John Ryan Posted May 27, 2009 Share Posted May 27, 2009 Ever since I rebuilt the car, it has suffered from 'popping' via the carbs, especially on the over-run. I thought this was a carb related issue. However, I have just fitted some 7 Workshop copper exhaust gaskets, to cure a persistent leak from the centre exhaust manifold, and not only has that worked, but the popping has gone too. Any ideas why? Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted May 27, 2009 Share Posted May 27, 2009 Air being drawn in to the exhaust does just that ! Link to comment Share on other sites More sharing options...
John Ryan Posted May 27, 2009 Author Share Posted May 27, 2009 How? Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted May 27, 2009 Share Posted May 27, 2009 ........ ask your father !! (ok, so I don't know, it just does !) Link to comment Share on other sites More sharing options...
John Ryan Posted May 27, 2009 Author Share Posted May 27, 2009 I think I was posting on the Tech forum to find out the reason, but thanks for the reply..... Link to comment Share on other sites More sharing options...
DaveMorris Posted May 27, 2009 Share Posted May 27, 2009 John, I've been told that spitting from the carbs, especially when cold, could be as a result of a weak mixture (normally jetting related). If you had a leak on the outlet then POSSIBLY you were drawing some air in from both sides and resulting in a weak mixture in that way - hence your popping / spitting back. ?? Dave Link to comment Share on other sites More sharing options...
JamesZ Posted May 27, 2009 Share Posted May 27, 2009 My understanding is that you are chucking unburned fuel in the exhaust, but exhaust gases have used up all their oxygen in the combustion process. Let fresh air in though and suddenly there’s a source of oxygen allowing the unburned fuel to go pop. As to how fresh air can get in – I don’t know, I’d have thought that the exhaust was under higher pressure so the exhaust gases should be flowing out of your holes, rather than fresh air flowing in! Oops - just read Dave's reply after typing mine, re-read your original post and spotted that you're referring to popping on the carb side. Edited by - james xflow on 27 May 2009 16:28:55 Link to comment Share on other sites More sharing options...
John Ryan Posted May 27, 2009 Author Share Posted May 27, 2009 Hi James That makes sense. I was convinced it was popping from the carbs, but perhaps not.. The gaskets are really good, but scarily expensive! The car is now running really well, but still VERY noisy! Link to comment Share on other sites More sharing options...
JamesZ Posted May 27, 2009 Share Posted May 27, 2009 I'd say there's quite a difference between my carbs popping (they don't any longer fortunately) and my exhaust. The carbs are much higher sound and sort of coughy, whereas the exhaust is a definite pop pop pop BANG! Link to comment Share on other sites More sharing options...
Normans_Ghost Posted May 27, 2009 Share Posted May 27, 2009 Unburnt fuel in the exhaust will explode on the overrun as described above. However new manifold gaskets shouldn't cause or cure that. If it was spitting back through the carbs it sounds like an air leak on the induction side which has now been cured. Norman Verona, 1989 BDR 220bhp, Reg: B16BDR, Mem No 2166, the full story here You and your seven toThe French Blatting Company Limited Link to comment Share on other sites More sharing options...
Roger King Posted May 27, 2009 Share Posted May 27, 2009 Carb popping (more like sneezing really) normally occurs on light throttle at cruising speed and is usually cured by richening the mixture a tad. If it's on the overrun, that is usually unburnt fuel in the exhaust. Certainly an air leak could be pulling in extra oxgen and burning off the fuel. In a way this is doing the same job as a catalytic converter, so just claim you're being green! On the overrun, the gas pressure in the exhaust will often be lower than atmospheric due to the small amount of exhaust gas moving at speed; therefore air is sucked in. Under load, the exhaust gas pressure increases and the leak goes the other way. My assumption therefore is that the problem was actually with the exhaust but it could have been something weirder I suppose. Edited by - Roger King on 27 May 2009 19:56:44 Link to comment Share on other sites More sharing options...
Bricol Posted May 27, 2009 Share Posted May 27, 2009 My old, original, proper, shape Renault 5 used to do exactly the same thing - the silencer box was in the nearside front wing and the leaking joint to it used to cause the popping - tightening the joint sorted it. I assumed then that hot exhaust, unburnt fuel exciting the cylinders on the over-run (no fancy fuel cut-off solenoids on that beastie) and fresh air = pop pop pop pop POP pop . . Bri Link to comment Share on other sites More sharing options...
dalongbloke Posted May 27, 2009 Share Posted May 27, 2009 I LOVE my popping and banging!! Check out Hal... http://picasaweb.google.co.uk/dalongbloke Link to comment Share on other sites More sharing options...
John Ryan Posted May 27, 2009 Author Share Posted May 27, 2009 Thanks Roger That explains it perfectly. I liked the effect too, but it was getting a little bit much! Link to comment Share on other sites More sharing options...
myothercarsa2cv Posted May 28, 2009 Share Posted May 28, 2009 All about the popping on over-run... On the 2cv, with some suitably leaky joints, or just the manifold on and no pipework, the popping became a banging, and now that was fun in tunnels... WARRRRRRRRRP-P-P-POP-POP-PARP-BANG! Taking the silencer off the 7 has a similar exciting effect, accept it sounds like a spitfire - loverly 😬 😬 😬 John _________________________ Bugsy: '82 2cv6 (Back on the road!!! 😬) Talloulah: '08 1.6K Classic (Grubby ) Link to comment Share on other sites More sharing options...
Doug Posted May 28, 2009 Share Posted May 28, 2009 My old Mk2 Midget used to pop on the over run and I've heard lots of A-Series engined cars doing the same (Moggie Minors and Minis in particular) - it was always something of a signature sound with those cars! Once the Midget had been rebuilt and the engine, carbs and distributor properly rebuilt, it still did it. I assumed it was unburned fuel in the exhaust... 'Have you any idea what it's like to have the wind rushing through your hair!' (Quote:Sq Cdr the Lord Flashheart) Link to comment Share on other sites More sharing options...
eric Posted May 28, 2009 Share Posted May 28, 2009 very difficult to stop bang from the exaust as I have an A2 cam, under the 3000 rev the engine is not on cam and if it is weak the carbs spit and if it is rich the exaust bang and if the trottle is off, sometimes it is a spit in the carb followed by a bang in the exaust But most of times no spit and no bang So it is the charm of the engine modern engine don't do that, except the race engines .... Link to comment Share on other sites More sharing options...
Steve C Posted June 2, 2009 Share Posted June 2, 2009 Cure it? Film it 😬 'Wet the Webers, light them and blat!' Link to comment Share on other sites More sharing options...
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