Lean running and backfires on deceleration

OK, so I know that a lean-running engine tends to backfire on a
trailing throttle (under engine braking, when you close the throttle at
speed). But I've been wondering - how does it happen? What exactly
causes the backfire, and why does it only happen at closed throttle?
Why on a lean-running engine? What exactly happens in the engine?
Anyone know?

-Gniewko

 

There are always unburned hydrocarbons in your tailpipe. When you chop
the throttle and running lean this means an excess of oxygen available
for combustion in the exhaust stream. That + the localised hot spots
inside the system ignite whatever fuel remains in the exhaust thereby
making a popping noise on deccel. In and of itself this does not harm
your engine, rather because it is an indication of lean and running so
can burn your exhaust valves everyone thinks it to be a bad thing.

Tuning your bike richer means an excess of fuel and very little to no
oxygen. Thus the unburned fuel passes through the pipe without ever
igniting. So you can think of the popping as a sort of environmental
control on your bike, combusting excess fuel that would have otherwise
not gotten burned.

I have noticed on the T5 gen motors there is very poor efficiency.
Tune them "right" for max power and they have a lot of unburned HCs in
the exhaust stream. Japanese performance motors do not stink as badly
as a properly tuned triple.

 

Most riders probably wouldn't know much about the breakdown and
disassociation of hydrocarbon fuel molecules during combustion, but
almost everybody knows that all the fuel doesn't burn, some of it gets
kicked out the exhaust pipe unburned and the EPA is against that ever
happening, so manufacturers are going to fuel injection to stop the
problem that carburetors have.

Carburetors always suck some fuel through the idle circuits or around
the tapered jet needle even if the throttle is closed, and that's a bad
thing for air pollution and backfiring.

But even fuel injected motorcycles could backfire on trailing throttle,
if enough unburned mixture manages to recombine into burnable
molecules. Honda investigated
hydrocarbon recombination for a fuel-efficient 2-stroke they ran in the
Dakar Ralley several years ago. It was a 400cc 2-stroke that got an
amazing 40 miles per gallon when conventional 2-strokes only get 20
mpg.

So, whatever the fuel supply may be, carburetors or fuel injection, an
engine has ways
of getting a burnable mixture into the wrong part of the engine, i.e.,
the exhaust pipe.

Besides the unburned hydrocarbons in the exhaust pipe (it may take
several revolutions of the engine for enough unburned mixture to build
up), the backfire also requires an ignition source. This may or may not
come from the spark plug, which may fire a waste spark during the valve
overlap period on carbureted engines.

This is less likely to be the case on fuel injected engines which would
usually fire only one spark, and that spark would be on the compression
stroke only.

However, a source of ignition for backfiring could also come from a
hot, glowing piece of carbon in the combustion chamber, or the ground
electrode of the spark plug can glow hot enough to light off any
mixture near it.

 

Like any good sportbiker, the first thing I do whenever I buy a new
motorcycle is to store away its portly, restrictive stock exhaust and
install an aftermarket pipe. In the case of my ZX7R, the bike was
already lean from the factory, and freeing-up the back pressure made it
go even leaner, causing backfire on roll-offs. So I installed a BMC
filter and a big, fat, Factory Pro jet kit.

Doing so improved the bike's mid and high range performance
dramatically, and it's actually slightly rich all the way through the
rev range now. But you know what? It backfires just a bad as it ever
did. Go figure.

Cheers,

Tim Kreitz
2003 ZX7R
2000 ZX6R
DoD #2184
http://www.timkreitz.com

 

According to these folks, it's not excess oxygen that
causes it, but unburned fuel. Also, the unburned fuel
isn't ignited by hot spots in the in the exhaust system.
Other than that, great reply!

http://www.mikuni.com/tg_backfires_in_exhaust.html

Backfires in Exhaust

Note: It is normal for many high performance exhaust systems to
moderately backfire or pop when the throttle is closed from
mid-to-high rpm. In fact, one should expect a well-tuned high
performance engine to "pop" and "crackle" when the throttle is
closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean
when the throttle is closed and the engine is rotating well above idle
speed. It is also necessary that the exhaust system have rather open
mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not
flow out of the main system (needle, needle jet, main jet). Fuel is
only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm
is to create a fairly strong vacuum in the intake manifold. This
vacuum, in turn, causes a high air flow rate through the small gap
formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver
enough fuel to create a normal, combustible air/fuel ratio. The
mixture becomes too lean to burn reliably in the combustion chamber.
It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent,
still burning, into the exhaust system where it sometimes ignites the
raw mixture that has collected ---- the exhaust then pops or backfires.


-


http://www.physics.byu.edu/research/energy/htm7.html

 

One thing that should be pointed out is that while the throttle may be
closed, leaning up the mixture, and it may take several revolutions of
the engine to build up a burnable mixture in the pipe, the engine will
still be running at a high enough RPM for the ignition advance curve to
be advancing the ignition. So, if you have an ignition system that
fires a waste spark, it's going to fire that spark during the valve
overlap period on one of your four cylinders, and that's enough to make
the exhaust backfire.

 

Angels and trumpets, Hankerstain is talking motorcycles!

snip

I used to subscribe to that theory. But it doesn't explain how EFI
which can turn off the fuel completely on closed throttle still pops
on deccel and yet the pops go away when you enrichen it. If anything
making it richer should make it pop even more according to their
theory.

Ergo the only reasonable explanation is the free oxygen. Which there
is lots of in a lean burn. I mean hello they use downstream oxygen
sensors to determine stochiometric.

Richer = too much fuel present to ignite and no popping even though
you're dumping even more of it unburned into the exhaust. But the free
oxygen is consumed. And shit don't burn in the absence of an oxidizer.

 

Conversely, oxygen doesn't oxidize anything without some kind of shit
to oxidize.

I know about what it takes to burn shit. I have actual experience
burning shit at various rates, including sub-stoichmetric, stoichmetric
and with excess oxygen. It's certainly a lot easier to do if you have
the latter two conditions.

Back to "backfires on deceleration":

It's true that there are upper and lower limits of explosibility or
burnability for hydrocarbon molecules. You can look up the LEL and UEL
for various hydrocarbon gasses and see which ones have the widest
range, but those tables are always for
*gasses*, not for liquid hydrocarbon molecules which will disassociate
under high temperature and then *recombine* as they cool, allowing the
molecule that was once torn apart by heat to form again and be an
acceptable fuel for the spark it finds if there is enough air to
oxidize it.

 

I talk bikes plenty of times, Demie! But they're
not all I talk about. Life has so much to offer,
so why limit our areas of interest?

I suppose EFI can turn off fuel completely on closed
throttle, but it doesn't. If it did, the engine wouldn't
idle.
Also, according to the explanation in the link,
enriching the mixture makes the popping stop because
the mixture is now rich enough to burn more completely
during high rpm closed throttle operation. They say
that an already lean mixure gets too lean to burn in
the cylinders during high rpm closed throttle operation,
so the unbrunt mixture doens't ignite until a bunch of
it accumulates in the exhaust.

I've gotta go with Mikuni's explanation over yours.
Nice try, tho....


--

Here's what happens to steel framed buildings exposed
to raging infernos for hours on end.

http://davesweb.cnchost.com/nwsltr69c.html

On 9-11-01, WTC7, a 47 story steel framed building, which
had only small, random fires, dropped in perfect symmetry
at near free fall speed as in a perfectly executed controlled
demolition.

http://911research.wtc7.net/talks/wtc/videos.html
http://www.physics.byu.edu/research/energy/htm7.html
http://wtc7.net/articles/FEMA/WTC_ch5.htm


"You're doin' a heckuva job, Brownie!" - bu$h, a few days
before his FEMA chief, Micheal Brown was forced to resign
because of his gross incompetence.

"The tools that enable Cuba to save lives and preserve
human dignity during hurricanes are socialist values
and organization." - Dr. W.T. Whitney Jr

Ever wonder who benefits from the 150 MILLION
U.S. taxpayer dollars spent each DAY in Iraq?
http://www.commondreams.org/views04/0223-08.htm
http://www.corpwatch.org/article.php?list=type&type=21

"They are waging a campaign of murder and destruction. And
there is no limit to the innocent lives they are willing to
take... men with blind hatred and armed with lethal weapons
who are capable of any atrocity... they respect no laws of
warfare or morality."
-bu$h describing his own illegal invasion of Iraq.
http://www.robert-fisk.com/iraqwarvictims_mar2003.htm

 

I hope this adds to the thread:

My KAW Vulcan 1500-88 has twin Keihin CV carbs.

Each carb has a pilot screw that meters fuel to the carb's slow or idle
circuit.

When I chop the throttle, the intake butterfly valve just behind the
air filter closes and the main jet needle drops down to close off fuel
flow from the main jet. The slow-idle circuit that has been flowing
air-fuel to the engine since start-up is then the only circuit feeding
the cylinder.

All the carb'd Vulcan 1500's can have popping and backfires when
chopping the throttle at high rpm's. The solution has been to turn OUT
the pilot screw on each carb to INCREASE fuel flow to the slow/idle
circuit. This works for all these bikes, with very few exceptions.
There's also a coasting enricher on each carb that cuts air to the
idle circuit on deceleration to decrease airflow to the circuit for a
richer air-fuel idle mix.

My theory (read:guess is that a too lean idle air-fuel mix has an
uneven/partial burn in the combustion chamber. Unburned fuel makes its
way into the exhaust pipes and is ignited by: a hot pipe; a hot carbon
spot in the pipe; air leak into the exhaust pipe; and (my favorite
all the above.

Manjo

 

Because not everything under the sun is interesting.

I meant as you're coasting on closed throttle in gear. The fuel map
obviously squirts gas in when the rpms are at idle but not necessarily
if it's at 4,000rpm.

Yeah but you can have an overly rich mixture which also puts loads of
unburned HCs in the exhaust stream. And it doesn't pop. Why? The only
explanation is a lack of available oxygen to complete the combustion.

Mikuni is still talking carbs. Half of what they said doesn't even
apply to EFI (like fuel metering on closed throttle).

But I'm not surprised you seemingly latched onto one source of
information and are clining to it as though your life depended on it!