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MG Midget and Sprite Technical - simple mssfire problem?

nothing to do with my other threads, this is a completely new problem! haha
Suddenly developed a missfire when in traffic; running on 3.
Pulled leads one at a time & it is on number one cylinder. (ie no change when lead disconnected)
Put a spark tester on top of plugs, connected leads and all are firing.
Cleaned plugs, removed weber jets & cleaned them and went for a spin and after 10 mins it suddenly cleared: All 4 firing.
Today, same problem happened. Now it was preceded by a number of backfires. It's not cleared by the way.
What is this telling me?
Mechanic had the dizzy out recently. Before I start on engine timing, do you think it could be out?
Shes not pinking, would a retarded ignition cause this?
I was driving around for an hour or so before this occurred.
Im asking you guys before I venture out there to play with the thing again.
Its 44 oC here (111F for those Americas out there)& I have no shade!
haha




Mallorcaben

A misfire that can be invoked and then does not go away doesn't sound like an adjustment problem to me...

- do you have a filter on the petrol line?
- is there any cr*p in the float chamber?
- have you looked at other tubes in the carb?

It sounds to me like something that gets sucked into a part of the carb and stops it working properly; and then later it finds its way back to where it was before.

Years ago, had a problem on a Scimitar (Ford V6 and Weber) whereby the car would randomly stop; but be OK next day for random time. Turned out (after rebuilding carb and everything else in sight) it was blobs of fuel in the fuel pump gradually attaching themselves to the filter in the fuel pump, blocking flow; they'd detached in their own time and the car would be fine... until next time...

HTH

A
Anthony Cutler

Im with antohny,

You would be shocked at the junk floating in the gas tank, thats why I always run the car on full, and have 2 filter before the carbs

Prop
Prop

If it's truly and consistently only the # 1 cyl, wouldn't a fuel issue be unlikely? Bad plug wire? Bad plug? Bad gap? Bad dizzy cap?

It's 107 degrees here yesterday, 106 today. Too hot to play.

R
Richard 1500 5 speed

Try running it in the dark and look for tracks of blue light. I would suspect plug lead insulation is breaking down. Fuel would be unlikely to cause a problem with one cylinder. If you don't want to wait until dark, run the engine and hold each lead in turn to check for electrical leaks.
Guy Weller

Looks to me like you've a puncture, i would sort that first.
Brad (Sprite IV 1380)

Brad.... puncture??? What on earth are you talking about?
.......
When the missfire cleared the other day, it "felt" like fuel, but as I said, when I pulled number 1 lead nothing changed, pulling other leads caused her to stall.
I do get problems with crap in the fuel, if I run her low.
Ive cleaned all the jets, where else to look?

Also, as I said, it backfired a number of times just before she started to missfire. What is that telling me?
Mallorcaben

Back fire is often caused by unburnt fuel in the exhaust.
Unburnt fuel from #1 cylinder because it isn't firing - because of no spark.
Almost certainly either the plug, plug cap or plug lead are failing under power, even though they appear Ok.
Try swapping plug #1 over with one of the others and see if the problem follows the plug or remains with cylinder and lead for #1.

Guy
Guy Weller

Just cleaned all jets again, removed filter king & cleaned. Took filter out for now.
Ran the balls off her to try to clear, nothing.
Took K&N off and held hand against ram pipes while revving to try to suck crap through.
Still misfire.
Looking at the plugs, ran her with number one out & held against block, I'm getting a spark.
It really does seem to be number one though, as again she runs with one disconnected, but if I take off any of the others, it stalls.
I've cleaned & swapped the plugs round, its still number one thats at fault (I believe)
getting pissed off now.
By the way, I think I mentioned that the dizzy had been out recently (while swapping starter motor)
But she was running fine for a few days before this misfire started.




Mallorcaben

Change your plugs, they break down under load but appear to fire on light loads. Same for leads.
Robin Cohen

If swapping the plugs around doesn't move the fault, then it is likely to be the plug lead or dizzy cap. It isn't timing or fuel which would effect other cylinders. It could be a faulty valve, maybe sticking, but my money is still on an HT electrical fault. If the leads are not too short you may be able to swap over #1 lead with one of the others (I mean the whole lead, not just swapping onto the different plug!!)
Is the dizzy cap properly clean inside?
Guy Weller

OK, just swapped the dissy cap, rotor arm, leads & plugs still misfiring.
I'm not convinced that its the one cylinder now. I just tried pulling leads off again while running and it seemed the same whichever I pulled!
May be back to the fuel blockage.
aaaarrgh.

Mallorcaben

Why is your car leaning so much to the right then?
Brad (Sprite IV 1380)

Brad, it's on a slope! (full pic attached)
More of a mountain really.
Remember the Vauxhall zaphira adverts, with the two kids. Parents on the beach, "think I'll have a little nap" ??
It was filmed just here. (Alcudia, Mallorca)




Mallorcaben

Hmmm,
would appear to have been a simple problem after all!
After cleaning everything once again (carb) I tried another set of plugs.
I had already swapped them about to see if the problem moved (it didnt appear to)
I had an old set of (missmatched) plugs kicking about so I fitted them.
Not misfiring now!
Still got a backfire occasionally though.
Done a short test drive and all seems well.

Need to get some more plugs, can't decide what type.
I will start a new thread!
cheers guys...
Mallorcaben

I was about to say plugs, but I was on your other thread. What you describe is a classic symptom of a plug fouling. There are a few of reasons why it can be just the one plug:

First, the spark plug may just have half died for what ever reason. It happens occasionally. Cheap & simple to fix - let's hope you have!

Second, it could be minor ring wear/damage allowing a small amount of oil into the chamber. Cure for this is expensive, so until you do, keep a plug spanner and spare plugs in the car!

Third reason is related to your other thread, and is unique to no 1 plug. If the heat range of the plugs is too cold, they will foul. Generally no 1 runs coolest (being first to get the cool water from the radiator and also having the best airflow around the block) and will foul before the others if the spark plug heat range is marginally too low. The cure for this is to fit a slightly hotter running plug in this cylinder only (assuming the others are OK). It is not uncommon for race cars to need cooler plugs in the centre cylinders as they run hotter. In fact BMC specified this in the B special tuning book.

This cure is also good for very light oiling in reason 2 - howver, take care to listen for pinking etc under load that wasn't there before. It is a sign that the heat range of the replacement plug is too high and if you continue to drive on it you may hasten the end of your rings.
Paul Walbran

The first thing I thought of was plug fouling. I cleaned the plugs but still had the same problem.
Just given her a good run & she seems fine.
I've got missmatched plugs though. Waiting to see what advice comes up on my other thread before I buy new plugs.
Mallorcaben

Sometimes cleaning the plug doesn't seem to work even though the plug looks fine. After cleaning plugs I always put the suspect plug back into another cylinder to see if the problem moves with the plug or stays with the cylinder.
Paul Walbran

If the problem continues after you sort out the plugs, you might want to look for a vacuum leak.

Charley
C R Huff

Any deficiency in the HT side will show up under heavy load, and the heaviest load from the ignition's viewpoint is at lowish engine speed and heavy acceleration. Checking spark at the plugs at atmospheric pressure in air is not at all like what happens in the cylinder. All the stuff you have done gets the common problems. What plug gaps are you using? Wide gaps load the system more.

One point is that coils really like to be mounted top down (as it was originally), or at least horizontal with the LT terminals also horizontal. There is an air space in the coil, and you want the "bubble" to not include the LT terminals. The oil in the coil then covers the internal connections at the LT terminals, and prevents arcing from the HT terminal. If the coil has been subject to abuse from bad leads, bad plugs, wide gaps, or amateur diagnosis by "seeing how far the spark jumps" the coil can track internally, just as caps do, and with the same symptoms. This is much more likely when the coil is mounted top up as yours is.

FRM
FR Millmore

Well noted FRM, my coil points downwards.
Brad (Sprite IV 1380)

Interesting to hear that coils are that sensitive. Some months ago - I think it was last autumn - I left the ignition switched on, the coil overheated and blew the end cap off, spraying all the hot oil out. I pushed the innerds back inside the cannister, but now with little or no oil left. I drove the car to go and buy a replacement coil but as the thing ran Ok I have never yet got round to fitting the new one. I use this car regularly and will have done 3 or 4 thousand miles since then and my air-cooled coil seems perfectly happy. The spare is still in the boot!

Guy
Guy Weller

That was an old photo, taken just after the rebuild. I spotted the coil had been fitted wrong just after this photo was taken & changed it. lol
Car still seems fine now. Have ordered some BP8ES plugs.
Mallorcaben

Guy-
It's not that "coils are that sensitive", but just that if pushed near their design limits, the oil thing can make the difference. Sort of like fitting a higher pressure cap to the cooling, mostly it doesn't matter if all is otherwise well.
It was formerly the case that "ordinary" coils did not have oil, and the "Oil Filled" label was a mark of a better than usual one. The latest thing seems to be that the coils are epoxy coated, but otherwise not in a container at all. Judging by Mazda, they last forever at modern high voltage outputs.

Mostly I was pointing out reason(s) that coils can have internal problems yet still seem OK on the usual checks.

I've noted several reports of exploding coils lately, which was a virtually unknown thing when coils were made by real electrical companies. Cheng Feng Gee must be getting old and sloppy!

FRM

FR Millmore

Thanks Fergus, I wasn't challenging or questioning your advice which from past experience here I know to always be very reliable. Just thought it was of some interest that I seem for the moment to be getting away with a less-than-perfect coil.

The problem is that when the car does develop a misfire I won't know if it is the coil or summat else. Or more likely I will forget about the coil completely and spend days replacing all the other components! Even more likely, it is waiting for some cold wet and dark wintry night before it packs up - just to teach me a lesson!

Guy
Guy Weller

This thread was discussed between 28/07/2009 and 03/08/2009

MG Midget and Sprite Technical index

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