Over - run Backfiring ?
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Senior User
Joined: Mar 2018
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From: Lindsey Suffolk England
Over - run Backfiring ?
A question,.... When my truck is say descending a gradient and my foot is off the gas pedal I get backfiring from the exhausts. What would cause this problem, and how can I solve it please.
Suffolkman
Suffolkman
Logistics Pro
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From: Fallon,NV & Bainbridge OH
Backfiring on de-acceleration is primarily a result of a lean condition. One way to determine if that is the cause is to hold the throttle open slightly (push the pedal slightly), if it goes away you are experiencing a lean backfire condition and one way to fix it is to richen the carb.
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From: Bothell & Silverdale, WA
A couple of things, or a combination of both. Are you just coasting or using engine compression to slow down? Is it a rapid type of backfire like popitty-pop, or just a random BANG? Timing could be too retarded, check breaker plate. Dumping raw fuel into a hot exhaust collecting in the muffler will do it, check carb float, possibly sticking. Sticking exhaust valve(s).
Post Fiend
Joined: Feb 2008
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From: St Albert, Alberta
Deceleration back firing is from by a lean condition on deceleration. When you suddenly close the throttle, the incoming air is choked off and the closed throttle also causes the motor to return to idle on the very lean idle circuit. However, momentum of the truck is driving the engine, and the engine is still spinning at rpm that needs far more gas than the closed throttle position can provide. It’s a waste of time messing with the ignition timing, or plugs or other items, because it is still a lean fuel/air mix on deceleration that is the cause. Stock mufflers are fairly restrictive and tend to mitigate the issue. One thing you can try is richening up the idle air mix this will usually help mitigate it if not outright stop it. .
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Fleet Owner
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I would bet on the PV too.
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Thread Starter
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Senior User
Joined: Mar 2018
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From: Lindsey Suffolk England
Thanks to all for your theories. It is a sort of poppetypop. The truck runs twin Stromberg 97's which were fitted just before I purchased the truck. The exhaust systems are also newish and are pretty straight through. As soon as I put my foot back on the throttle the problem ceases. I wondered if maybe one plug was not sparking therefore putting fuel into the exhaust.
Suffolkman
Suffolkman
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Joined: Feb 2008
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From: St Albert, Alberta
Thanks to all for your theories. It is a sort of poppetypop. The truck runs twin Stromberg 97's which were fitted just before I purchased the truck. The exhaust systems are also newish and are pretty straight through. As soon as I put my foot back on the throttle the problem ceases. I wondered if maybe one plug was not sparking therefore putting fuel into the exhaust.
Suffolkman
Suffolkman
The 97 has no power valve so that is off the table,
And it's not a plug issue.
And to be honest this is quite common as soon as you uncork an exhaust.
As I stated before you can try backing out the idle mix screw to see if this will alleviate it or help or minimize it. I have good success doing this on bikes, be warned it will richen up the idle mix and may run rich at idle, potentially fouling plugs if you go to far and idle it for any length of time..
Post Fiend
Joined: Feb 2008
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From: St Albert, Alberta
I can certainly see that a wiper motor leaking vacuum would lean the fuel air mix and this would even more noticeable on deceleration when intake vacuum was high and the fuel mix was lean to start then add in a vacuum leak leaning out even further.
Logistics Pro
Joined: Nov 2011
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From: Northville, MI
The usual cause is timing but a lean mixture can contribute also. If you have an other than stock cam adjustment can be tricky. You will need to look at mechanical and vacuum advance along with fuel mixture.
Post Fiend
Joined: Feb 2008
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From: St Albert, Alberta
I have to disagree here the usual cause is not timing.
What happens exactly is that the mixture leans out enough when the engine is over run with the throttle closed to where the mixture it fails to ignite consistently. This, in turn allows some un-burnt fuel to get into the exhaust. Then when the engine does fire, these un-burnt gasses are ignited in the exhaust, causing the backfire/popping. Some carbs have an enricher circuit which cuts in on deceleration to help alleviate this problem
Enriching the idle air will alleviate it or help mitigate it as it will add more fuel to help ensure more consistent ignition of the fuel air mix when the engine is over run with the throttle is closed..
