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Daniel

LOTS of popping on the over run

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Does anyone else experience this on carb'd none turbo cars?

 

I'm hoping it's nothing more than poorly tuned carb's; which I'll sort, but it could also point to valve damage.

 

Factors to consider:

 

  1. There is a ticking 'blow' from the exhaust. I think it's a cracked manifold.
  2. I do get some smoke on cold start up (valve guides?).

Planning a full head rebuild in the winter, but wondered if anyone else experienced this?

 

Cheers, you lovely set of lovely people.

 

Lovely........

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If the fuels old and stinky - get some nice new fresh stuff and try again

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I've owned both NA & turbo carb'd Esprits & never experienced popping of the exhaust, it's probably due to the cracked exhaust manifold.

 

The symptoms of worn valve guides is smoky exhaust on the over run because the vacuum in the inlet ports sucks oil down between the guides & valve stems & also smoke when she's been left overnight allowing oil to run down.  :thumbup:

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Mine pops on the over-run but I have a larger stainless exhaust, so its not unexpected...

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The Lotus Sports exhausts do actually do that. My Exige sounded like a race car on overrun....but that was how the exhaust was meant to sound.

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Mine pops and bangs on over-run and has the SJs stainless exhaust. I believe its a characteristic so enjoy!!!! 

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Mine also pops on the overrun and always has done, with both an original exhaust and the SS one I have now. As Mike says, enjoy it.

 

Chris

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I have an SJ aftermarket stainless system on my S1 on the original cast iron manifold - it pops occasionally on overrun. I put it down to a weak mixture, maybe its just characteristic ?

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Hmm, I too have the SJ exhaust and was wondering about the added sounds I now have.

 

However, can someone please explain exactly what you mean by 'overrun' - I get the sound when I lift off the gas pedal  - is that it?

 

Si

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Yup - thats what I get, on low speed deceleration or light throttle/low revs.  I have only the rear silencer section.

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That's always been my understanding Si,

Did you go for the straight thro mid section in the end, or stick with both silencers?

Dave

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The overrun is when you take your foot of the accelerator while leaving it in gear :thumbup:

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Bib's old Esprit would "pop" out big fat flames on overrun. Twas awesome.

 

not sure of the reason why but it's just excess fuel igniting (apparently). Many of the standard Lotus exhausts don't do this because of the tight restrictions on noise/Co2 etc. However, most aftermarket and Factory build sports exhausts do it.

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That's always been my understanding Si,

Did you go for the straight thro mid section in the end, or stick with both silencers?

Dave

Hi Dave

Both silencers at the mo. 

 

OK, so now I know its OK and meant, I'll go and have a listen!!

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My 912  2.2 engine has currently a K&N better flowing filter,portmatched head, SJ stainless steel exhaust manifold with in betweentube without resonator and with a free flowing silencer.

The carburators had idle jets 55.

ignition advance is 12° at idle , 32° above 3000rpm

head is recently been overhauld and all valves are freshly grinded in their seats.

In this configuration, I have quite some loud popping on the overrun,especially if the engine and exhaustsystem are hot after a firm trip.

 

Recently I have been playing and testing with carburator jetting.

I changed the idle jets from 55 to 58, synchronised the carbs again and one of the things I could notice is that the loud popping sound on the overrun has changed in an obviously more muted burbling sound.

 

I thought I'd share this.

 

But I have to be clear, first make sure the exhaustsystem is tight , without cracks or holes. they are a main cause for popping and banging.

Free flowing systems with less backpressure are also more prone to popping..

 

Now on a bike (HD) forum , I did find someone who explained very clear how exhaust decel popping is created and amplified.

I copy it here:

 

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.

 

 

Geert

Edited by gvy

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