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EFV-8 Club Forum / General Ford Discussion / Backfire in exhaust

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Posted By Discussion Topic: Backfire in exhaust -- page: 1 2     Dustbowl 06-29-2013 @ 12:51 PM reply profile send p.m. Member Posts: 74 Joined: Feb 2011 What causes our cars to backfire in the exhaust when de-accelerating the engine? I have posted this question several times over the years and have never been satisfied with the answers. For example; air leaking into the intake manifold. If this were the case, why would the fuel air mixture not ignite in the combustion chamber if the same mix will ignite in the hot exhaust pipe? Instead, is it possible that the spark is retarded so much (by high manifold vacuum) that the spark fires too late to ignite the mix in the combustion chamber, so it passes on till it hits the hot exhaust pipe? Any theory will be carefully considered. supereal 06-29-2013 @ 7:17 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 An air leak into the intake manifold is a bypass around the throttle plate in the carb. This creates a lean fuel mixture that ignites in the exhaust system. Manifold vacuum is at its peak on deceleration. That is why backfire happens then. The advance control, a snubber, is activated by low vacuum, and retards the ignition. If backfire is frequent, it is likely that the throttle shaft bushings are worn, or the butterfly plate doesn't fit the carb throat. Occasionally we find those have been filed down, or otherwise modified for some reason, allowing air to enter when the throttle is released. This message was edited by supereal on 6-30-13 @ 11:34 AM Dustbowl 07-05-2013 @ 4:49 PM reply profile send p.m. Member Posts: 74 Joined: Feb 2011 Your comments are very thoughtful, Supereal, but I still don't understand why the lean mixture you mention does not ignite in the combustion chamber before igniting in the exhaust. buford8 07-05-2013 @ 6:16 PM reply profile send p.m. Member Posts: 50 Joined: Oct 2009 An explosion in the exhaust system can be caused by a leak anywhere in the system... as there is always some unburned fuel present in the exhaust the introduction of air (oxygen) into the pipes causes the unburned fuel to ignite with a bang...hench the backfires you hear...good luck! If you make sure there are no leaks in the system I think your backfire problem will go away... You mess with me, you mess with the whole trailer park! Stroker 07-05-2013 @ 6:24 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 Dustbowl: The ideal air-fuel mixture of air and gasoline that is designed to burn properly in an internal combustion engine is called "stoichiometric" or a 14.7:1 ratio by mass. A lean mixture burns much slower than a stoichiometrically "correct" mixture, so it is still completing its' burn late in the exhaust cycle, often continuing after the exhaust valve closes, and the combustion byproducts have entered the exhaust plumbing. These byproducts obviously contain yet un-burned fuel, which upon contact with the hot exhaust plumbing, ignite. So...it's simply a matter of timing; eg how long does it take for the mixture to completely burn. TomO 07-06-2013 @ 8:26 AM reply profile send p.m. Senior Posts: 7264 Joined: Oct 2009 A rich mixture will also cause backfires in the exhaust. The power valve should be closed during deceleration and if the gasket or the valve is leaking, there will be excess gas in the exhaust system that will explode when it hits the hot muffler parts. Tom Stroker 07-06-2013 @ 4:51 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 TomO/Dustbowl: I agree that a rich mixture will also retard the burning rate. For our old Fords, the "Goldilocks" mixture is probably closer to 16:1 rather than the more environmentally acceptable 14+, wherein the mixtures burns where it is supposed to and finishes burning before the exhaust valve opens. Ignition timing is also a factor, as a late spark obviously delays the onset of combustion. Modern fuels, particularly those which contain ethanol alter this ratio as well, requiring more fuel to air to achieve a burn rate that will complete on time. Contrary to popular notion, the higher the octane the fuel, the slower the burn rate. Back in the 60's, Honda had some 250cc 4 cylinder motorcycle racing engines that basically would outrun the burn rate of contemporary racing fuels, and had to run quicker-burning fuels that were actually very low octane. I've used acetone mixtures in the past to accelerate the burn rate. shogun1940 07-09-2013 @ 6:53 PM reply profile send p.m. Member Posts: 464 Joined: Feb 2010 my 48 backfires because it has leak in the exhaust header,, hot exhaust plus unburned fuel plus plus new oxygen is an explosion Dustbowl 07-12-2013 @ 6:43 PM reply profile send p.m. Member Posts: 74 Joined: Feb 2011 OK, I will almost give up and accept above comments. My reservation is re the leak in exhaust system. The gas in the exhaust system is at above atmospheric pressure, or otherwise the gas will not flow out the tailpipe. If so, how does air lead in from lower to higher pressure? I am, incidentally, an old chemical engineer who worked in oil refineries so I think I understand combustion and fluid flow fairly well. supereal 07-12-2013 @ 6:53 PM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The answer is that exhaust flow thru the system creates suction at any breaches in the conduit between the combustion chamber and the taiipipe. This allows unburned fuel to enter and be ignited with the air and be ignited, producing the "back fire".

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