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 - Misfire at 110

I've been using my MkII Sprite (HAN7 1098cc) almost daily without a problem since its purchase in September this year.

However, over the past month it's developed a misfire.

The engine starts first time, every time, and runs perfectly until the temperature on the gauge hits 110 degrees.

Thereafter, the engine continues to run, but only on three cylinders. As soon as the temperature rises to 150 degrees, the misfire clears and the engine runs perfectly.

Car behaves perfectly out on the road.

Happens every time I use it now!


Any help/advice gratefully recieved.


Jonathan.
J Whitehouse-Bird

WOW,

No thats intresting...Only a pure guess, but maybe a sticking valve stem. My thinking is there maybe a small amount of oil on the stem, the temp raises and the oil becomes gummy, as it gets hotter it burns off...a pure guess on my part

are the degrees in F. or C.


What a fun stumper to think about, looking forward to what the others think of as the problem.

Prop
Prop

Have you checked the plugs for either fouling or lean condition?

intake manifold to head misfitted or loose causing manifold leak when parts are different temps until both pieces reach same temp? I would think that would give you 2 lean cylinders, you could check the 1-2 pair of plugs vs. the 3-4 pair for different coloration.

As Prop says, interesting problem!

R

Richard 1600 5 speed

Prop. 100 deg C is 212 deg F. Thus, if the system were the centigrade system, at 150 deg we would be looking at engine melt down.

Jonathan. You say that the engine begins to run on only three cylinders when the temp gauge hits 110 deg and, when it his 150 deg, it again runs on four cylinders. This is interesting.

The first question would be how did you determine that it was only running on three cylinders?

Second question is can you determine which cylinder is not functioning properly when the miss begins? If so, is it the same cylinder each time?

I have some information on how to check out the ignition system and spark plug wires on the MG section of my website, www.custompistols.com/ which might be of use to you. At least, it will help you eliminate some of the possible causes of miss firing and help us to focus on what might be causing your problem. If you decide to perform those checks, please pay particular attention to the inside of the distributor cap, examining it under good light and carefully. Moisture, from condensation or a leaky hose/cylinder head/cylinder head gasket could cause the problem you describe. But, more information would make it easier to offer focused suggestions rather than a "shotgun" approach.

Les
Les Bengtson

Or...

Crack in the head opens/closes with temp variations?

Condensation in the dissy migrating around to the cap with temp rise?

Agree with others... interesting.
Anthony Cutler

Not a solution, but a useful way of determining if a cylinder is misfiring, without otherwise altering any of the conditions operating, is with one of those hand-held laser-spot remote temperature sensors. The rogue cylinder will show a lower temperature to the others.

My suspicion would be a sticking valve.

Guy
Guy Weller

I am terribly sorry JWB but I am a member of the MGCC and as such we are not very technical club(apparently), however may I try and comment.

The 2 temperatures you mention are surely just part of the warm up cycle and are therefore in the region of where we might use the choke aren't they?

I think most engines would run at least at 170F and hopefully 190F so by then your engine is running superb.

So therefore it must be mixture related and I would ask how did you set up your carbs?

Also how do you use the choke

How long does it take to warm up to above 150?

It has become considerably cooler this last month has it not?

Regarding other ideas, such as a sticking valve I have had that happen and you would know immediately that you had that as the rattling noise is unmistakable.
Robert (Bob) Midget Turbo

Prop, Richard, Les, Anthony, Guy and Robert (Bob),

Thanks for all your thoughts as to what this could be.

On return home from work this evening, the 'warming up' misfire sadly turned into a continual misfire (kiss of death, me saying that the engine ran perfectly once hot)!

I'd driven about 7 miles before the problem started and was almost home. Managed to limp the remaining mile or so of the journey.

Unfortunately, it'll be tomorrow evening now, before I get the chance to investigate further.

Richard, I had removed the plugs on a couple of occasions over the last month, and noticed that No. 2 plug was repeatedly fouling.

I had a compression test done (along with a 'Crypton' tune-up) at Archers Garage, Tividale, early in November. Compression was excellent/perfect on each cylinder, but the carb's did need fettling. However, there's still a flat spot on initial acceleration, which I suppose could indicate an air leak at the manifold to head joint? (although I've checked and all nuts are tight).

Robert, your technical brilliance is something I continual to marvel at when reading your posts. You've probably forgotten more than most of us will ever know!!

Not sure who's been bad mouthing the M.G.C.C. - but they're the oldest and best (IMHO). The only reason I'm not a member is that sadly I no longer own an M.G.!

Re. using the choke. I would agree with you, if it not for the fact that I always push the choke (cable) back into place as soon as the engine fires - I never warm the engine up using choke.

When I took the car up to Archer's, someone in the workshop mentioned the possibility of a sticking valve, but as you say the noise would be unmistakable - and there's no noise.

I'll get up to the garage tomorrow and report back.

Thanks again.

Jonathan.

P.S. Should have mentioned in my original post: On initial start up there's some blue smoke (without choke), but this disappears after a minute or so. Would this account for the fouling plug? Or worn rocker gear/oil seals maybe? Excuse my very poor technical knowledge!

J.
J Whitehouse-Bird

The initial start up blue smoke would indicate worn valve seals

Im not sure i full understand the what happened on the drive home last night...

was it misfiring on one cly. all the way home, and what was happening the last mile home that it was limping and barely made it into the drive way

Just curious...when you mean misfire, are you implying that the engine is not firing on one cly. at all and the engine is running rough and out of balance ...thrashing side to side?? like only one carb is working...(Ive seen that before)

sorry just a little confused.

Prop
Prop

Anthony Cutler, Gt Malvern, UK
Or...

Crack in the head opens/closes with temp variations?

Condensation in the dissy migrating around to the cap with temp rise?

Agree with others... interesting.


I had exactly this once. I only managed to diagnose the problem when I opened the bonnet at night and saw the arc of sparks.
The heat of the engine evaporated the moisture so no sign during daylight which foxed me and several garages!!
R Fowler

Jonathan.

There is NO requirement to own an MG to be a member of the MG Car Club. In the midget Register, we welcome Austin Healey Sprites just the same as MG Midgets.....

Oh, I've lost your phone numbers, could you PM to me please....

Mark.
M T Boldry

Well that is very kind of you to say so Jonathan and that means alot I must say, thank you, I will not disclose who on here suggested the above but it certainly hurt myself and also a number of other excellent MGCC people. How on earth would someone suggest that the likes of Peter May, Dennis Wharf, Peter Burgess and Don Hayter to name but a few had no technical ability is beyond words but there will always be someone.

Anyway enough of that nonsense what about your car.

Now that it has packed in altogether almost a new set of problems arise.

Firstly why did it hesitate on acceleration? was that mixture related? could have been, surprised Archers did not sort that.

Anyway tomorrow is another day and it is worth as described above getting in a darkened garaged and checking the dissy cap, if it is faulty white tracks will appear on the surface and that will be the culprit.

Must admit my second Abingdon car was an MGB. I used to use it to travel to work each week as I was always away from home and digging it! If monday mornings were wet it would always give trouble with misfiring due to the dissy cap breaking down in the manner described. I must have changed that cap at least 3 times but can not explain why even today! :)
Robert (Bob) Midget Turbo

If it were worn valve stem oil seals, you would expect it to misfire right from start up, but you say it DID run perfectly until it reached 110.

Now that it has a permanent misfire, it should be easier to diagnose.
Dave O'Neill 2

Did I miss something someware?


Not to make an issue, but Im not seeing where anyone put down the MGCC...All the people I know that are members I envey, it is my hope to be as talented as they one day...granted Ill be 88 when that happens, But personally im glad there aboard here on the BBS.


Prop
Prop

J. I hope you motor problem turns out to be minimal.

When I read the heading "missfire at 110" I thought you where refering to MPH. Mine doesn't start missing till I'm going about 125 :o}
Phil

My 1972 RWA developed a miss which worsened quickly. It was a burnt exhaust valve and once this was fixed, the miss persisted. It was present throughout the rev range from start up. It improved as the engine heated up and would almost vanish sometimes when the engine was hot.

Similar problems are described in the archives but no solution is given. I replaced the distributor cap, rotor, coil, points, condenser but nothing improved the miss. Finally I tested my plug leads with a meter and noticed a resistance in the lead to the coil. On removing the metal cap on the coil end of the lead I noticed a green deposit on the wire. Once this was cleaned off, the resistance went.

I also cleaned the wire on the ends of the plug leads removing slight deposits between the lead and the clip for the spark plug. This also lessened the resistance. I gapped the points to 15thou and put everything back together.

I am pleased to say everything is now running perfectly. Hope this helps someone else get rid of their annoying miss.
Warren

Jon
Just in case they have been overlooked,--- Have the valve clearances been checked recently. It could be possible that a tight tappet could give this sort of problem ---- maybee ? Willy
WilliamRevit

Just removed the plugs from the cylinder head.

They're photographed in the order they were fitted L to R.

4-3-2-1

1 and 2 appear to be okay, but three (and particularly) four seem a little oiled up.

Oil appears to have worked its way up the entire length of the threads - is this usual?

About to go and fire up the engine in the dark!

Will let you know what happens.

William, I have checked all valve clearances and they're spot-on, but thanks for the suggestion.

Jonathan.


J Whitehouse-Bird

I would say that 3 & 4 look to be slightly richer rather than oily. The centre electrode and insulation look dry.
Dave O'Neill 2

Jonathan. If Dave is correct and the plugs show a dry, sooty residue, your back carb may be off. This can be caused by it being slightly rich or it may be caused by the jet not being fully raised after the choke is moved to the off position.

Again, my website has information on how to check out the ignition system, including how to test the spark plug wires, which may be of use to you.

Les
Les Bengtson

Managed to get the engine fired up after several attempts. Still running on three cylinders only.

Turned off all lights and in the pitch black (with the engine at fairly high rev's), I could just detect a faint glow from plugs 1, 3 and 4 only. No sign of white tracks or shorting from dissy cap or elsewhere.

However, having done all that; once I'd switched the lights back on, I spotted a previously unnoticed film of fresh oil on the water pump, so grabbed the lead light and investigated further.

Hidden from view and behind the radiator, carb's and manifolds, it appears that oil is seeping from between the head and block. Head gasket gone?

Would a failing head gasket account for the original temperature related misfire?

Jonathan.


J Whitehouse-Bird

And at the rear...

Jonathan.


J Whitehouse-Bird

Les,

Thanks once again for the advice.

Have just re-read your first post where you mention a leaky hose/cylinder head/cylinder head gasket!

Hmmm...

Thanks for the link to your website also; looks as though I'll be making a further visit!

Jonathan.

J Whitehouse-Bird

I see it's a factory reconditioned engine Jonathan, when was it done?
David (davidDOTsmithAT stonesDOTcom)

Retorque the cylinder head right now. If you haven't had major water loss, you may be able to save it. I've saved a few like this. Don't forget to readjust valves.
Especially likely with a recently replaced head that has not been retorqued.

FRM
FR Millmore

Jonathan, I've had trouble before with oil plugs on 3&4 due to the crankcase vent being plumbed into the rearside of the single HIF carb I had. Rather than distributing the oily air across all 4 cylinders, it seemed to stay on one side of the airflow and hence only went into the back 2 cylinders.
Tarquin

David (DOT smith),

I'm not sure as to the history of the engine, as I've only had the car since September.

Does the engine number relate to a factory reconditioned unit?

The PO claimed that he bought a fully-rebuilt unit and fitted it himself (I have no reason to doubt this). as the engine that was fitted in the car when he bought it (in 2000), was losing oil through the rear oil seal at an alarming rate.

Looking back through all the previous M.O.T. certificates, it looks as if the car (with the rebuilt engine), has only done about 900 miles in the last 9 years, and judging by the general cleanliness of the unit, this wouldn't surprise me.

Maybe through lack of use, the head wasn't ever re-torqued?

Tarquin, thanks for the heads-up on that. Would something like that occur with twin S.U.'s? I'm not sure.

I think I may just strip everything down and remove the cylinder head and systematically go through everything. That way I'll know exactly what's what! Then, if it still misfires after that...!

Thanks for all your help everyone.

Will let you know how it goes.

Jonathan.
J Whitehouse-Bird

If it was my engine Jonathan then before I took the head off I would be checking a number of things. (bearing in mind the fault is there all the time)

I would compression tst each cylinder. I would remove each HT lead and listen to the effect Etc Etc.

Although it is not an onerous job to remove the A Series head is it.
Robert (Bob) Midget Turbo

yes the format of the engine no. denotes either a Gold seal or Silver seal unit. But of course if the block did not need refacing then the tag could date back to the last-rebuild-but-one.
David (davidDOTsmithAT stonesDOTcom)

This thread was discussed between 18/11/2009 and 21/11/2009

MG Midget and Sprite Technical index

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