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 - Exhaust noise when you shut the throttle.

So what exactly causes exhaust noise, including popping, on the overrun?

Aside from popping, I also get a long continous kind of rasping sound when I rev up and lift the throttle. We've all heard it. Sounds quite nice. But what exactky causes it?

I asked AI google about -- "deceleration exhaust noise in old 1960s british car engines"

AI says.

Deceleration exhaust noise (popping, crackling, or "burbling") in 1960s British cars (like MGs, Triumphs, and Austin-Healeys) is caused by unspent fuel entering the hot exhaust and igniting. This is primarily a result of high-vacuum engine braking, aggressive camshaft profiles, and inherent traits of carbureted fuel systems.

Why the "Burble" Happens

High Vacuum: When you close the throttle while moving at speed, the engine pistons continue to pull air, creating a massive vacuum. This draws a rich air-fuel mixture through the intake.

Unburnt Fuel: Because the throttle is closed, there isn't enough oxygen to fully combust this mixture inside the cylinder. The unburnt fuel passes through the exhaust valves into the hot exhaust manifold.

Ignition: The hot exhaust pipes ignite these stray fuel droplets, creating the characteristic "pop" or crackle

Common Mechanical Factors

Carburetor Setup: 1960s British sports cars usually rely on SU or Zenith-Stromberg carburetors. Unlike modern fuel-injected cars, they lack a deceleration fuel cutoff, meaning fuel continues to flow when the throttle closes.

Valve Overlap: Many of these engines use performance camshafts (especially in models like the MGB or Triumph TR series) that hold the intake and exhaust valves open simultaneously. This promotes better high-RPM breathing but allows some unburned mixture straight into the exhaust.

Ignition Timing: Slightly retarded ignition timing causes the spark to occur late, meaning combustion continues as the exhaust valve opens.

When to Troubleshoot
A gentle burble is normal and a sought-after feature of classic British motoring. However, loud, violent backfires usually indicate an underlying issue:

Exhaust Leaks: A leak at the manifold, downpipe, or joint sucks fresh, oxygen-rich air into the exhaust, which fuels and amplifies the popping.

Air Leaks: Worn carburetor throttle spindles, leaking manifold gaskets, or bad intake seals can pull excess air into the engine, causing a lean condition that leads to severe backfiring.

Ignition Issues: Faulty spark plugs, incorrect timing, or failing ignition wires.

https://bitly.cx/VNpiT

Right. So any of the above? Lol.

anamnesis

We could play spot the error πŸ˜‚
Paul Walbran

Compared to modern SUVs a Spridget is the same size as rice crispie ( and about as robust) so why not expect a little " snap, crackle and pop"


Already getting my coat...
David Cox

🀣🀣. I like coco pops myself. Just a bit more resilient than a Spridget David. Lol.

anamnesis

Don't believe what an AI produces. It commonly amalgamates distinctly different explanations as one, producing false 'facts'.
GuyW

In days gone by there were three arched streets running under London Bridge Railway Stn now actually lost as part of the new station complex
Panda drivers, in Morris Minors, would often run the car up to high revs. Turn off the ignition then turn it on again. The β€˜bang’ from the exhaust would reverberate along the archways.
Alan Anstead

I know Guy. That's why I posted the AI answer.

Yep I remember those arches too Alan. All the charm of London Bridge, like so much of London, has been demolished. May well have been needed, may well be better, but not in my eyes.

I used to do that ignition trick for the big bang too. I seem to recall it risks blowing a bloody great hole in your exhaust. πŸ˜…. Good job you reminded me.

So my earlier suggestion in Guy's parting thread, of --- " if I switch off the ignition next time I'm going down a decent hill with my foot off the pedal, the noise should continue ", isn't such a good idea. The pop I get when I switch on again, may bigger than I expect. 🀣🀣.



anamnesis

I can confirm it.

Going downhill closed throttle, circa 3000 rpm, switch off ignition, looooong growling uuuuuuuuurrrrgggghhhh exhaust note continues. BUT. NO snap crackle and pop. NO SPARK, but still the long growling uuuuurrgggghhh. So what is generating that noise?

When I DO switch on again, BANG. Exactly as Alan remembered. A loud sharp crack though rather than a big boom, and the popping returns.

And another thing I realise now.

When I get up to higher revs, say 4000/4500 rpm, on a long descent, and close the throttle completely, IGNITION STILL ON, the long growling exhaust note is sustained WITHOUT any popping and banging, UNTIL THE RPM DROPS to circa 2500 ish and below, and stops somewhere below 1500 rpm.

So my popping is ignition/spark and rpm dependant. It seems to me it must be timing. I wonder if my timing is out, but it goes fine and has a nice idle. Is my vacuum advance knackered? I don't think so, but maybe my spec is wrong.

I've got a 1300gt head, 30 thou over, cooper s pistons, and a 1300gt distributor complete with the correct vacuum advance can.

I don't think (I'm pretty sure it doesn't in fact) it pops and bangs like this when I run twin hs2 SUs. I wonder if changing to a single HIF44 has upset my vacuum timing.

I know it did before. So I had to drill my HIF to create the distributor vacuum port. I'm certain it's in the right place.

But maybe I need the same Vaccum can Guy has on his MG Metro distributor. Or better yet the same distributor.

'Ere we go again. Maybe I'll just live with the snap crackle and popping.

Edit.
Close throttle coming down from higher rpm, my engine is pulling over 25inHg of manifold vacuum.

I wonder what it's pulling at my distributor port. Think I'll check at the port.






anamnesis

I was always under the impression it was caused by back pressure from the exhaust because of the exhaust manifold design, but thinking about it it would disappear or change when you switched to a tubular manifold so that blows that theory out the window.

Trev
T Mason

Too rich ?

I remember when first commissioning my midget I was getting a lovely snap, crackle and pop on the over-run (just like racing cars). I adjusted the mixture (leaned it down) and then all was OK.
Tim C

Maybe Tim. I'll try leaning it out. But somewhere in my head I thought too weak caused popping.


Yep, found it.

"Why does my exhaust pop on deceleration?

That loud popping or crackling sound when you roll off the throttle might sound aggressive, but it usually signals a fuelling issue. Popping on deceleration is almost always caused by a lean mixture, an exhaust leak, or both."

https://www.thecarbshack.co.uk/pages/why-does-my-exhaust-pop-on-deceleration

Now that's interesting. A leak or a weak mixture.

The HIF has a bi-metal device that weakens the mixture. I wonder -- πŸ€” ???


anamnesis

Watch this video. Brilliant. Doesn't answer my questions, but great explanation of echausts and backpressure.

https://youtu.be/3zfLnOnnW2A?si=RCxxITqZ4DSS0glt

anamnesis

A lot of later carburettor cars (80's) had a spring loaded vac pot on the throttle connected to manifold vac. to hold a little throttle on at high vac. -overrunning, but the spring in them is strong enough (sometimes adjustable) to override normal vac. at idle and let the throttle return to normal idle.
For a car like a sprite/midget it's going to be an exhaust leak or ign.timing too far back most likely. but could be a lean idle mixture.
try pulling the choke on a bit for a try.
The bi/metal in a hif is to adjust mixture compared to fuel temp-------but
Some hif's had a spring loaded valve in their throttle butterfly to stop the manifold vac. sucking excessive fuel on the overrun--does your hif have that.
William Revit

"I thought too weak caused popping"

You could well be correct. Maybe I was too lean and had to richen the mixture. Oh for the days when I could remember everything :)
Tim C

No, solid hif butterfly Willy.

Yep good idea about the choke, been meaning to try that but wasn't sure it was relevant. Now you've confirmed it may be, I'll have to back the fast idle screw off so it doesn't hold the throttle open. Next time I'm out I'll give it a try.



anamnesis

An -if your exhaust is good and sealed and it's popping out the back on overrun i'd be checking your ignition timing, just knock it up a tiddle and see if it fixes it---
As far as normal exhaust noise on the overrun, it's just pumping noise, the same as you might hear through carburettors on throttle without air cleaners. or same as gets produced from a compressor--or from your chainsaw when you're trying to start it. it's just wind pumping. You would have been impressed by our old Chev racer, it had 180deg headers and a propper tuned exhaust with two 3 1/2" collectors poking out the side. It was on methanol and we ran the idle mixture fairly fat to get a stable idle temperature. on the track on throttle it screamed and fairly noisy on the overrun too. and on trailing throttle it would flame and flutter in the exhaust like a blowtorch with a flame about a foot long--used to get plenty of comments from fellow racers like, -that thing scared the sh** out of me when it went past and that flame came at me----lol
William Revit

Thanks Willy. I'll double check the timing.

That Chevy must have been a sight to see then. A foot long flame; fantastic.

My Sprite already turns heads with the popping. Flames too would blow their minds. 🀣🀣.

Holy flame Batman.
https://youtu.be/5V8tX1TXOvQ?si=Bi6MEzwZS1pqVrJP






anamnesis

It's not all that long ago you were playing with your exhaust----are you sure you haven't got a little leak up the front there somewhere.
With the chev ,yeah it used to create a bit of interest. Arrived at a track on the mainland late once and scored the end spot in the pits up against a wire fence. all good but annoying as well because of the millions of questions from the spectators up against the fence. Anyway when you start this thing up it starts with a 'BANG' out the exhaust-lol-that got em, they're back off the fence now. then you have to warm it right up for ages(the methanol keeps it from warming up quickly) but it has to get up to temp to warm up the conrods and gas the methanol out of the engine oil--the oil gets really milky cold until it gets up to temp then it steams out and we have clear oil ready to race. Usually once it's warm it gets a couple of good revs to clear it out , brrrm,pop on the overrun and flame ,brrrm,pop, flame----ready to race ---turned it off, put the bonnet on and clip it all down and went for a feed. As we were walking back through the pits checking the opposition a guy goes racing past with a fire extinguisher-in a bit of a hurry --looked around and a big patch of grass outside the pit fence was on fire. Bit like claiming your patch like a pussy cat -lol
William Revit

Yep new inlet/exhaust manifold gasket Willy.

Had a seriously knackered gasket. It was popping then just the same, and I thought that was the cause. But the new gasket has made no difference at all, other than made it quieter in the engine bay and stopped me being gassed by fumes getting into the cabin.

3 branch. Into long pipe with no joints into the single silencer. Been under it multiple times looking for leaks. The 3 branch was new only last year or 2024. So unless that has a dodgy weld, I'm pretty sure no leaks.


Lol. Sounds like you need to take a portable fire deflector with you. Like those things you see when they launch rockets. Lol.

Stand back folks, get yer burgers ready for cooking. πŸ˜ƒ.

Complicated stuff going racing then. Shame all that excitement will be lost by going electric. Just turn on and drive away.



anamnesis

No electric racing here-well not yet anyway
Motorsport Aust. has decided in their wisdom that if any track wants to run electric cars they have to have an imersion pond big enough to drown a car in the event of fire---what a load of bu****it, so you crash, wreck your suspension or whatever and your battery decides to start burning, now where's that pond, oh s*it it's way down there 200metres away -hmm. I'd been thinking what'll happen when the F1 cars come with their ever increasing powered batteries but realised the circuit in Melbourne is built around a lake, but somehow I can't see that many will get thrown in the lake if they crash out.
William Revit

Yeah that's interesting about all the ev cars catching fire lately. Chinese cars in the news today.

"More than 30 Chinese-made Jaecoo EVs that went up in flames 'had ALL been bought by customers' "

https://www.dailymail.com/news/article-15871903/More-30-Chinese-Jaecoo-hybrids-went-flames-bought-customers.html

As you say, not very practical fire safety if a racer catches fire and has to be immersed to extinguish it.

And apparently there are now concerns about solar panels too, setting fire to roofs here in the uk.

I suppose the move to everything electric is becoming unstoppable. But just maybe that headlong rush should be put on hold for a good while, until common sense kicks back in.




anamnesis

According to some quick research, most causes of solar panel fires are a result of poor installation I.e. cabling and connection faults. We both know the risks with poor electrical installation so a competent installer must be used. When I had the smart meter installed I watched the guy, who seemed to know what he was doing. But, when asked some basic technical questions, it was clear he hadn't much electrical knowledge but had just been trained to put this wire there and that wire somewhere else etc.

It's a problem when a new technology is rolled out quickly with insufficiently trained people or insufficiently tested components. At least my meter is in a metal cabinet outside so should be a lower risk. My PV was connected up by a "proper" electrician - ok so far after 15 years.

Sorry about the thread drift - back to exhausts....
Bill B

You'd think installing a meter and solar would be done by people who are more than just 'fitters' and so called 'competent persons'. But increasingly it's thought, anyone with a single brain cell can be 'trained'. But personally like you I want someone who understands more than just how to bolt or clamp, and follow instructions.

Some new tech' though is so complex, that only designers understand it, and pethaps all you need is a fitter because it's just a black box to the fitter. Plug and play.

The drifters.

Ground level, and a bit higher up.

https://youtu.be/a7enhnt9ZVA?si=43bJpCxNURChR_lU


https://youtu.be/LOmZMycN0Gg?si=NU8D33K3OsHwprOX






anamnesis

Got a mate here who's a firefighter, or was,very recently retired. His worry with solar panels is/was who wired them in and how did it get connected to the mains--- So there's a house on fire, first thing the firies do is screw the fuse out, out at the power pole, all good but you've still got charging panels and that dreaded battery feeding the house---he reckoned the unknown of it was a real worry for most firefighters.-
And electric cars--he reckons just walk away you can't stop them burning.
As far as plug and play fitters go---The federal govt. here a few years ago did a heavily sponsored rollout of retrofit insulation for older houses (read-ceiling insulation) It went ok for a while with insulation batts going in houses left right and centre. THEN some wise 'fitters' decided they could do it quicker and make more money out of it if they used the foil backed sheets(rolls) that usually go under roofing--All good probably would have been semi ok but the low paid fitters found out the hard way not to stretch it out and nail it in place. yep ,nice big sheet of aluminium foil with a nail through into a live wire, instant big barbecue plate. Then, although there had been a few deaths from it the govt. kept letting it happen blaming untrained fitters---poor buggers were probably working flatout for some shadey operator making a quick buck out of the system. They ended up pulling stumps on sponsoring the rollout instead of banning the foil for ceilings--didn't bring any of the dead fellas back, too late for them.
William Revit

This thread was discussed between 29/05/2026 and 05/06/2026

MG Midget and Sprite Technical index

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