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MG Midget and Sprite Technical - Another misfire thread!

Whilst trying (and failing) to fix a faulty Revotec thermostatic fan switch, I noticed the front carb float chamber leaking.

The solution there was to replace the sticking valve with a new Viton tipped one (thanks for the heads up about Southern Carbs Anam).

However soon after a misfire started around 3,000rpm. Fine at idle and light load but it misses badly and won't rev about 3000 out on the road.

Replaced plugs BP5ES with BP6ES - no difference except a lower idle speed. Both front plugs were sooty with a slight petrol smell which means I think, the front carb is running rich?

it has electronic ignition, Piranha, a red rotor arm (needed a rub down with wet & dry and ditto the dizzy cover (Distributor Doctor). No cracks apparent on the cover either.

Haven't tried the coil to see if it's running hot as yet. Connections secure and clean on HT leads.

One of the Archive suggestions is to turn the mixture screws up 10 flats and see if that makes a difference; if it does it suggests a failing coil.

My initial thoughts are that it's more likely to be ignition than fuel but what do you think?
Jeremy MkIII

Electronic ignitions can scar rotor and cap, take advantage by widening the sparkplug gap (+3 thou was found to be best on mine but yours might be different).

Why did you have 5 heat and not 6 plugs?

Were both front plugs sooty with a slight petrol smell after you'd sorted the front carb or after?

Have you cleaned (or replaced) those two plugs and put them back in and tried again, wet plugs won't have run well.

I wouldn't bother messing with the carbs in this weather (but I'm not the best at setting the carbs) as the engine warms the mixture will get rich and then be lean latter, that could lead to misfiring. 😉


Nigel Atkins

Have you checked valve clearances, Jeremy. In hot weather particularly, if they are a bit tight then valves can overheat and stick. The effect is almost like a rev limiter coming on with the engine appearing to go from pulling well to suddenly braking. Uprated cams are often specified to be set with slightly wider clearances than 'book'.
GuyW

Is it only a coincidence that the misfire and sooty plugs occured after the new float vlave was installed?

Could the new viton float valve sit higher and be allowing the float to sit higher, raising the fuel level, causing richness?

anamnesis

Thanks for the responses.

Nigel, I didn't know that so will regap the plugs and see what happens.

Guy, you could be onto something as I did the tappets in the Spring as they were really noisy so may have been a little enthusiastic on the clearances. I'll redo them tomorrow and err on the side of loose.

Anam, the front two plugs have always been a bit sootier than the rear two.
But yes you're right the misfire started shortly after the new valve was fitted. I'll try the other one and see what that produces.

Thanks again, plenty to think about and check.
Jeremy MkIII

Is your new float valve one of the new batch which has done away with the spring pin?
Paul Walbran

Paul,
yes it is.
It's one of these (although it wasn't that price when I bought it!)
http://www.southerncarbs.co.uk/onlineshopproducts_53353/prod_2148574-VZX1101-WZX1101-NeedleSeat-Kit.html
Jeremy MkIII

I thought about putting my usual of checking tappets, (points) plugs, timing and mixture in that order but thought it might sound excessive but I find when there's an ongoing issue that has been had a go at already rechecking the process from start can often turn up changes/errors/stuff missed.

As such I'll also add in checking low and high side of in and out from coil.

Also stealing from Guy if you noticed the rev counter needle "jumping" it could be an indication of a fault with the ignition low tension side.

I think you might have already done this if the front is rich swap the new needle valve to the rear and see if the issue moves too.

Swapping the heat of the plugs would mean adjustment changes anyway.
Nigel Atkins

Thanks Nigel,

the first viton tipped valve didn't resolve the flooding issue. Examination showed it was sticking open (no spring to push it downwards as Paul has noted) but a change to the second spare sorted it. At least it stopped it flooding.

A Minispares valve has been acquired. This is all brass, has holes drilled in it and has two different thicknesses of washer supplied to help get the correct float level. It may be pressed into service if all else fails.

It was running well before this, still ticks over nicely and pulls away without hesitation. So annoying!





Jeremy MkIII

Ok

Weakened the front carb by a single flat.

Replaced the viton tipped valve with the one above and with a spacer to ensure it's the same fuel level in the float chamber.

Replaced the float - just in case.

Reset the tappets and checked them twice. A couple were on the tightish side, ie I could push the feeler gauge through but there was resistance to it.

Checked all electrical connections including cleaning the distributor cap and leads. No cracks nor splits.

At rest it ticks over perfectly and will rev beyond 3,500 without hesitation and without misfire.

On the road as soon as load is applied say in 2nd or 3rd, the misfire returns.

Baffled, moi? Oui.

Jeremy MkIII

I had similar symptoms once with a split pump diaphragm ( 1500, so a mechanical pump) Car ran fine on the flat up to motorway speeds and would 'snap' on the throttle to 5500rpm with no load. Then on a moderate climb (up out of Moffat on the Devil's Beeftub road), it refused to pull at all. Dropping the gears didn't help, but it ran fine again when back on the level.

I repaired the diaphragm at the roadside with a piece cut from an Asda carrier bag (don't get excited Nigel, it's not good for you) The repair lasted the remaining 2000 miles of a touring holiday around Scotland - NC500 included, before the tourist board had named it as such and filled it with camper vans.

Anyway the point of this was fuel starvation, but only when the engine was being greedy and the pump with its split diaphragm couldn't keep up. Maybe a parallel there Jeremy? Maybe check fuel line between tank and pump. Splits in the rubber don't always leak out fuel but can draw air in when the pump is working hard, reducing fuel delivery.
GuyW

Cheers Guy,

that idea was not on my radar so will check the hose from back to front. It could do with replacing as some of it must be 15+ years old and I've had to use ethanol fuel for the past 12 months (with Nigel's choice of additive of course).
Jeremy MkIII

Can I suggest when the misfire occurs under load you try pulling the choke out a tad and see if it clears it.
Would indicate a fuelling issue!

R.
richard b

Jeremy, the critical bit is between the tank and the pump. Its possible for a split in the rubber pipe which doesn't weep fuel out as it's only under gravity with a few inches if "head" from the level in the tank. But the suction from the pump is greater and can draw air in, reducing the rate of fuel supply.
GuyW

Thanks Richard, will try that - that would be an easier solution so will try it as soon as it stops raining.

Thanks again Guy. Your suggestion brought back memories of a similar issue with a Rover Metro which broke down 6 times on a journey of 50 miles.
It would run then splutter to a halt. It turned out to be a tiny split in the fuel hose where it ran through the wheel arch (not the greatest design) and was letting in water.
When running it was a mixture of fuel and water, then when stopped the petrol and water separated, it would start and run for a couple of miles the repeat.

I'll get some E10 proof fuel line and change that section first.

Jeremy MkIII

The rear carb on the 1971 midget "spits" fuel occasionally at around 4,000 RPM. I have changed needles and seats, the float, etc. I can smell the fuel when it starts missing, which usually is under a full throttle load and then everything smooths out. After a float broke outright and dumped fuel directly onto the exhaust manifold several years ago at speed, I have vented both carbs to the wheel well past the shock.

Even so, this occasionally poor running condition is not good for fuel economy or my sense of having everything in perfect running condition. Since I have a 5 speed installed, I am usually running under the 4,000 RPM and tend to forget that the condition has not been resolved.
Glenn Mallory

Still misfiring!!

So far replaced:
Float valve
Float (it floats within Haynes tolerance)
Gasket obviously
Fuel filter
All fuel hose from tank to carb (Cordan J9)
Plugs

Checked:
Tappets
Plug gaps
Plug leads
Dizzy cap
for a manifold leak, none spotted (using brake cleaner)

Just returned from a test run. Ran well up to 3,000rm then splutters. Tried Richard's suggestion of pulling out the choke, no difference noticed although shortly afterwards I smelt petrol so stopped and saw fuel on top of the float chamber.

It's a bit weird as sometimes the misfire is bad whereas other times it feels as if the car is going to clear itself and drive through it but doesn't quite achieve that. Below 3,000 rpm everything is fine.
The float chamber occasionally leaking is on my mind but I'm also wondering if it could be a failing coil?
Jeremy MkIII

After a quick scan read, I'm throwing in this speculative punt...

Have you had the air filters off? Have you blocked off the "breather" holes by accident?

Cheers,
Malcolm.


Malcolm

could be. I had a similar sort of problem many years ago and a new coil fixed it, but that was with points and a condenser and I changed those as well.
You seem to have changed everything else apart from that and the electronic ignition!
Bob Beaumont

Cheers Malc and Bob,

yes the air filters are off atm (much easier to remove the float chamber lid and that's been on and off a dozen times...) Will have a look and maybe blow an airline through them to be sure.

Interesting you had a similar issue Bob and a change of ignition parts cured it. Think I'll order one and at worst it can be used as a spare.
Jeremy MkIII

Another thought, is the advance and retard working ok in the distributor??
Bob Beaumont

This the equivalent Lucas Sports Coil I'd now go for and Minimine I've dealt with before and were good. Intermotor are SMPE (Standard Motor Parts Europe ltd), see buy-out info. - https://www.ebay.co.uk/itm/185499070621

Elta is one brand owners of the Lucas name (licence holder) - https://www.catmag.co.uk/elta-automotive-in-management-buy-out

Elta VXPRO Sports High Output Ignition Coil - https://www.ebay.co.uk/itm/142146435277

https://www.demon-tweeks.com/elta-lucas-style-sports-ignition-coil-eltee5001/

Then there's the newer Lucus classic in red labels and boxes.

You pays yer money and make your choice of brand name and label colour.



Nigel Atkins

Just to rule it out, try another rotor arm, I had one go bad and that caused an intermittent misfire and yes it was supposed to be a good one from Aldon in one of their own dizzies.

R.
richard b

You've probably checked and eliminated this possibility, but .... I once had a bit of debris trapped in the fuel feed line between the float chamber and the jet itself. At tick over and low revs the needle pushed the debris down allowing a near normal flow of fuel past it and out of the jet. At high revs, with the piston at WOT position the needle lifted allowing the lump of dirt to float/ get washed up into the jet itself where it restricted fuel flow. It wouldn't clear until I fully removed the jet and plastic tube and washed it out.
GuyW

You might try a full lift test on the pistons in case the needle isn't centered or there's some sh*te preventing full lift.
f pollock

Thanks Bob, haven't checked that so next on the list.

Nigel thanks for the links for coils. So many to choose from.

Richard, I'll add a red rotor arm to the coil order as a spare - just in case!

Guy, Fergus, yes that's an idea a friend has also suggested - dirt somewhere in the float chamber or feed line as you suggested Guy.
I've blown through (with a compressor) the float lid including the overflow depression and backwards through the needle guide into the float bowl so hopefully that should have dislodged/removed any lurking crud.

I noticed the float pivot pin was scored where the float pivots, so cleaned it with very fine wet & dry. They're only 70p so will add a couple to the order as I'm not sure if they are the original 51 year olds.

Paul, sorry misread your earlier post. No the new float is the traditional type with the metal spring. The one it replaced was as you described.

It's due to rain next week so no doubt the misfire will disappear only to return when/if the sun returns!
Jeremy MkIII

Dirt!

After blowing through with an airline it seems the misfire has gone (at least for now).

So well done Guy and Fergus for suggesting that and thanks for all the helpful responses; I've done jobs which needed doing (tappets, fuel hose replacement, new plugs) which had been in the 'defer' category!
Jeremy MkIII

Well done Jeremy!
Someone once showed me how to clear partial blockages in the jets or the fuel to jet feed tube on an SU carb. It involved running the engine at medium revs (2500?) and then alternately covering and opening the carb mouth with the flat of one hand. When covered the intake air flow is strangled and momentarily sucks a surge of fuel through the feed tube and jet, before the engine hiccups and falters, which is when you remove your hand to let it recover. A few repeats of this may clear the blockage if you are lucky and it's worth a try as no dismantling is needed - other than remove the air filter.
Late now, but chalk it up for next time!
GuyW

That's a good tip Guy and committed to memory now for, as you say, the next time.
Jeremy MkIII

Jeremy
Perhaps you dislodged a spec of debris when changing the float valve - faults are often related to something you've just done I find. Do you have an in-line fuel filter? I think they're worth having.
Bill Bretherton

Very possibly Bill. You're right it makes sense that something you've just done is the culprit but I'd got to the stage where all options were exhausted but not quite on this occasion.
Yes I've had an inline fuel filter for donkey's years (changed fairly regularly).
Jeremy MkIII

This thread was discussed between 16/07/2022 and 12/08/2022

MG Midget and Sprite Technical index

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