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MG Midget and Sprite Technical - Loss of power, spluttering, then OK again

After a summer of work and upgrades, took the midget out to run in the new diff, so a 20 mile run around the lanes, gentle driving. All went well until about 2 miles from home, on a straight but hilly road, doing @ 55mph, the car started to lose power and a little spluttering, struggled up the hills. Felt a bit like losing a spark to one or two of the plugs. Fortunately no-one being me.

Told car to at least get me back to the village, which it did, then the problem cleared up. Got home, engine revved freely without issue. Not dived under the bonnet yet.

Methinks fuel, perhaps one of the float chambers? I have a new Hardi fuel pump.

That aside, pleased with the upgrades: new radiator has solved hot running, stock 1275 single box exhaust now much quieter, diff nice and quiet too. Brakes OK, but still need work as have long pedal travel.
Tim Carter

Also possible that fuel filter at wrong angle, so when demand is high no fuel gets through?

I have the glass one like in image

Will check this and floats and needles in chambers.


Tim Carter

Tim
Is the fuel low? I've had that a time or two but only with low fuel. Still haven't found anything wrong. I have a filter like yours just below the carbs (1275 Frogeye, mechanical pump).
Bill B

I've never used a fuel filter. Just something else to cause problems. I dont think they are necessary with UK fuels, unless you have a suspect rusted fuel tank.
Vent on fuel tank cap clear?
GuyW

Plenty of fuel, just filled up.

Original cap, plenty of venting there.

New fuel tank

Crud gathering in fuel filter, so needed at present. Will clean out in case blocking outlet holes.

Probably won't get a chance to play again til Wednesday...
Tim Carter

Ignition? Have you got electronic or original
Bob Beaumont

Have you got the filter connected in the right direction of flow--I've seen these before round the wrong way and the little filter element gets chockers full inside while the glass bowl is squeaky clean
Also some have a multi size fitting on the ends that has to be cut back to suit the size hose you have, If you have thje piddly little ends still on there stuck up inside the hose it will definately starve for fuel under any sort of load
William Revit

I had a piece of rubber get shaved loose when the fuel line was slipped onto the rear carb inlet pipe. This eventually obstructed the inlet to the needle valve, leaving the front carb only operational. Once this little rubber flap was removed, all was well.

I have a filter mounted between the tank and the S.U fuel pump since there is always potential for rust, sand, etc., albeit in small quantities.
Glenn Mallory

In my youth I had an MGA that suffered from an intermitant problem. I’d be going along okay and then the spluttering would start. After a stop all was back to normal. My Dad and I tried all kinds of solutions but nothing seemed to work. Eventually, by chance, I I found that the rubber washer where the fuel float chamber connected to the SU carb body and jet had broken in two. That washer had a section like a round L and it broke at the corner such that the top bit formed a ring. Under certain circumstances like a rapid throttle opening that ring would get drawn up and choke off the fuel but later drop back allowing normal service to be resumed. It took weeks to find that most annoying problem!!
C Hasluck

Dicky rotor arm? Have you a real, red one?
Your symptoms sound familiar...
Jeremy MkIII

@Willy - doesn't appear to matter which way around it is fitted, looks the same at both ends. Am going to take it off though, clean it up, and the pipes to and from will be checked.

Still leaning towards the angle / position where fuel input is lower than fuel output, so that there is air at the output end.

@Glen - looks like I will have to have it all apart

@Bob - original ignition

@Jeremy - fairly new distributor cap / leads and rotor arm - a real black one (what is a real red one?)

@ C Hasluck - useful story, again fairly new rubber washers to the float chamber, but worth looking at.

Tim Carter

Tim, an early clue is to watch the rev counter as the car is faltering. If the needle jumps and dances around it is very likely to be a fault with the LT side of the ignition. That is, ignition switch, rev counter ignition warning light, coil and points plus wires and connections to those components.
If the needle just drops as the revs drop then it is unlikely to be electrics, but will be fuel related. It doesn't pin point the fault, but can point you in the right direction for investigation.
GuyW

Tim,

a real red rotor arm is one like this from the Distributor Doctor https://distributordoctor.com/collections/top-rotor-arms
- although they're available elsewhere.
The issue with some replacement rotor arms is the rivet on the top can short out and cause irregular running.
The red ones have a proper small rivet and don't short out.
Jeremy MkIII

@ Guy

rev needle fell with engine speed, no erratic jumping (electric tacho).
Tim Carter

Could still be ignition HT side with tacho behaving like that.

Do you still have your old rotor arm?
Dave O'Neill 2

Yes the rotor arm could be the problem. I would also renew the condenser with a replacement from the distributor doctor as these can fail causing erratic running
Bob Beaumont

Tim

"@Willy - doesn't appear to matter which way around it is fitted, looks the same at both ends. "

It DOES matter which way round---

With it fitted correctly the fuel enters from the end that is open to the glass housing and then goes IN through the outside of the filter element then out the outlet from the inside of the filter element--All the rubbish can be seen in the glass bowl of stuck to the outside of the element--

If it's fitted wrong way round then the fuel enters the inside of the element first where it is trapped and goes no further- the element can be chock a block full of rubbish while the outside of the element and the glass bowl will remain clean ,tricking you into thinking the filter is clean when really it is blocked up-

Just recheck and make sure it hasn't got them piddly little undersize ends left on it too while you have it off-
Pic #1 shows the glass filter which I take you have fitted --With this one in the pic the fuel would/should enter into the RH open side and out the left--The housing 'should' be marked in--out on the ends
Pic#2 shows the troublesome stepped universal fittings that need cutting back to get a decent size--not all filters have these trouble makers
willy






William Revit

Exactly as Willy says. Apart from functioning properly that way, how else would you know there's crud?

Are you sure yours isn't marked Tim?

https://youtu.be/7zm-yM9rIXk?feature=shared

You can also take it apart to clean the filter, and buy new elements for it.

https://classiccarbs.co.uk/product/replacement-fuel-filter-elements-x3-for-pro804-pro805-in-line-fuel-filter/


Unlikely an angle problem. It's pumped, not gravity fed.







anamnesis

You are quite right, on closer inspection the fuel filter has an IN and OUT end. I have it fitted the correct way around.

Dismantled and cleaned it out, lots of tiny grains in there, which I could see would accumulate in the nooks and crannies of the float bowl if filter not present.

Did some testing with engine running, holding the filter at all sorts of angles. With IN down and OUT up, the filter appears to empty, I can just see a slight pulse of fuel from the pump, but after waiting for the bowls to empty - they didn't and engine kept running OK. With IN up and OUT down, filter was mostly full up, no running issues.

Dismissing fuel filter as the problem.

Float chambers apart no obvious issues, other than the front float had some petrol in it! about 4mm from the bottom. Took float off and held at all angles but no petrol would come out. Possible cause? Will need a replacement for that, I will change the needles as well, they are working, but look tired. What are the good ones to get ? Maker starts with a V ?

Pipework all checked and clear, no flappy bits I could see.

Will move on to ignition tomorrow.
Tim Carter

Forgot to mention, I did go on my run without the air filters fitted. Possible I sucked something into a carb, which was eventually consumed by the engine ?
Tim Carter

<<rev needle fell with engine speed, no erratic jumping (electric tacho).>>

Unlikely then to be LT side of the ignition. But doesn't therefore rule out rotor arm, plugs or plug leads. The poorer quality black rotors tend to fail when hot, and then recover as they cool. But it still sounds like a fueling problem to me.

Btw, running with air filters off also causes weakened mixture. Weak mixture will also cause overheating, especially under load. And if your valve are running too tight. They should be not less than 16thou, possibly more if you have a faster cam.
GuyW

Tim
Guy could be onto something with running a bit lean without the aircleaners on---it could well be enough to make the valves stick in their guides----question -you mentioned that you've been doing work/upgrades to the car over summer---can I ask what work/mods you've been doing.
A friend has a 1275 Midget and it'd get stuck valves, He could run the downhill run about 5 Klm from his house and as soon as he got out on the main road, slightly uphill, it'd loose one or two cylinders, let the throttle off for a bit ,then it'd clear and be ok. we went to a slightly richer needle and it solved it, went better all up as well.
Was it a cold day--If I take off from home in our elan on a crisp day, up the main road about 2Klm and the carbs ice up, really weird, the throttle goes soggy and idle speed is way up--Pull up on the side of the road and let the engine heat melt the ice and it's all good for the rest of the day-
I'll be interested to know what you've been doing to your car-
willy
William Revit

@Willy

Was cool (14c) rather than cold (2c)

Issue arose after @ 2omins of driving about.

I didn't do any work on the engine. New radiator, refitted oil cooler, new diff (3.7) - the reason for the run, replaced straight through noisy exhaust with new stock exhaust, brake servo (so vacuum ?), lots of non engine related things.

I believe I am already running a bit rich, but was thinking about maybe a carb piston sticking ?
Tim Carter

Nothing jumps out there then--had thought if you'd done exhaust mods the other way round ,like an open exhaust replacing a std. setup could lean it off but that's obviously not the issue,
Yes, I'd remove the dashpots and clean the pistons and reassemble and check for free movement, refit the aircleaners and go for a repeat of your previous run and see if that, (pistons/aircleaners) was the culprit.
William Revit

Could start a flame war with what oil I put in the dashpots. I have always, on all my cars, used engine oil.
Tim Carter

That’s what was recommended by BMC.

Last page of the driver’s handbook, column E.


Dave O'Neill 2

I had similar issues , and chased it as a fuel problem. It was the condenser in the end (as mentioned by Bob).
Mark '74 Midget

Ordered (at great expense) a new rotor arm and condenser from the Distributor Doctor.
Tim Carter

Re-reading this. Lots of good suggestions, all of which have explained symptoms like this on different occasions. Two more to add to the list:

Coil. When they start to fail one symptom can be (like condensers) perfect when cold but start to grumble after about 20-25 min running. When we run a coil on our tester we leave it there for at least 30 min, break-down usually doesn't occur till at least 20min.
Not sure whether it applies in this case as the problem cleared, but it might be the lower loads from reduced speed being less demanding of HT voltage.

Spark plug temperature. Too hot can cause a misfire under prolonged loads. Eventually this kills the plug. I have seen a change of silencer type change the plug temperature, with more restrictive silencers increasing it and causing this sort of misfire. Unusual, but can happen. Plug appearance will be the tell-tale, ideally you'd need to do a plug check on the roadside while the problem is present rather than later as subsequent running can change the appearance. Photos for us to read the tea leaves would be fun.
What plugs and CR are you running?
Paul Walbran

Another run today after checking over the fuel side of things (not changed the flooded float yet), similar weather conditions. All was going well until coming out of the next village up a hill after @ 15mins of driving, in 3rd, doing @ 30, @ 2200rpm. Spluttered! Was able to pull over almost immediately to a bus stop, problem cleared. Set off home, gave it some wellie up the next hill (no issue) then slowed to @ 40 in 4th, @ 2300rpm, up the next one, still no issue, and fine the rest of the way home.

Distributor Doctors Condenser and Rotor Arm next...

@ Paul, Coil is the Sports Coil from MGOC, probably only done @ 500 miles.
Tim Carter

@Paul

Plugs: NGK BPR6ES

CR (compression ratio? No idea, would need to do compression test ?)

Engine 1275 bored out 40thou, leaded head, running Shell Premium fuel with Castrol Valvemaster Plus additive
Tim Carter

Hmmm--
I'm game --going to stick my neck out and blame the sparkplugs
Why are you using resistor plugs.
What plug gap are you running.----??
With the combination of an upgraded ign. coil and resistor plugs the tendency is for the spark to jump down the outside of the plug at the time when the plug is under most stress, which is usually when lightly accelerating from lean cruise or at full hook.Your 2200rpm light throttle is the perfect time for this to happen.
I'd almost bet if you pull the plugs out and have a real close look there will be a pencil line down the outside of the insulator on at least one plug from up at the top post down to the earthed body.

I'd try a set of BP6ES at .025" or maybe up to .030" max. if you have electronic ignition.

Have you got the correct ign. coil for your car.

willy
William Revit

IF--They sent you a coil for a ballast system and your car is non ballast, then that can cause the spark to be too powerfull for the rest (plugs) as well---while the coil lives.
William Revit

Coil:

"Non ballasted Sports ignition coil from Intermotor manufactured by the original sports coil manufacturer. Sports coils provide a stronger spark for improved economy and performance. Primary resistance 3 Ohms non-ballasted continuous 12 volt supply. Specified for use on vehicles with non-ballasted ignition e.g MGA, chrome bumper MGB & Midgets with either points based or electronic ignition systems."

Resistor plugs intended to reduce "ignition chatter" on the radio. Can try non resistor plugs. Will have a look for tell tale lines on the insulators.

Difficult to keep testing because the problem only happens once, but it does seem that when the problem arises, if I lift off the accelerator and let the revs drop, the problem clears, as if something is stuck and then gets unstuck. The temptation when the problem arises is, of course, to give more gas, in order to maintain speed, or resolve issue via the Italian tune up method ;)

Still waiting for my new floats and needles, supplier being difficult about payment...
Tim Carter

No signs of any tracking on the plugs
Tim Carter

Oh the joys of intermittent faults!
And for us pundits trying to chip in helpfully each subtle variation symptoms can have, which we can't experience directly, can point in a completely different direction. 😂
Paul Walbran

(I'm still flip flopping between fuel and ignition... )
Paul Walbran

Maybe I misunderstood, but if so far this has been a one-off occasion of power loss/ spluttering, then I wouldn't be chasing for any fault at all. I would continue to use the car as normal, be sure to have a charged phone with me and a breakdown "trailer me home" service.

Then just get on with using it. Use the ever increasing spiral circuit of your local roads so you are never far from home. Be alert to a repeat at which moment observe the rev counter needle for behaviour and play with the choke control to see if it will respond to a richer mixture. A simple explanation could be a bit of debris blocking a jet, that then cleared itself. Not an uncommon event with Su carbs.

If I have misunderstood and it has happened repeatedly already, then just ignore these comments and substitute with silence!
GuyW

Cheers Tim
Sounds like the coil is correct ,although , "never take anybody's word for anything". If you've got a multimeter I'd measure across the primary terminals of the coil (wiring circuit disconnected)just to check that it is in fact a 3ohm coil-being a sports coil it might be a little less down to 2.5 would still be ok -I'd still run the plug gap at .025"
If you do have a meter I'd measure up the plug leads and coil lead as well and make sure they're all plugged in properly--measure the coil lead from the carbon pencil in the inside middle of the cap out to the coil fitting and same for the plug leads ,measure from each post inside the cap out the the lead end.
Something to do while you wait for parts.
willy
William Revit

@ Willy

"measure" ? Do you mean resistance in mOhms ?

@ Guy

I probably need to do a longer run after it happens, to see if it will happen again. At the moment, I am just keen to get back home, to avoid having to call out my lovely assistant with the Volvo and towrope!
Tim Carter

Sounds to me like an 'induction' type problem.

Recap.

It's happened more than once.

Same symptoms. Same-ish distances, and same times to clear. Weather cool-ish.

Tim you say.---
"All was going well until coming out of the next village up a hill after @ 15mins of driving, in 3rd, doing @ 30, @ 2200rpm. Spluttered! Was able to pull over almost immediately to a bus stop, problem cleared. Set off home, gave it some wellie up the next hill (no issue) then slowed to @ 40 in 4th, @ 2300rpm, up the next one, still no issue, and fine the rest of the way home."

And you say ---
"the problem only happens once,-- (I ASSUME YOU MEAN PER TRIP) -- but it does seem that when the problem arises, if I lift off the accelerator and let the revs drop, the problem clears, as if something is stuck and then gets unstuck--".

***** as if something is stuck and then gets unstuck--". *****

Interesting.

From all that then, when 'it' happens, it happens once per trip, the engine is at/approaching the same-ish temperature, and if you pull over, and let the engine idle, the problem resolves itself for the rest of your journey; and I'd hazzard, for the rest of the day if you continued driving.

Think about engine oil. Think about how it's viscosity changes as the engine warms up.

I'm thinking PCV. I'm assuming you have a working connected pcv system.

If you don't have a working/connected PCV system, don't bother reading on, and ignore me.

Otherwise --

Have you got a mushroom into the manifold, or a y piece into the carbs?

The symptoms you describe, (I and others have had exactly the same before), are EXACTLY those of a momentary ingestion of engine oil into the induction. It DOESN'T always produce clouds of blue smoke.

Rule nothing out until you check.

Temporarily disconnect your PCV for a few days, for a series of early morning cold start runs.

Still got the problem? Or has it gone?





anamnesis

How about this then:
Bit of debris (grit) in the jet of one carb. At low or constant revs the carb piston remains low(ISH,) and the needle pushes the debris down the jet. Fuel flows past it. All is well.
Accelerate hard, or at least open the butterfly by pressing the throttle, and suction lifts the carb piston and needle. Bit of debris rises up the jet and obstructs fuel flow. Loss of power. Engine stutters.
Leave it for a bit and debris drops back down the fuel tube where it lurks until the next time.
Fix: either dismantle jet and clear through the tube.
Or, a roadside repair, if lucky with engine running, rev it up, repeatedly covering and uncovering the carb inlet throat and it may well suck the debris clear
GuyW

@ Guy

Yep, new floats and vales coming today, so will have the jets off for a check over and clean out when I fit the new parts.

@ Anam

Although engine is 1275, I don't have the mushroom. I do have a catch tank, which is fed from the timing case, and then feeds the carbs.

Have just done a oil change, so perhaps a little too much oil in there. I filled to the top line on the dipstick, but this seems possibly too much. I have read elsewhere about wrong dipstick/measurements for high/low.

I am getting some oil in the catch tank...
Tim Carter

Yep, too much sump oil 'could' be a cause.

"I am getting some oil in the catch tank."

How much? It's possible some is carrying through to the y piece.

I had that on my Capri. I had a closed pcv system like my Sprite on it, but controlled by a valve (much the same ooeration as a mushroom).

And like you, I put in a catch tank, that then fed into the weber inlet manifold.

When cold, small amounts of oil made the engine 'splutter'.

Just as a test. Disconnect the feed to the y pice. Close the pipe so you don't have an air leak.

Question. Why do you have a catch tank anyway? You shouldn't need one.

anamnesis

@ Anam

You asked: Why do you have a catch tank anyway?

Because I was getting oil from the timing case being sucked into the carbs. The catch tank helps me monitor what is happening.

You asked: How much? (oil in catch tank)

Only about a couple of capfuls (e.g. 5 litre oil container cap).

There is a 4" gap between the in/out pipes and the near bottom of the tank, so unlikely oil is getting to the Y piece. But will review when I have the carbs apart again.

I wasn't getting any oil in the catch tank previously, with an oil level in sump at @ 4mm below the full line, which makes me think the oil level in the sump is too high at the moment. No oil leaks from anywhere underneath (apart from a small ooze from the drain plug - forgot to put ptfe on it!)
Tim Carter

"Because I was getting oil from the timing case being sucked into the carbs."

How did that, -- did it affect how the engine was running?

Take you almost no time to elliminate the pcv.

Disconnect it and test for a few days. Onky temporary, abd worst case you get a bit more oil from the rear scroll.
anamnesis

"(apart from a small ooze from the drain plug - forgot to put ptfe on it!)"

PTFE won't really help on the sump plug, assuming that's what you meant by drain plug.

Sump plug doesn't seal on the threads but with a copper washer under the head.
Dave O'Neill 2

Oh no!!!! The oil sucking drama rides again?
Rob
MG Moneypit

Tim
Yes resistance in ohms- around 3 (2 1/2 --3 1/2)

Guy raised an interesting point--as he does--
I had an SU carby playing up randomly once. not sure which model carbs you have but this ended up being that someone had had the jet tube and it's plastic hose off to replace the little 'O' ring in the bottom of the float bowl.They hadn't cleaned out the hole and there was a small piece of O ring left in there and when the hose with it's new o ring was fitted the little piece got itself into the plastic tube which in turn blocked the fitting on the bottom of the jet tube whenever it felt like doing so, very frustrating trying to find it and lucky it was a clear plastic tube on this one so luckily spotted the little nointer floating around in the hose--
William Revit

So, Tim.

Having rebuilt your carb's, have you established the cause and solved your mystery?

Or, could it indeed be a 'sucking' drama? 😉

Wassup?


anamnesis

I have been delayed by other real world matters. Need to do the re-tune, then take for a test run. Maybe Monday or Tuesday.
Tim Carter

Yep. Life does tend to get in the way.
anamnesis

This thread was discussed between 21/09/2025 and 10/10/2025

MG Midget and Sprite Technical index

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