running so well, and then just a day or two later it wasn't.
Anyway - the mechanic felt it was probably fuel related and so, with the issue seemingly on just cylinders 1 and 2, and with the car now at home - I removed and cleaned the front carburettor float chamber and the pipe through to the jet head/needle.
A compression test was next on my list too as I did wonder about a valve problem, or the head gasket - but on the latter I don't think I'm seeing any other symptoms.
Thanks again
Rod
R Pearce

Sounds like some fuel delivery issue to me.

Blocked carb jet - blow it out, but probably just as well to give the whole carb and components a good clean.
Blocked fuel filter in the carb float bowl - if it has has one

If not then

Hose leak/union somewhere
Cracked intake manifold
Oggers

Thanks Oggers
I removed and cleaned through the connecting pipe and the jet when I took the float chamber off.
R Pearce

Rod,
try spraying some carb cleaner over the exhaust/intake manifold gasket with the engine running and warmed.

Then the compression test - or the other way round.

I think you are right to never discount there being more than one thing causing or contributing to a problem and that the parts you put on could also be faulty.

Having had similar last year my money is on a possible HGF, altho' it might turn out to be something simpler, and in my case I found other issues (at least one self inflicted) contributing.
Nigel Atkins

'popping noise from the exhaust' is that on the over run?
I had the same noise and it turned out to be a failed exhaust manifold gasket, identified by spraying carb cleaner around both carbs (as others have suggested) and the engine revs increased when sprayed around the rear carb. In your case it seems it may be the front carb.
Try that first and see what results you get.
Jeremy MkIII

Thanks Nigel, Jeremy
Yes, from talking to a mini enthusiast neighbour, a leak test around the front carb inlet manifold connection with carb cleaner or WD40 seems a good idea for tomorrow, before then moving on to compression tests and possibly valve clearance checks.
My neighbour also said he had a similar issue with an old mini and that turned out to be the start of a head gasket failure between cylinders.
The car has always popped a bit on overrun, but this new issue is popping all the time....
R Pearce

An update.
This morning I've run the car and sprayed everstart and carb cleaner around the inlet manifold to check for leaks and nothing detected.
Then I did the compression tests:
Cylinder 1 - 90-92psi
Cylinder 2 - 98-100psi
Cylinder 3 - 160psi
Cylinder 4 - 152psi

Next steps are to take the rocker cover off and check valve clearances and look for anything else.
It is looking like it will be a head off job - hopefully just for head gasket replacement and nothing worse.
Rod
R Pearce

..Further update
All rocker/valve clearances checked and are fine.
The top of the head/valves/rockers/shaft are all spotlessly clean too.
Head off next I think.
If it is the head gasket, what would the tell tale signs be?
R Pearce

Gasket may look something like below, or worse.

You'd also want to check for marking on the head and block.

The Payen gaskets were in short supply or possibly (still?) not available at the moment but there are others.



Nigel Atkins

If you have compressed air source, maybe try a leakdown test? Remove rocker cover, and spark plugs and turn engine until valves are closed on a cylinder then pressurise it with air through the plug hole and listen to see if/where it comes out. There are proper adapters available or you can bodge something.

You haven't mentioned any overheating or smoke from exhaust so either burned valves or leak between cylinders seems most likely at present, although this usually happens between 2&3.



AdrianR

HG failure - Classic symptoms are gas bubbles in your coolant. Look in the overflow tank. That or it reeks of exhaust fumes.
Oggers

I'll get in quickly before the chorus of ' fit a Payen HG'.

By all means fit one but torque and then immediately heat cycle and re-torque. Someone I know fitted and torqued then filled with water before retiring for the night. The intension was to run the car in the morning and retorque then. In the morning the sump was full of water. A phone call to Payen allegedly revealed that they were aware of the problem. This was on a 1/2 engine I built before last Christmas that the owner then fitted the head to on installation.

Copper HG's are not free from fault. Again last year. An acquaintance who admits the gasket was of some age and from a supplier whose name was lost in the mists of time. Another sump full of water. The sealing rings held the head away from the gasket face. Perhaps they had hardened with age?
Alan Anstead

Rod. The lower readings from cylinders 1 and 2 indicate that it is not an electrical nor a fuel related problem. It, also, indicates that there is some form of leakage between cylinders 1 and 2 which is causing those two cylinders not to function as effectively (perhaps not even function) as the other two cylinders.

There are a couple of possible causes for this condition with, as others have noted, a head gasket failure being the most probable. But, a warped cylinder head, a cracked cylinder head, a burned cylinder head, or a burned block between the two cylinders are also possible causes of such a problem.

Remove the cylinder head and use a straight edge on the upper surface of the block and the mating surface on the cylinder head. Use a feeler gauge to determine how warped the block and cylinder head may be. If you can get a .002" gauge between the block/cylinder head surface you should have the areas reinspected by a professional automotive machine shop and follow their recommendations. (As a straight edge, a length of cold rolled steel may be used initially. For final investigation, a machine shop should use a straight edge made of tool steel with a possible deviation from perfect of .001" per foot or less.)

As some have noted, water in the oil may (but not always is) be a result of a head gasket failure. Much depends on where and how the gasket fails. But, as a general matter, it would be a very good idea to drain and refill the oil and replace the oil filter as part of your repair process.

Les
Les Bengtson

I don't have a Payen, just the standard one supplied by Moss. Never (touch wood) had a problem(10.5CR). I follow Alan's procedure. I agree some of the old stock gaskets can be a problem.
Bob Beaumont

Finally got the head off and the gasket is worse than the image posted by Nigel above. There is a 2 or 3 mm wide gap completely burnt away between cylinders 1 and 2. It has resulted in a small amount of copper being effectively welded to the head where the gap in the gasket is.
I've spent some time with Carb cleaner, rags, a brass wire brush and careful scraping with a Stanley knife blade trying to clean off the raised area of copper.
I need to look again at the block to see if there is a similar area of material welded to that.
Has anyone seen this before and/or got any thoughts on how to clean and prepare the head and block surfaces before fitting a new gasket?
R Pearce

Use a long straight edge (metal) across the cylinder head from front to back. Then, if you see any light between the cylinder head (the bottom edge that mates with the block) use a feelers gauge to determine how much your cylinder head is warped--most of them are to some extent. If more than about .003", the cylinder head should be machined back to perfectly flat so that it will mate properly with the block. You can do the same with the block, but cylinder head warpage is far more common than block warpage.

As for cleaning the top of the block, insert some paper towels into the cylinders, then use a wire brush on the upper surface of the block, preferably with an assistant holding a shop vacuum hose near the brush to, hopefully, pick up anything that is brushed loose.

Les
Les Bengtson

It's not unusual.

From the very post it was obviously a head gasket failure.

Do you know the history of the engine? When was the head last off?

"The issue is that cylinders one and two don't seem to be doing much (there is some but very little effect when the plug cap is removed and replaced on either) - three and four are OK it seems as the same removal of cap on either causes the engine to nearly stop altogether."

"Anyway - the mechanic felt it was probably fuel related ". Mechanic? Hmmmm.

To the issue. If all you have is remnants of the 'copper' head gasket, stuck/welded to the head and block faces, then that's good news. I'd use a file, or course emery wrapped around something flat, or just a scraper, and gradually remove it from both surfaces.

Then as suggested by Les, a straight edge across both surfaces to check for 'dips', through which you can see light, or slip a small size feeler gauge blade. If acceptable, bung on a new hg.

If not, either a machine shop for skimming, head and or block, -- the best but more expensive way to do it. Or if very small gaps, then copper wire or even JB WELD, if less inclined to spend money on it (it's been done with success before).

Considering the copper. Is the old hg copper? If not, has someone done the copper wire trick before on this engine?

BUT anyway, before fitting new hg, you might as well inspect your valves and seats, and grind or renew as required.
anamnesis

Thanks Les, anamnesis,

I was concerned about using a steel wire brush for fear of scratching the head/block surfaces.
The engine was rebuilt be a reputable specialist about 12 years (~10k miles) ago.
Yes the head gasket was the copper type so I'm pretty sure the copper on the head comes from that rather than a previous bodge.

I shall continue the slow process of scraping/careful emery-ing the copper residue and then use a straight edge to check
Thanks again
Rod
R Pearce

10k miles is nothing. So either it wasn't done well, or wasn't torqued after.

Possibly it already had block/head face irregularities when last built, that weren't addressed then.

So now's the time to do it properly, and NEVER need to do it again, -- given your low annual milage.

Shame you aren't in kent near me, if you need help.

Anybody in West Sussex who could render eyes on advice? - Assuming it's wanted/needed that is.😉
anamnesis

A quick check for your valves Rod.

Put the plugs in the head. Invert the head and support it level.

Fill the compression chambers with petrol, and see if it leaks past the valves. If not, your ok.

anamnesis

It was done by Lackford in Cowfold, or at least he used his specialist builder.
The car definitely went back for its re-torque too.
Having looked back through the records, the mileage since the work in 2009 is circa 7k.
R Pearce

My first HG only lasted 7k-miles from getting the car with a rebuilt/reconned engine which really annoyed me, it's replacement only lasted 46k-miles which pissed me off even more as both times the block was checked and head skimmed.
Nigel Atkins

Yes, quite common.
Ever since asbestos was nixed the copper sandwich gaskets have been variable, only as good as the non-asbestos fillers. Some have been excellent, others shockers.

There is likely some damage to both head and block, though in many cases it is very minor and wont show up nuch if at all with the straight edge. So I strongly recommend having the head resurfaced regardless of what the straight edge test shows. The process is relatively cheap and the head is already off the car. (still worth doing the straight edge test so you have a record of what happenned.)

With the head dead true, that gives the gasket a much improved chance of coping with any irregularity on the block, greatly reducing the potential for further failure and having to pull the engine and get the block machined true - much more effort and expense than a head resurface.

I suggest checking out factors that may have contributed to the failure, in particular does the ignition advance rate (full curve, not just static or idle) suit the combination of compression ratio and fuel octane used.

Paul Walbran

Paul is spot on, but I appreciate it is not that easy to find a decent machinist especially in the thicko UK. Clean all mating surfaces off - plastic scraper and light abrasion, check for flatness with a pane of glass and use a Payen gasket. I again agree with Paul that efforts to find root cause should be made. 7K is nothing, unless whoever did it made a real mess of it. I reckon the cause of my HG failure was the car running too hot due to a weak mix.
Oggers

And don't overtighten the head bolts! Do the 2 stage process as Alan describes but only to the 'book' specified torque.
GuyW

Ahhh, but which "book"? What is correct for one gasket may not be correct for another....
Oggers

Oggers, I wouldn't want to bore you with chapter and verse. Any competent owner can establish the correct torque for their set-up.

But there is a temptation to 'add a bit for good luck'. This crushes the fire rings and distributes the pressure over the whole gasket, thus reducing the pressure / unit area and giving a weaker seal.
GuyW

I was stirring things somewhat, but it strikes me that all torque settings are not the same. It is somewhat dependent on which gasket is used. I am guessing the MG "book" would use torque settings appropriate to gasket material used at the time - copper perhaps?. Things have moved on, so it may be a different torque setting for a Payen - for example. In most cases I am sure it won't matter a great deal, but HG is one to get as right as possible.
Oggers

I quite agree Oggers. That's why I said don't over- torque just for good luck! Haven't got the link handy just now, but there is an interesting discussion about this on the Kieth Calver web site.

And yes, I recognised the bait ! ;-)
GuyW

Rod, you should have noticed by now, that there is a commonality to the answers.

All agree on cleaning and surface preparation. All agree on the need for a new head gasket. All agree that it should be tightened in sequence, and to an 'appropriate' torque.

You have probably noticed too, that as many as there are answers, there are almost as many opinions on how tight, which head gasket, how to scrape it clean, and why it failed in the first place.

And of course, we are ALL right. 🤣.

But seriously. IMO, these are simple engines, and a hg replacement isn't complicated, and there's no need to complicate it. Use a Payen if you like, and if you have one to hand. They're a good brand. But I've found a hg from Sussex classic cars works just as well on my bored out 1275 with cooper s pistons.

I will say, I definitely agree, don't over tighten it. You not only risk the new hg, but also risk breaking a stud. And there's another debate. New studs or re-use old? I've re-used mine, more than once. Haven't broken one for yonks, and haven't had a hg fail for yonks either. 😉

anamnesis

Agreed. I have always re-used studs.
But do check you have the correct hardened flat washers under the head nuts. Normal washers are too soft and will compress in time, reducing the head stud clamping pressure.
GuyW

Good point Guy, wrong washers that collapse are often a contributing factor. If in doubt get a new set of 12H2178. They should be about3mm thick and not deform or gouge when torqued to spec.
Paul Walbran

'check for straightness using a ruler'

I'm not convinced this is a reliable DIY check. How do you know your ruler, piece of glass, or straight edge is straight to within, say, 3/1000 ?

At best, it might show a badly warped head, but is it good enough to tell you if the head would benefit from a skim - I doubt it.

Chris Madge

Chris, probably not, if you are an engineer. But it has been the standard recommended way for everyone else for the last 50+ years! If you shine a light from behind the rule it is surprising how small a gap can be picked out. It is particularly useful for checking for volcanoes around the stud holes on the block.
GuyW

It is sometimes difficult not to over-torque if you don't understand what the correct torque setting should be! - hence my question. Of equal importance to not over-torquing is the necessity to obtain an even torque on all studs/nuts.
Oggers

Chris

A pane of glass is amazingly flat - especially if it is float glass which effectively finds its own level. I think both glass or a straight edge are sufficiently accurate and fall within the acceptable tolerance which the gasket would take up when torqued down. A ruler? - not for me though.
Oggers

And, QED. 😉
anamnesis

Quite agree Oggers. And probably even more variation between mal-adjusted torque wrenches, dry or oiled threads, dirt on threads, worn threads . . . But start at least from the specified data for your engine. And don't "add a bit for luck!"
GuyW

I think we have torqued this one to death....I'll fetch my coat
Oggers

:-)
GuyW

Thanks for all the comments since I last posted.
An update:
I took the head around to a friend who has a good flat plate and dial gauge set up and we determined that the head itself, having only done the circa 7k miles since the full engine rebuild that included a skim, is flat and the issue is/was a raised section of deposited copper ~3mm wide and about 1.5 thou high between cylinders 1 and 2.

I then went to our local garage where one of the mechanics has rebuilt sprites, midgets and MGB's over the years. Looking at the head his advice was to very carefully use fine emery, lubricated with duck oil/wd40, to remove that raised section. I have done that on the head and the block (which had a similar copper deposit) and all looks pretty good.

On the recommendation of another acquaintance I've ordered a silver steel straight edge to check the block and double check the head, but with the appearance and checks so far I'm expecting it to all be OK.
How clean does the head and block need to be? I've used Carb cleaner to remove a lot of carbon deposit but there are still dark areas - presumably it doesn't need to be spotless as much of the deposit is within the still visible machining marks on the head and block.

I was recommended, by a reputable mg specialist, a Mini parts supplier for head gaskets and have sourced a couple (in case I mess up one) of copper on the upper side head gaskets. These are made to original Payen (no longer available) specification.

I'm unsure whether to use any additional gasket sealant, or fit dry. The gasket supplier has said the gaskets are fine to fit dry, but also said that Wellseal can be used if required for a highly stressed unit. I'm not sure my standard 948cc engine qualifies for that description.

Regarding the head stud washers - 8 of the 9 had good thick washers, but the rocker pedestal with the rocker shaft locating tab washer had deformed/gouged and in fact was a difficult to get off. I've ordered two new locating tab washers. I've also ordered a set of the flanged cylinder head nuts which spread the load better.
R Pearce

I've used Wellseal for as long as I can remember. One extra thing I do is to remove the studs to fully check the block is flat. Often the threads will pull up a little, and these can be dressed flat and the top of the threads chamfered slightly. The removed studs and threads want to be squeaky cleaned and checked for level when they go back, check also that your new flanged nuts can spin on with little resistance. With Wellseal I retorque after the first heat cycle (always cold) and again after a hundred miles. With any dressing of the block face or head be very careful no detritus can get into the various oil ways.
f pollock

This thread was discussed between 17/08/2021 and 09/09/2021

MG Midget and Sprite Technical index

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