What are the principal causes of back-firing on overrun? I notice that there has been several articles in the Octagon Club Bulletin in the past, so is this a common problem?
What are the principal causes of back-firing on overrun? I notice that there has been several articles in the Octagon Club Bulletin in the past, so is this a common problem? Peter
Not common, but by no means unknown. (You haven't said what model/spec spec it is?)
Possible cause(s): Weak mixture. Exhaust manifold/system leak. Ignition retarded. Valve timing incorrect. Ignition fault. In that order I should think.
The car is a PA. The exhaust manifold is cracked and I am acquiring another one. I am hoping that this will solve the back fire as well as the leak. However having noticed that there are several articles in the Octagon bulletin I was just wondering whether it was a common problem. I do not have acess to the articles as I am not an Octagon member.
Back firing is not that common on over run(*) and is usually confused with cold air expansion. back firing is caused by unburnt fuel entering the exhaust system and igniting this causes an explosion in the exhaust system causing a very sharp loud bang, the causes as Kimber says. More common is cold air being drawn in to the exhaust on over run, as you lift off the throttle, the supply of very fast hot exhaust is reduced causing a partial vacuum within the exhaust which sucks cold air in through any leaks or cracks in the front end of the exhaust system, this cold air then is heated rapidly and expands with a loud popping/cracking sound that burbles on 'till the pressures equalize.
* on modern race engines however, backfire on overrun is normal.
I have taken on board the replies on this thread plus information from other sources and conclude there are several contributory factors to 'backfiring'. Air getting into the exhaust system on a trailing throttle is one of them. Excess fuel getting into the system is another. I expect to have a replacement exhaust manifold ready for installation next week an this week-end I have found that the fuel level in the rear float chamber was too high. Identified by blackened No 3 & 4 spark plugs and this no doubt lead to excess fuel getting inot the exhaust system. So finger crossed that these two corrrections will solve the anoying problem.
Having ran Norton motorcycles for many years it was common for the exhaust downpipe to vibrate loose at the cylinder head. You always knew when it had come loose because it started backfiring on the overrun. Once it was tightened up all was well again. Hope this confirms your suspicions.
The repaired exhaust manifold is now installed and the backfire is cured. No more hiding my face as I slow down for roundabouts. Thanks for all your input. The old manifold was very badly cracked and someone had tried to braze it. The new manifold was gas fusion welded by Technilock & Welding Services Ltd. Not a cheap process but is the right way to repair a casting and they give a 12 month guarantee.
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Posted - 07/04/2005 : 16:55:48
