Spark plugs indicate lean mix on carbureted V6 [Technical]

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From: jim on 6 Dec 2008 18:42

Kevin wrote:
>
> Mike Romain <romainm(a)sympatico.ca> wrote in news:493af7ae$0$26770
> $9a6e19ea(a)unlimited.newshosting.com:
>
> > Jack wrote:
> >
> >> Chrysler refers to these carbs as "lean burn carburetors", so there
> may
> >> be something in the design that causes a lean mix, But the only real
> >> difference between this "lean burn" variety and the feedback variety
> >> that came stock on this vehicle is the extra main jet controlled by
> the
> >> mixture control solenoid, and the auxiliary idle air bleeds that are
> >> also controlled by the mixture control solenoid.
> >
> > Isn't the intake different on the lean burn carb also? Or maybe the
> EGR
> > or even the cam is different? On my Jeep there are 5 flavors of EGR
> > that can be used according to other emission components. I had one
> lean
> > burn once and remember something odd about it, beside it not being
> able
> > to get out of it's own way.
> >
> > Mike
> >
> >
>
> I would be the only one here to believe it was fine and dandy all
> along. the new carb was ok, unless you have another indicator other than
> the plugs it is lean, I doubt it is. white plugs unless bubbled are
> common with todays gas. get a airfuel ratio check if you must, but I bet
> it was ok before you started messing with it. KB

He could rev the engine and dump a little sand down the carburetor. He needs to
get it to burn a little oil to get that nice tan glow he wants in his spark
plugs:^)

-jim

>
> --
> THUNDERSNAKE #9
>
> Protect your rights or "Lose" them
> The 2nd Admendment guarantees the others

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From: Mike Romain on 6 Dec 2008 18:48

From: Jack on 6 Dec 2008 21:28

Mike Romain wrote:
> Jack wrote:
>
>> Chrysler refers to these carbs as "lean burn carburetors", so there may
>> be something in the design that causes a lean mix, But the only real
>> difference between this "lean burn" variety and the feedback variety
>> that came stock on this vehicle is the extra main jet controlled by the
>> mixture control solenoid, and the auxiliary idle air bleeds that are
>> also controlled by the mixture control solenoid.
>
> Isn't the intake different on the lean burn carb also? Or maybe the EGR
> or even the cam is different?

Could be. I may be wrong but I seem to recall that Chrysler called it a
lean burn "system", so it may have included more than just the carb. At
any rate, the replacement carb body is the essentially the same as the
feedback variety with minor porting differences. For instance, the port
designated on the replacement carb for "spark vacuum line" is the one
that I use for the EGR valve. It is stronger than the port designated
for EGR. The designated EGR port was not strong enough to fully open the
EGR valve used on this truck. The spark vacuum on this truck uses
manifold vacuum, so that port was free.

At this point, I am tempted to remove the main jets to see if I can
force a rich mix before I yank the the whole carb, but that would not
explain the lean mix at idle. Like you pointed out, it seems like a
massive vacuum leak, but other than the white plugs there is no evidence
of one. At any rate, if I can force a rich mix in the main system by
removing the jets, I can tune it with the jets.

> On my Jeep there are 5 flavors of EGR
> that can be used according to other emission components. I had one lean
> burn once and remember something odd about it, beside it not being able
> to get out of it's own way.
>
> Mike
>

--
"Sometimes we have Nyquil for dinner." -Dot Goddard

From: Jack on 6 Dec 2008 21:39

Mike Romain wrote:
> Jack wrote:
>
>> Chrysler refers to these carbs as "lean burn carburetors", so there may
>> be something in the design that causes a lean mix, But the only real
>> difference between this "lean burn" variety and the feedback variety
>> that came stock on this vehicle is the extra main jet controlled by the
>> mixture control solenoid, and the auxiliary idle air bleeds that are
>> also controlled by the mixture control solenoid.
>
> Isn't the intake different on the lean burn carb also? Or maybe the EGR
> or even the cam is different? On my Jeep there are 5 flavors of EGR
> that can be used according to other emission components. I had one lean
> burn once and remember something odd about it, beside it not being able
> to get out of it's own way.
>
> Mike
>

After reading up on the lean burn system, I am beginning to suspect the
wire from the carb that signals the computer whether the throttle is
opened or closed. If the computer never senses that the throttle is
opened, the spark advance would never kick in. As I recall, this wire is
housed in the same plug as the wires from the old mixture control
solenoid. I will have to check to see if I got this wire screwed up when
I changed carbs.

But I doubt that I would be getting over 21 mpg on the freeway without
spark advance. Well, my day is spoken for tomorrow :--)

From: Jack on 6 Dec 2008 21:50

Kevin wrote:
> Mike Romain <romainm(a)sympatico.ca> wrote in news:493af7ae$0$26770
> $9a6e19ea(a)unlimited.newshosting.com:
>
>> Jack wrote:
>>
>>> Chrysler refers to these carbs as "lean burn carburetors", so there
> may
>>> be something in the design that causes a lean mix, But the only real
>>> difference between this "lean burn" variety and the feedback variety
>>> that came stock on this vehicle is the extra main jet controlled by
> the
>>> mixture control solenoid, and the auxiliary idle air bleeds that are
>>> also controlled by the mixture control solenoid.
>> Isn't the intake different on the lean burn carb also? Or maybe the
> EGR
>> or even the cam is different? On my Jeep there are 5 flavors of EGR
>> that can be used according to other emission components. I had one
> lean
>> burn once and remember something odd about it, beside it not being
> able
>> to get out of it's own way.
>>
>> Mike
>>
>>
>
> I would be the only one here to believe it was fine and dandy all
> along. the new carb was ok, unless you have another indicator other than
> the plugs it is lean, I doubt it is. white plugs unless bubbled are
> common with todays gas. get a airfuel ratio check if you must, but I bet
> it was ok before you started messing with it. KB

I would be fine with that, Kevin, but on the last California smog test,
it failed the CO portion on both 15mph and 25mph. After fiddling, it
*barely* passed the HC test at 15mph. Unfortunately, I would have to do
some serious memory recall to remember what I fiddled with.

Plus, there is the part about seeing the plugs in another engine in the
area with toasty tan plugs.

But I will keep in mind what you posted about white plugs not being out
of the ordinary. The only things that I am concerned about are 1) not
killing the engine, 2) passing the next smog test and 3) getting good
mileage and performance. Plug color is way down the list, as you rightly
point out that it should be.

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