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MG Midget and Sprite Technical - Popping On Over run

Hi all
as the title suggests I get a lot of popping from the exhaust on over run, not a backfire just lots of popping . Is this normal or should I look at a mixture or timing adjustment. Car seems to run fine.

Thanks Ian
I Pickering

Hi Ian, is your engine standard? I get the same but I'm running a sportier cam/head/exhaust etc.

I believe its a normal thing to some extent/

Thanks
Mark
Mark Whitmore

Mine does and I love it (with a fairly worked on engine, carbs and exhaust).
G Lazarus

Hi Ian, I agree with Mark. Assuming you have an A-series engine, there's nothing to worry about. In Minors, A-30, A40 etc you get the same thing - a cross between a pop-pop and a raspberry - while in Minis and 1100s the gearbox whine tends to drown it out. It's loudest when the valves are worn or valve clearances inadequate or a sporty cam as Mark says (my Minor was louder on overun with a mild sporty camshaft) but they all do it to an extent.
Nick Nakorn

Mine does it and it's a Datsun engine ....wish I had some switch that could make it do it more...
Andy Phillips (frankenfrog)

Thanks for the replies

The engine is a marina 1275 I think it is standard but it has been rebuilt at some point. I have the early exhaust fitted with only 1 silencer, it does sound very raspy. I just wanted some reassurance that all was good so I am not going to start fiddling, If It Aint Broke Don't Fix IT......

Thanks Ian


I Pickering

Completely normal and one of the charms of the A-Series.
On the current Mrs Clark's Cooper S (BMW), the engine management is deliberately mapped to make the same noise on over run - apparently.
Steve Clark

No not normal...

Esp if you have over run, a good chance your running to lean, or your timing is set to advanced, or you have a vacume leak somewhere in the induction, yes worn valves can cause this symptom also, it can also be a case of the wrong temp or worn spark plugs, even gapped incorrectly, even poor quality fuel can cause this



The big draw back is this condition can increase the damage to the valves as well as increase running temp. And should be considard a form of detentation

Prop
Prop and the Blackhole Midget

As Prop says I always knew it as a sign of a lean mixture on overrun and have had it on a few occasions and always cured by correcting the mixture.
David Billington

Prop... really... remember the throttle is near-as-damn-it closed on over-run... :-) ..all you have is tickover fuel; it's normal, honest.
Nick Nakorn

Popping is normal in most cases however check there are no exhaust leaks.
Bob Beaumont

Agree with Bob. Exhaust leaks are likely.

Nice though, isn't it?
Nick and Cherry Scoop

I love the sound it makes, gets plenty of looks.
I Pickering

Ive had popping in the past, but I found the souce and fixed it

Yes leaky exhaust but not as likely on an a-series,the back flow suction is not that common on an a series...of course it would also depend where the exhause hole is leaking from to draw in fresh air on the back suction of the exhaust...

Agian... peter burguss would have the inside track on this one... im just aware of the concept

Prop
Prop and the Blackhole Midget

Ex leaks can let em pop Prop. So does weak idle mix, as long as it doesn't annoy not really a problem. We had one customer wanting us to drill a B series ex system to pop on over run, told him he would have to do that himself as it wasn't really the thing to do.

Peter
Peter Burgess Tuning

Read this for the low down on popping.

http://www.mikuni.com/tg_backfires_in_exhaust.html

4: Backfires in Exhaust

Note:
It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.


Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.
Lawrence Slater

lawerance...thats everything i said... the 3 key words everyone is jumping over are ((high performance engine))

If we a re talking about hopped up chevy short 350...then yes there will be some popping, but for ours to do this as a normal operation they would have to be radical engines

Of all the A and A+ engines ive been around, none of them pop....if you got popping, then something is out of adjustment

But I digress to peter, if he says popping is fine with no damage, then im not going to question him

But to say they were popping as they rolled off the assembly line and into BMC show rooms around the planet.... no im not buying that

They only way popping can be discribed as normal is because no one has traced down and fixed the problem

Pinking is normal also if the timing is adjusted incorrectly and no one corrected it

Prop
Prop and the Blackhole Midget

Prop, my family's first Mini in 1962 - it popped; our Minor Van, my Austin A30, my Minor and every A-series car I've driven over many decades - they all popped.
Nick Nakorn

I dont know what to tell you nick....

considering your just 1 person X several million people that purchased so many new bmc cars and if they all did that may have been the start of the end to bmc

add some butter to it maybe, and call it a jiffy pop corn engine

Maybe its just a UK Flaw. Or the usa cars are just better setup after shipping

Prop
Prop and the Blackhole Midget

Mine used to pop. It was lovely and I miss it but I was suffering a bit from running on so I either advanced or retarded the timing and enriched or leaned off the carbs. Can't remember which! Stopped the running on but lost the pops. I think I might try and get them back!
Matt1275

There's a bit of road dropping down into Ashbourne on the A515 where you can drop a gear and lift off. The road runs between high walls and my custom straight through single box system sounds fantastic down there, crackling and popping for about 400 metres.

Bernie.
b higginson

This thread was discussed between 22/06/2014 and 24/06/2014

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