What causes an engine to backfire?

[Tinster]

No, I have not opened the hood of the car.

Simply a curiosity question.

When I downshift I get a short, muffled, burst
of backfire that sounds like it is from the tailpipe.

Simply curious,

Thanks,

dale

 

[AweMan]

Any number of things Dale:
1. could be timing advanced a little too far
2. could be carbs running a little rich loading up the muffler with un-burnt fuel.
3. could be egr valve not operating properly.
BUT ..........
Most of that stuff will cause a slightly more pronounced backfire on both acceleration and deceleration.

<span style="font-weight: bold">If it is only slight, it could very well be normal. Many engines produce slight backfire {more of a poping sound than a backfire} on loaded de-celeration.</span>
Keep an eye on your spark plugs {not daily by any means hahahahahaha} if they are running on the brown side of tan ...... you are fine!
chalk white to light tan ... too lean .... real dark brown to black ... too rich ... but then .... you should already know that by now.

<span style="font-weight: bold">Dont go tearing into you perfectly fine runing engine now .... ya hear me?</span>

 

[M_Pied_Lourd]

Tinster

A common cause of back fire on deceleration is a faulty Throttle Bypass Valve, or more particularly, the diaphram surrounding the actual brass valve.

M. Pied Lourd

 

[swift6]

Your likely hearing more of what could be called a "burble" than a backfire. Slight popping from the exhaust while engine braking (loaded deceleration). Backfires sound more like a gun shot. If your hearing the burble, don't worry about it and drive. Think of it as a "feature".

 

[TR3driver]

For a bit more theory, what happens is that the fuel/air mixture does not burn in the cylinder due to extremely low pressure and/or poor distribution, lean mixture etc. Literally the engine stops running, but there is no noticeable loss of power since it's not pulling the car at the moment.

Then when the unburned fuel/air gets put into the exhaust system, the residual heat eventually ignites it and what you hear is the pop made when it burns. Under some circumstances it accumulates long enough to make a louder bang, which can cause problems, but generally it (the burble Shawn is talking about) does no harm.

But your engine is supposed to have functioning throttle bypass valves, which are supposed to prevent the problem by, in effect, opening the throttle slightly and raising cylinder pressure enough for the mixture to burn in the cylinder (thereby reducing exhaust emissions) during overrun. As M. Pied says, a problem with the bypass (or someone deliberately blocking them off, as many do) brings back the popping.

If memory serves, you think you have a TR250 engine, which I think would have the external vacuum lines to the bypass valves. If so, you might check that it's securely connected to both the valves and the manifold.

 

[Brosky]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Think of it as a "feature".[/QUOTE]

Think of it as "Exhaust Patina".

 

[Tinster]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Think of it as a "feature".

Think of it as "Exhaust Patina". [/QUOTE]


<span style="color: #990000">Yup, it's more like a dull popping sound.
All the pollution gear and various canisters and bypass
valves were long ago stripped from my engine by POs.

Thanks for the responses.

d</span>

 

[BobbyD]

My first thought was too many jalapenos on your tacos.....they always make me back fire........

And it could be more exciting.....just look at the NASCAR guys when they lift off of the throttle.....Flames & Fire out the exhaust!

Actually I love to back off the throttle going through a tunnel or under an overpass.....it gives off a great throaty rumble

 

[71MKIV]

If I may,

An air leak in the exhaust will cause this as well.

But if all is tight...

Knew a fella that would deliberatly turn off his ignition on the overrun, then turn it back on, and listen to the bang.

Human nature being what it is, each time got a little longer to produce a bigger noise,

until one day he split the muffler of his Toyota pickup wide open. Then it wouldn't do it anymore.

Steve Spangler

71 Spitfire MKIV "Jehu"

You want me to go where? with who? and do what?

 

[poolboy]

Dale, I don't doubt that the PO (singular or plural) may have removed some components that really weren't necessary, but the carbs would not work if he removed the throttle bypass valves. There would be (2) 1/4 inch holes doing nothing but sucking air so your engine would not idle.
It is possible to block off the internal passage way so that air can not bypass the throttle disc. BUT it is that little extra air/fuel mix that bypasses the throttle disc when you let off the gas pedal that inhibits the tendency for the fuel to explode in the exhaust.
Explosion though it is, it is not as severe or loud as you would normally think of as a backfire.
The Throttle Bypass Valve on the ZS carb, doesn't really open the throttle disc. It just provides a port or pathway to transfer an air/fuel mix from the mixing chamber to the intake manifold, bypassing the nearly closed throttle disc. That's the "bypass" part of the Throttle Bypass Valve.
The valve can be adjusted or replaced and adjusted with Moss part # 365-745 @ $10.95 each + $500.00 shipping to PR.

 

[DNK]

That's the standard Triumph "Burble".

 

[Tinster]

Poolboy!!

Maybe I don'y know where to look for the throttle
bypass valves. Here's a top view of my engine.

Do you see the throttle bypass valves? If so,
point them out to me.

PS; My intake manifold sucks at 17 on my gauge.
The light blue are my twin vacuum pipes.

Thanks

 

[Brosky]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]My first thought was too many jalapenos on your tacos.....they always make me back fire.......[/QUOTE]

Dale, don't pay any attention to Bob. That's just another case of bad gas..........

 

[poolboy]

They are hard to see from that angle, Dale. For the CDSE versions they are on the right side of the carb between the temperature compensator assembly and the throttle shaft. There are usually 3 Phillips head screws and 3 slot headed screws holding the thing together.
There are good pictures in the Buckeye Triumph article on carbs.

 

[Brosky]

Dale,

Jeff would have rebuilt them as part of the carb rebuilding. Very unlikely that they need any attention.

Timing and lean mixture can cause backfiring.

Idle hands can make a nice burble turn into a bad back fire condition or worse.........

 

[poolboy]

Dale,

Jeff would have rebuilt them as part of the carb rebuilding. Very unlikely that they need any attention.

Timing and lean mixture can cause backfiring.

Idle hands can make a nice burble turn into a bad back fire condition or worse.........

I agree. Once they are adjusted that's pretty much it. I reckon, I replaced mine about 5,000 miles back and they're holding up well.
The diaphram that surrounds the brass valve will get brittle and inflexible, but it seems like that takes a long time.