Can someone explain exactly what causes backfiring?

[Mandelon]

We were talking about this today and while we all had ideas what happens, none of us could actually explain why and how an engine backfires.....
Please help me look smart tomorrow when I can explain it. :boxed: :boxed: :boxed:

[Squirtin Thunder]

Well is it back fireing through the carb or exhaust ????
Jim :cool:

[Fiat48]

Simple. Exhaust backfire is firing the cylinder with the exhaust valve open. Intake backfire is firing the cylinder with the intake valve open.
Trick is...you gotta find out why it's doing that 'cause it ain't supposed to.

[78Eliminator]

Simple. Exhaust backfire is firing the cylinder with the exhaust valve open. Intake backfire is firing the cylinder with the intake valve open.
Trick is...you gotta find out why it's doing that 'cause it ain't supposed to.
For fun times, go ahead and switch just two cylinders on your buddies BBC. Make sure they are not companion cylinders. See how long it takes him to figure out what's wrong.
*If you do this at night, it will shoot fire out the exhaust. Bring marshmallows.

[Infomaniac]

Yep the combustion pushes out the exhaust instead on the piston.
I hate it when that happens.
In aviation terms, backfire is out the carb and after fire is out the exhaust.

[Kachina26]

Ok, those are what allows it, however what causes it can vary. Some times it can be caused by carbon tracking in the distributor cap which send spark to a cylinder which is on a stroke other than the power stroke . Also, carbon in the cylinder that remains hot enough to light the incoming air fuel mixture can cause a backfire. Afterfire or backfire through the exhaust can be caused , or rather is usually caused by an exhaust leak allowing oxygen to enter the exhaust stream which re ignites unburned fuel in the exhaust which is hot enough to fire but lacks the oxygen to do so. This usually occurs on older carburated vehicles. In short, many different things can cause a "backfire" condition, these are only a few.

[Sleek-Jet]

... backfire is out the carb and after fire is out the exhaust.
Ahhhh... smoldering Cessna's. :D
Anyone know the proper procedure for a carburator fire??

[78Eliminator]

Ahhhh... smoldering Cessna's. :D
Anyone know the proper procedure for a carburator fire??
Yes sir. Shut fuel selector off, and continue to turn over motor....

[Sleek-Jet]

Yes sir. Shut fuel selector off, and continue to turn over motor....
Well, almost. Mixture to idle cut off and continue cranking with throttle open.
If it backfired and then started, run the engine up to 2000 rpm for a minute before shutting down to inspect the carnage.

[Fiat48]

Halon :p
When the fires out..put on injection. :D

[Infomaniac]

Ah yes, carb fire. Over priming. Depending on the degree of overpriming, carb, carb box and maybe nose gear fire. Even more than that grass fire. :wink:

[78Eliminator]

Ah yes, carb fire. Over priming. Depending on the degree of overpriming, carb, carb box and maybe nose gear fire. Even more than that grass fire. :wink:
Info, what about as bird's nest fire? :jawdrop:

[Infomaniac]

Bird's nest fire would be on the cylinders cooling fins around the baffles. That is what cowl plugs are for.
A poor pre flight inspection would be the cause of this. :cry:

[Some Kind Of Monster]

A screwed up timing will cause backfire as well.

[Sleek-Jet]

Info, what about as bird's nest fire? :jawdrop:
Fly the airplane more often. :D

[Fiat48]

That does it for me. Only thing I am gonna fly is my flat! Fires...sheesh...and you got different kinds and descrptions. :jawdrop:

[Sleek-Jet]

That does it for me. Only thing I am gonna fly is my flat! Fires...sheesh...and you got different kinds and descrptions. :jawdrop:
Those are the easy ones.
The exhaust tube failures (especially on turbo charged engines) that blowtorch the firewall or engine beam.... those get interesting.

[Infomaniac]

Fiat would like the mechanical fuel injection systems on those engines.
Very interesting fuel metering for a mechanical injection.
Do a search for Teledyne Continental or Bendix fuel injection systems. I can send some text books with theory of operation if an internet search does not come up with anything.
I taught that stuff at A&P school and worked on that stuff for a number of years.

[Sleek-Jet]

Fiat would like the mechanical fuel injection systems on those engines.
Very interesting fuel metering for a mechanical injection.
Do a search for Teledyne Continental or Bendix fuel injection systems. I can send some text books with theory of operation if an internet search does not come up with anything.
I taught that stuff at A&P school and worked on that stuff for a number of years.
Yes he would. Hot starts anyone?? :D

[Fiat48]

Yeah and you put out fires too!
LOL..Something tells me this ain't quite Stu Hilborns constant flow mechanical fuel injection. Never heard a Co pilot say "lean her 10 ".

[Infomaniac]

Yea it is continuous flow. Just a finer method of metering rather than varying pressure to an orifice. You really should read up on it.

[78Eliminator]

Short answer to Mandelon's original question:
Too much beans for lunch.

[Jordy]

Anyone know the proper procedure for a carburator fire??
To quote from Monty Python and the Holy Grail:
Run away. Run away. :D :D

[Sleek-Jet]

To quote from Monty Python and the Holy Grail:
Run away. Run away. :D :D
LOL, that works too. :D

[Hotcrusader76]

Ahhhh... smoldering Cessna's. :D
Anyone know the proper procedure for a carburator fire??
In an airplane? ........jump?:D:D:D....

[Danhercules]

In an airplane? ........jump?:D:D:D....
Man, I thought you had somthin to add.

[Mandelon]

Thanks guys. Now I can sound smarter than my friends... :hammerhea Not that hard to do....... :cool:

[Fiat48]

We covered everything from farts to fuel injection and even managed a little bit to stay on topic. Hope you do well. Education like this is priceless. :D