Question re fuel/air lean mixture

[Ext User(Jason James)]

In the "old'" days, it was common practise to turn off the fuel feed to the
carby on 2-strokes. The reason given that otherwise the fuel will evaporate
leaving oil in the fuel bowl,..fair enough. What I dont understand, is why
just a few seconds before the fuel runs out, the engine speeds up? It's
running lean, but wouldnt that give less power?

TIA,..Jason

[Ext User(John Tserkezis)]

Jason James wrote:

> In the "old'" days, it was common practise to turn off the fuel feed to the
> carby on 2-strokes. The reason given that otherwise the fuel will evaporate
> leaving oil in the fuel bowl,..fair enough. What I dont understand, is why
> just a few seconds before the fuel runs out, the engine speeds up? It's
> running lean, but wouldnt that give less power?

No expert, but running leaner, means running hotter with more power.

That was the hardest thing I had to wrap my mind around, when it was
repeatedly beaten into me by various sources that more fuel = more power.

Then we got into gokarts, and I had to learn all over again. The carby was
as crude and manual as you could get. If you didn't understand why things did
what they did, the kart either didn't run as well as it could, or ran into the
ground very quickly.

So I stopped listening to everyone (either they were all miss-informed, they
miss-understood, they were intentionally misleading us, or they were idiots).
I sat down with a bunch of books, looked at the carby itself (it was
relatively simple) and worked things out myself.

It all worked for me after that.

Generally, running rich means the engine runs cooler with less power, and
running lean means the engine runs hot with more power.

If you overdo it on the rich side, you'll gum up the engine with unburnt
fuel (likely the plugs), and if you overdo it on the lean side, you'll burn
the motor out by overheating over too long a time.
--
Linux Registered User # 302622
<http://counter.li.org>

[Ext User(Noddy)]

"John Tserkezis" <jt@techniciansyndrome.org.invalid> wrote in message
news:47ae4535$0$5200$afc38c87@news.optusnet.com.au ...

> No expert, but running leaner, means running hotter with more power.

That's it exactly.

Back in my drag racing days the "trick" to getting a bit of a kick out of
the engine as you were going through the traps was to pull the fuel shut
off. With supercharged engines running on methanol and shitloads of boost,
they're throwing fuel into the engine at a massive rate and shutting the
engine down by flicking off the mags runs the risk of dieseling. Pulling the
fuel shut-off just as you were going through the first mph beam leaned the
engine out to the point where it would pick up a few hundred rpm and make a
few more ponies before it ran out of fuel and shut down.

Some blokes used specific high speed lean-out controls to extend that lean
"power boost" for a longer period, but the problem then becomes one of
balance. It's incredibly easy to melt holes in pistons when you run a super
lean mixture, and you can usually tell when someone has done that as the
resulting explosion usually blows the supercharger off the top of the
engine.

--
Regards,
Noddy.

[Ext User(John_H)]

Jason James wrote:

>In the "old'" days, it was common practise to turn off the fuel feed to the
>carby on 2-strokes. The reason given that otherwise the fuel will evaporate
>leaving oil in the fuel bowl,..fair enough. What I dont understand, is why
>just a few seconds before the fuel runs out, the engine speeds up? It's
>running lean, but wouldnt that give less power?

How to tune a 2-stroke for maximum reliable power (racer's trick from
the "old" days)....

1) Progressively lean mixture until a hole appears in the piston.
2) Replace piston.
3) Increase main jet size by one increment.

Should answer your question. :)

--
John H

[Ext User(hippo)]

John_H <john4721@inbox.com> Feb 10, 2008 at 02:53 PM wrote:


>Jason James wrote:

>>In the "old'" days, it was common practise to turn off the fuel feed to
the
>>carby on 2-strokes. The reason given that otherwise the fuel will
evaporate
>>leaving oil in the fuel bowl,..fair enough. What I dont understand, is
why
>>just a few seconds before the fuel runs out, the engine speeds up? It's
>>running lean, but wouldnt that give less power?

>How to tune a 2-stroke for maximum reliable power (racer's trick from
the "old" days)....

>1) Progressively lean mixture until a hole appears in the piston.
>2) Replace piston.
>3) Increase main jet size by one increment.

>Should answer your question. :)

>--
>John H

Sounds like the old Suzuki GT250... 158Km/h... Pop!... limp home at 35-40
on one cylinder. Change piston & repeat process/result. Up main jet and
hey presto, 160 with no pop. Cheers

--
Message posted using http://www.talkaboutautos.com/group/aus.cars/
More information at http://www.talkaboutautos.com/faq.html

[Ext User(Jason James)]

"John Tserkezis" <jt@techniciansyndrome.org.invalid> wrote in message
news:47ae4535$0$5200$afc38c87@news.optusnet.com.au ...
> Jason James wrote:
>
> > In the "old'" days, it was common practise to turn off the fuel feed to
the
> > carby on 2-strokes. The reason given that otherwise the fuel will
evaporate
> > leaving oil in the fuel bowl,..fair enough. What I dont understand, is
why
> > just a few seconds before the fuel runs out, the engine speeds up? It's
> > running lean, but wouldnt that give less power?
>
> No expert, but running leaner, means running hotter with more power.
>
> That was the hardest thing I had to wrap my mind around, when it was
> repeatedly beaten into me by various sources that more fuel = more power.
>
> Then we got into gokarts, and I had to learn all over again. The carby
was
> as crude and manual as you could get. If you didn't understand why things
did
> what they did, the kart either didn't run as well as it could, or ran into
the
> ground very quickly.
>
> So I stopped listening to everyone (either they were all miss-informed,
they
> miss-understood, they were intentionally misleading us, or they were
idiots).
> I sat down with a bunch of books, looked at the carby itself (it was
> relatively simple) and worked things out myself.
>
> It all worked for me after that.
>
> Generally, running rich means the engine runs cooler with less power,
and
> running lean means the engine runs hot with more power.

Lean then means more O2 to burn the available fuel?


>
> If you overdo it on the rich side, you'll gum up the engine with unburnt
> fuel (likely the plugs), and if you overdo it on the lean side, you'll
burn
> the motor out by overheating over too long a time.

Hence adjusting the mixture control for max revs on a 4 stroke B&S or a
2-stroke whipper snipper is bad :-)

Jason

[Ext User(Jason James)]

"John_H" <john4721@inbox.com> wrote in message
news:lc0tq31cqftvd0c4e69dk4updsqai7k6us@4ax.com...
> Jason James wrote:
>
> >In the "old'" days, it was common practise to turn off the fuel feed to
the
> >carby on 2-strokes. The reason given that otherwise the fuel will
evaporate
> >leaving oil in the fuel bowl,..fair enough. What I dont understand, is
why
> >just a few seconds before the fuel runs out, the engine speeds up? It's
> >running lean, but wouldnt that give less power?
>
> How to tune a 2-stroke for maximum reliable power (racer's trick from
> the "old" days)....
>
> 1) Progressively lean mixture until a hole appears in the piston.
> 2) Replace piston.
> 3) Increase main jet size by one increment.
>
> Should answer your question. :)

To think the "Electronic lean burn" Valiants ran 17:1 instead of
stoichiometric (14:1?) that is until the metering rods got worn.

Jason

[Ext User(Jason James)]

"hippo" <johnh@chopthis.shoal.net.au> wrote in message
news:52b5de239faf41f356e8f37dc38288d5@localhost.ta lkaboutautos.com...
> John_H <john4721@inbox.com> Feb 10, 2008 at 02:53 PM wrote:
>
>
> >Jason James wrote:
>
> >>In the "old'" days, it was common practise to turn off the fuel feed to
> the
> >>carby on 2-strokes. The reason given that otherwise the fuel will
> evaporate
> >>leaving oil in the fuel bowl,..fair enough. What I dont understand, is
> why
> >>just a few seconds before the fuel runs out, the engine speeds up? It's
> >>running lean, but wouldnt that give less power?
>
> >How to tune a 2-stroke for maximum reliable power (racer's trick from
> the "old" days)....
>
> >1) Progressively lean mixture until a hole appears in the piston.
> >2) Replace piston.
> >3) Increase main jet size by one increment.
>
> >Should answer your question. :)
>
> >--
> >John H
>
> Sounds like the old Suzuki GT250... 158Km/h... Pop!... limp home at 35-40
> on one cylinder. Change piston & repeat process/result. Up main jet and
> hey presto, 160 with no pop. Cheers

Thanks for the answers,..now I understand...

Jason

[Ext User(John McKenzie)]

Jason James wrote:
>
> "John Tserkezis" <jt@techniciansyndrome.org.invalid> wrote in message
> news:47ae4535$0$5200$afc38c87@news.optusnet.com.au ...
> > Jason James wrote:
> >
> > > In the "old'" days, it was common practise to turn off the fuel feed to
> the
> > > carby on 2-strokes. The reason given that otherwise the fuel will
> evaporate
> > > leaving oil in the fuel bowl,..fair enough. What I dont understand, is
> why
> > > just a few seconds before the fuel runs out, the engine speeds up? It's
> > > running lean, but wouldnt that give less power?
> >
> > No expert, but running leaner, means running hotter with more power.
> >
> > That was the hardest thing I had to wrap my mind around, when it was
> > repeatedly beaten into me by various sources that more fuel = more power.
> >
> > Then we got into gokarts, and I had to learn all over again. The carby
> was
> > as crude and manual as you could get. If you didn't understand why things
> did
> > what they did, the kart either didn't run as well as it could, or ran into
> the
> > ground very quickly.
> >
> > So I stopped listening to everyone (either they were all miss-informed,
> they
> > miss-understood, they were intentionally misleading us, or they were
> idiots).
> > I sat down with a bunch of books, looked at the carby itself (it was
> > relatively simple) and worked things out myself.
> >
> > It all worked for me after that.
> >
> > Generally, running rich means the engine runs cooler with less power,
> and
> > running lean means the engine runs hot with more power.
>
> Lean then means more O2 to burn the available fuel?

I'd suggest that a contributing factor in a lot of cases is that the
idle a/f ratio is deliberately rich - which will ease starting, cold
running (whilst getting up to temp) and also help get around the fact
that the mixture will go transiently lean as the throttle is opened up.
Which means that as the fuel runs out, for a small amt of time, it's
actually got a more desirable a/f ratio.

Once you go well somewhat leaner than stoich, the exhaust temp drops
again (most people think it only happens when you go richer) - when you
go richer than stoich, the extra fuel has more cooling effect, so the
temp before combustion is lower, and then the final exhaust temp will be
slightly lower too, even though more work might actually be done (since
the denser, by virtue of the cooling effect, intake charge will still
have some push on the piston after it travels down the bore, as it takes
more cylinder volume to bleed off the pressure). AFAIK the fuel will
still actually mostly burn at stoich, and some of it won't burn at all,
or completely, which is why going rich can make more power but is
atrocious on fuel economy.

Going leaner than stoich, there's less fuel, so there's less heat
energy, so the exhaust temp again drops. The reason going lean makes for
better fuel economy is not so much to do with the reaction (it's still
burning similarly, with the slight exception that there will be some NOx
created) but something else - for the same relative amount of fuel being
used, there's more air in there. That might mean that to maintain a
particular road speed/rpm with a rich mixture you need say 25% throttle.
To achieve it with a leaner mixture, you need say 35% throttle opening.
This extra throttle opening will put more air in there for the same (or
less overall most likely) amt of fuel - so there's more
dynamic/running/effective compression - which leads to both a quicker
cleaner burn and (sort of) more 'leverage' on the piston due to the
greater mass of air to expand. This is incidentally why EGR can actually
lead to improvements in fuel economy - it's introduced into the
cylinder, and won't burn itself, but does increase the effective
compression ratio. And it does it via egr, bypassing the carb. If the
egr was removed, with no other changes, you can see a drop in fuel
efficiency, and the way around it would be either re-introduction of egr
(at part throttle cruise conditions only of course) or revision of the
a/f curve. The difference being that leaning it out will, as said
increase NOx emissions, since some of it manages to burn with enough
temp/pressure and the unused oxygen from the air. Hence the egr
operation has some upsides - similar effect on economy, but less nox
emissions.


> Hence adjusting the mixture control for max revs on a 4 stroke B&S or a
> 2-stroke whipper snipper is bad :-)

I suppose you'd also have to factor in how close the comp ratio was to
anything dangerous. Given the fuel is already contaminated with oil (for
2 strokes at least) and their inherent design, the actual running comp
ratio isn't particularly high, but even if it could physically be
achieved, it wouldn't be a great option. On most apps it's probably the
case that you can definitely damage them if you go lean enough. I'd
wonder if some of the really early or otherwise low-po 2strokes would
actually not be capable of melting a piston crown via lean mixtures.

The worst I came across (and experiences differ) was the motors in the
rg250s (prior to the rgv 'slingshot' models). I dunno if it was just my
unsympathetic treatment, but I damaged a few of them, and knew of a few
friends who did likewise. In the manual they did list various needle
positioning and so forth, and I found that anything other than 'full
rich' and at a minimum factory main jet was asking for trouble. As a
result the ****s of things were particularly inefficient on fuel.


--
John McKenzie

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[Ext User(John Tserkezis)]

Jason James wrote:

> Lean then means more O2 to burn the available fuel?

As I understand it, yes.

>> If you overdo it on the rich side, you'll gum up the engine with unburnt
>> fuel (likely the plugs), and if you overdo it on the lean side, you'll burn
>> the motor out by overheating over too long a time.

> Hence adjusting the mixture control for max revs on a 4 stroke B&S or a
> 2-stroke whipper snipper is bad :-)

Normally, I'd say here would be a good time to keep an eye on the
temperature of the motor, and throttle things back a bit when it gets close to
maximum.

But a whipper snipper is not a mission-critical application.

Besides, a goat is auto-manoeuvring, auto-searching, and mostly
self-maintaining. And usually quieter than a whipper snipper anyway.
Best of all, the air-fuel mixture is internally self-controlled.
--
Linux Registered User # 302622
<http://counter.li.org>

[Ext User(atec77)]

John Tserkezis wrote:
> Jason James wrote:
>
>> Lean then means more O2 to burn the available fuel?
>
> As I understand it, yes.
>
>>> If you overdo it on the rich side, you'll gum up the engine with
>>> unburnt
>>> fuel (likely the plugs), and if you overdo it on the lean side,
>>> you'll burn
>>> the motor out by overheating over too long a time.
>
>> Hence adjusting the mixture control for max revs on a 4 stroke B&S or a
>> 2-stroke whipper snipper is bad :-)
>
> Normally, I'd say here would be a good time to keep an eye on the
> temperature of the motor, and throttle things back a bit when it gets
> close to maximum.
>
> But a whipper snipper is not a mission-critical application.
>
> Besides, a goat is auto-manoeuvring, auto-searching, and mostly
> self-maintaining. And usually quieter than a whipper snipper anyway.
> Best of all, the air-fuel mixture is internally self-controlled.
Best thing about a goat is the multi use , it snips quietly , keeps
uncle billy away from the kids and tastes bloody good with baby roast
spuds and peas .

[Ext User(Toby Ponsenby)]

On Sun, 10 Feb 2008 11:28:37 +1100, John Tserkezis blathered on in :

> Jason James wrote:
>
>> In the "old'" days, it was common practise to turn off the fuel feed to
>> the carby on 2-strokes. The reason given that otherwise the fuel will
>> evaporate leaving oil in the fuel bowl,..fair enough. What I dont
>> understand, is why just a few seconds before the fuel runs out, the
>> engine speeds up? It's running lean, but wouldnt that give less power?
>
> No expert, but running leaner, means running hotter with more power.
>
> That was the hardest thing I had to wrap my mind around, when it was
> repeatedly beaten into me by various sources that more fuel = more
> power.
>
> Then we got into gokarts, and I had to learn all over again. The
> carby was
> as crude and manual as you could get. If you didn't understand why
> things did what they did, the kart either didn't run as well as it
> could, or ran into the ground very quickly.
>
> So I stopped listening to everyone (either they were all
> miss-informed, they
> miss-understood, they were intentionally misleading us, or they were
> idiots).
> I sat down with a bunch of books, looked at the carby itself (it was
> relatively simple) and worked things out myself.
>
> It all worked for me after that.
>
> Generally, running rich means the engine runs cooler with less power,
> and
> running lean means the engine runs hot with more power.
>
> If you overdo it on the rich side, you'll gum up the engine with
> unburnt
> fuel (likely the plugs), and if you overdo it on the lean side, you'll
> burn the motor out by overheating over too long a time.

Worthy of consideration...
There's the added wheeze whereby in two-stroke engines with premix, the
lean/rich business is compounded by the proportions of the mix brought on
by the quality of the petrol component.
That's the stale fuel gag with these engines - stale fuel = lean
mix=****ed engine - even if the mix proportions are just fine.

I remember a guy I knew was having problems with the bearings in his go-
kart mill.
Much more so than he figured was 'normal'. Turned out that transporting
the infernal thing was causing damage to the bearings with trailer
bumpees doing the damage.
He built up a little frame that held the crank in a particular fixed spot
relative to the crankcase, and the bearings were just fine after that:-)

--
Toby

[Ext User(Paul Saccani)]

On Sun, 10 Feb 2008 11:28:37 +1100, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

>
> No expert, but running leaner, means running hotter with more power.

Up to a point. For economy with spark ignition aircraft engines, you
run them leaner of peak EGT till they start to run rough, then back
off a bit.

It runs cooler, with slightly less power.

Practical for engines running at constant revs. Not practical for
automotive engines. Well, maybe for a Pious...

If you are running at high power output at close to stoichiometric
mixtures, mixture will often be set to be rich of peak EGT, with some
of the excess fuel being used directly for cooling purposes.


--
Cheers
Paul Saccani
Perth, Western Australia.

[Ext User(Paul Saccani)]

On Sun, 10 Feb 2008 17:10:09 +1100, "Jason James" <at@peace.com>
wrote:

>
>"John_H" <john4721@inbox.com> wrote in message
>news:lc0tq31cqftvd0c4e69dk4updsqai7k6us@4ax.com...
>> Jason James wrote:
>>
>> >In the "old'" days, it was common practise to turn off the fuel feed to
>the
>> >carby on 2-strokes. The reason given that otherwise the fuel will
>evaporate
>> >leaving oil in the fuel bowl,..fair enough. What I dont understand, is
>why
>> >just a few seconds before the fuel runs out, the engine speeds up? It's
>> >running lean, but wouldnt that give less power?
>>
>> How to tune a 2-stroke for maximum reliable power (racer's trick from
>> the "old" days)....
>>
>> 1) Progressively lean mixture until a hole appears in the piston.
>> 2) Replace piston.
>> 3) Increase main jet size by one increment.
>>
>> Should answer your question. :)
>
>To think the "Electronic lean burn" Valiants ran 17:1 instead of
>stoichiometric (14:1?) that is until the metering rods got worn.

IIRC, ELB operated under low load conditions.
--
Cheers
Paul Saccani
Perth, Western Australia.

[Ext User(Jason James)]

"Toby Ponsenby" <me@privacy.net> wrote in message
news:fom42o$jtp$1@registered.motzarella.org...
> On Sun, 10 Feb 2008 11:28:37 +1100, John Tserkezis blathered on in :
>
> > Jason James wrote:
> >
> >> In the "old'" days, it was common practise to turn off the fuel feed to
> >> the carby on 2-strokes. The reason given that otherwise the fuel will
> >> evaporate leaving oil in the fuel bowl,..fair enough. What I dont
> >> understand, is why just a few seconds before the fuel runs out, the
> >> engine speeds up? It's running lean, but wouldnt that give less power?
> >
> > No expert, but running leaner, means running hotter with more power.
> >
> > That was the hardest thing I had to wrap my mind around, when it was
> > repeatedly beaten into me by various sources that more fuel = more
> > power.
> >
> > Then we got into gokarts, and I had to learn all over again. The
> > carby was
> > as crude and manual as you could get. If you didn't understand why
> > things did what they did, the kart either didn't run as well as it
> > could, or ran into the ground very quickly.
> >
> > So I stopped listening to everyone (either they were all
> > miss-informed, they
> > miss-understood, they were intentionally misleading us, or they were
> > idiots).
> > I sat down with a bunch of books, looked at the carby itself (it was
> > relatively simple) and worked things out myself.
> >
> > It all worked for me after that.
> >
> > Generally, running rich means the engine runs cooler with less power,
> > and
> > running lean means the engine runs hot with more power.
> >
> > If you overdo it on the rich side, you'll gum up the engine with
> > unburnt
> > fuel (likely the plugs), and if you overdo it on the lean side, you'll
> > burn the motor out by overheating over too long a time.
>
> Worthy of consideration...
> There's the added wheeze whereby in two-stroke engines with premix, the
> lean/rich business is compounded by the proportions of the mix brought on
> by the quality of the petrol component.
> That's the stale fuel gag with these engines - stale fuel = lean
> mix=****ed engine - even if the mix proportions are just fine.
>
> I remember a guy I knew was having problems with the bearings in his go-
> kart mill.
> Much more so than he figured was 'normal'. Turned out that transporting
> the infernal thing was causing damage to the bearings with trailer
> bumpees doing the damage.
> He built up a little frame that held the crank in a particular fixed spot
> relative to the crankcase, and the bearings were just fine after that:-)

Too true. The 50kva "no-break" gensets (3 ton flywheel used to drive the
alternator to maintain continued 415vac to site while the diesel came on
load) used to damage the rear main bearing shells in the diesel. 99% of the
time, an electric motor drove the flywheel and the alternator. The consant
s;ight vibration in the drive-axis which ran thru the 3 units battered the
diesel's main bearings. We were under instruction to start the Perkins
"off=load" just to get some oil to the main bearings when we were visiting..

Jason

[Ext User(Jason James)]

"John McKenzie" <jmac_melbourne@bigpond.com> wrote in message
news:47AE9CE4.528D@bigpond.com...
> Jason James wrote:
> >
> > "John Tserkezis" <jt@techniciansyndrome.org.invalid> wrote in message
> > news:47ae4535$0$5200$afc38c87@news.optusnet.com.au ...
> > > Jason James wrote:
> > >
> > > > In the "old'" days, it was common practise to turn off the fuel feed
to
> > the
> > > > carby on 2-strokes. The reason given that otherwise the fuel will
> > evaporate
> > > > leaving oil in the fuel bowl,..fair enough. What I dont understand,
is
> > why
> > > > just a few seconds before the fuel runs out, the engine speeds up?
It's
> > > > running lean, but wouldnt that give less power?
> > >
> > > No expert, but running leaner, means running hotter with more power.
> > >
> > > That was the hardest thing I had to wrap my mind around, when it was
> > > repeatedly beaten into me by various sources that more fuel = more
power.
> > >
> > > Then we got into gokarts, and I had to learn all over again. The
carby
> > was
> > > as crude and manual as you could get. If you didn't understand why
things
> > did
> > > what they did, the kart either didn't run as well as it could, or ran
into
> > the
> > > ground very quickly.
> > >
> > > So I stopped listening to everyone (either they were all
miss-informed,
> > they
> > > miss-understood, they were intentionally misleading us, or they were
> > idiots).
> > > I sat down with a bunch of books, looked at the carby itself (it was
> > > relatively simple) and worked things out myself.
> > >
> > > It all worked for me after that.
> > >
> > > Generally, running rich means the engine runs cooler with less
power,
> > and
> > > running lean means the engine runs hot with more power.
> >
> > Lean then means more O2 to burn the available fuel?
>
> I'd suggest that a contributing factor in a lot of cases is that the
> idle a/f ratio is deliberately rich - which will ease starting, cold
> running (whilst getting up to temp) and also help get around the fact
> that the mixture will go transiently lean as the throttle is opened up.
> Which means that as the fuel runs out, for a small amt of time, it's
> actually got a more desirable a/f ratio.

Yeah,..remember those vacuum controlled choke-butterfly things? While
wrenching that painful shoulder trying to start the B&S, the choke flap did
its thing,..then when it spluttered into life, the vacuum dragged the flap
open,..until you hit some heavy going,..then it throttled the intake as the
throttle went full on = little vac.


> Once you go well somewhat leaner than stoich, the exhaust temp drops
> again (most people think it only happens when you go richer) - when you
> go richer than stoich, the extra fuel has more cooling effect, so the
> temp before combustion is lower, and then the final exhaust temp will be
> slightly lower too, even though more work might actually be done (since
> the denser, by virtue of the cooling effect, intake charge will still
> have some push on the piston after it travels down the bore, as it takes
> more cylinder volume to bleed off the pressure). AFAIK the fuel will
> still actually mostly burn at stoich, and some of it won't burn at all,
> or completely, which is why going rich can make more power but is
> atrocious on fuel economy.
>

That's why the oil goes black so quick!


> Going leaner than stoich, there's less fuel, so there's less heat
> energy, so the exhaust temp again drops. The reason going lean makes for
> better fuel economy is not so much to do with the reaction (it's still
> burning similarly, with the slight exception that there will be some NOx
> created) but something else - for the same relative amount of fuel being
> used, there's more air in there. That might mean that to maintain a
> particular road speed/rpm with a rich mixture you need say 25% throttle.
> To achieve it with a leaner mixture, you need say 35% throttle opening.
> This extra throttle opening will put more air in there for the same (or
> less overall most likely) amt of fuel - so there's more
> dynamic/running/effective compression - which leads to both a quicker
> cleaner burn and (sort of) more 'leverage' on the piston due to the
> greater mass of air to expand. This is incidentally why EGR can actually
> lead to improvements in fuel economy - it's introduced into the
> cylinder, and won't burn itself, but does increase the effective
> compression ratio.

Interesting stuff John,..I'm only just staying with you :-)


And it does it via egr, bypassing the carb. If the
> egr was removed, with no other changes, you can see a drop in fuel
> efficiency, and the way around it would be either re-introduction of egr
> (at part throttle cruise conditions only of course) or revision of the
> a/f curve. The difference being that leaning it out will, as said
> increase NOx emissions, since some of it manages to burn with enough
> temp/pressure and the unused oxygen from the air. Hence the egr
> operation has some upsides - similar effect on economy, but less nox
> emissions.

The '76 2L OHC Corona had a tiny ID pipe jutting into the sec-venturri,
from the exhaust, controlled (I think) by a bellows. It didnt seem big
enough to do much?



> > Hence adjusting the mixture control for max revs on a 4 stroke B&S or a
> > 2-stroke whipper snipper is bad :-)
>
> I suppose you'd also have to factor in how close the comp ratio was to
> anything dangerous. Given the fuel is already contaminated with oil (for
> 2 strokes at least) and their inherent design, the actual running comp
> ratio isn't particularly high, but even if it could physically be
> achieved, it wouldn't be a great option. On most apps it's probably the
> case that you can definitely damage them if you go lean enough. I'd
> wonder if some of the really early or otherwise low-po 2strokes would
> actually not be capable of melting a piston crown via lean mixtures.

I always wondered how 2/stroke bikes blew a hole in their pistons


> The worst I came across (and experiences differ) was the motors in the
> rg250s (prior to the rgv 'slingshot' models). I dunno if it was just my
> unsympathetic treatment, but I damaged a few of them, and knew of a few
> friends who did likewise. In the manual they did list various needle
> positioning and so forth, and I found that anything other than 'full
> rich' and at a minimum factory main jet was asking for trouble. As a
> result the ****s of things were particularly inefficient on fuel.

Ta,..I've learned heaps



Jason

[Ext User(Chas)]

"Jason James" <at@peace.com> wrote in message
news:47af4b42$0$98040$c30e37c6@pit-reader.telstra.net...
> <SNIP>
>
>
> The '76 2L OHC Corona had a tiny ID pipe jutting into the sec-venturri,
> from the exhaust, controlled (I think) by a bellows. It didnt seem big
> enough to do much?
>
>
>
> Jason
>
The Toyota 18R EGR tube was approx 10 mm ID when clean, but had the habit of
blocking with carbon deposit over time. Most people in those days disabled
the EGR valve anyway, in the belief that EGR was detrimental to performance
and economy.
--
Regards,

Chas.

(To email me replace 'xxx' with tango papa golf)

[Ext User(Jason James)]

"Chas" <umlaut@xxx.com.au> wrote in message
news:47b011db$1@dnews.tpgi.com.au...
>
> "Jason James" <at@peace.com> wrote in message
> news:47af4b42$0$98040$c30e37c6@pit-reader.telstra.net...
> > <SNIP>
> >
> >
> > The '76 2L OHC Corona had a tiny ID pipe jutting into the sec-venturri,
> > from the exhaust, controlled (I think) by a bellows. It didnt seem big
> > enough to do much?
> >
> >
> >
> > Jason
> >
> The Toyota 18R EGR tube was approx 10 mm ID when clean, but had the habit
of
> blocking with carbon deposit over time. Most people in those days
disabled
> the EGR valve anyway, in the belief that EGR was detrimental to
performance
> and economy.

I had an earlier EGR system and carby,..as the exhaust injection point was a
small metal pipe which was tapered to about 2-3mm. If the car was doing
extended urban driving, a mound of solidified coke/carbon formed on the
other side of the sec-venturri. The later 18Rs also had an air-pump. My
carby had a small, round fuel-bowl level window.

Jason