CJ - backfiring problem help (a little long)

My 83 CJ7 with a stock (so far as I know) 258, T176, D300, 33's,
4.11's, Detroits, etc and "full" factory emissions crap meaning
computer, pulse air and all those vacuum lines etc (I have one more WA
state emissions test to pass before it "ages out" and it no longer
needs to be tested) is having a mild to severe "backfiring problem"
on deceleration. LOTS of popping and noises out the exhaust when I
decelerate or downshift.

The timing is set at "about 12 degrees" before TDC (which
Chilton's suggests is about right - (pretty sure it's a California
model). The timing mark sorta "bounces around" when I check it
witht the light (distributor vacuum line disconnected and plugged) -
possibly due to some wear in the distributor or ??. I have another
distributor to swap in to "test" the wear theory, but haven't had
the time to do so yet.

Anyway - if I advance the timing further which significantly improves
the driveability and idle, I start to get engine pinging (a little
difficult to hear over the tires) - so I don't really want to leave
it too far advanced. I'm running 87 octane - NOT from Arco.

But I'd like to improve the drive-ability, STOP the "backfiring",
and not damage the engine.

Any thoughts or guidance?

Thanks

Lynn in Vancouver, WA

That usefully means there's a exhaust gasket or crack leaking
oxygen back into the exhaust. Sometimes disconnecting the Exhaust
Recycling Valve, or Gulp valve will hide a problem in there SMOG
systems.
God Bless America, Bill O|||||||O
http://www.billhughes.com/

" wrote:
>
> My 83 CJ7 with a stock (so far as I know) 258, T176, D300, 33's,
> 4.11's, Detroits, etc and "full" factory emissions crap meaning
> computer, pulse air and all those vacuum lines etc (I have one more WA
> state emissions test to pass before it "ages out" and it no longer
> needs to be tested) is having a mild to severe "backfiring problem"
> on deceleration. LOTS of popping and noises out the exhaust when I
> decelerate or downshift.
>
> The timing is set at "about 12 degrees" before TDC (which
> Chilton's suggests is about right - (pretty sure it's a California
> model). The timing mark sorta "bounces around" when I check it
> witht the light (distributor vacuum line disconnected and plugged) -
> possibly due to some wear in the distributor or ??. I have another
> distributor to swap in to "test" the wear theory, but haven't had
> the time to do so yet.
>
> Anyway - if I advance the timing further which significantly improves
> the driveability and idle, I start to get engine pinging (a little
> difficult to hear over the tires) - so I don't really want to leave
> it too far advanced. I'm running 87 octane - NOT from Arco.
>
> But I'd like to improve the drive-ability, STOP the "backfiring",
> and not damage the engine.
>
> Any thoughts or guidance?
>
> Thanks
>
> Lynn in Vancouver, WA

I have converted my 86 to a manually tuned engine with about 9 deg
static timing and a ported vacuum advance. Mine pops and burbles when I
decelerate down a steep hill.

If it actually 'backfiring', (firing on the back side of a valve) that
can mean your exhaust and/or intake manifold is loose. This isn't
uncommon. You can test for it by spraying a mist of water along the
seam or carb cleaner or I think even WD40. If any of these make the
engine stumble, then you need to re-torque the intakes back down.

You also could have a broken mechanical timing advance spring or a
seized up advance plate. The advance plate inside the distributor needs
to be lubricated regularly. To do this you pull off the rotor and put
oil on the sponge or felt pad you should find in the center of the
distributor shaft. You then can hand work the advance plate to be sure
it moves freely and springs back hard and fast when released.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)


" wrote:
>
> My 83 CJ7 with a stock (so far as I know) 258, T176, D300, 33's,
> 4.11's, Detroits, etc and "full" factory emissions crap meaning
> computer, pulse air and all those vacuum lines etc (I have one more WA
> state emissions test to pass before it "ages out" and it no longer
> needs to be tested) is having a mild to severe "backfiring problem"
> on deceleration. LOTS of popping and noises out the exhaust when I
> decelerate or downshift.
>
> The timing is set at "about 12 degrees" before TDC (which
> Chilton's suggests is about right - (pretty sure it's a California
> model). The timing mark sorta "bounces around" when I check it
> witht the light (distributor vacuum line disconnected and plugged) -
> possibly due to some wear in the distributor or ??. I have another
> distributor to swap in to "test" the wear theory, but haven't had
> the time to do so yet.
>
> Anyway - if I advance the timing further which significantly improves
> the driveability and idle, I start to get engine pinging (a little
> difficult to hear over the tires) - so I don't really want to leave
> it too far advanced. I'm running 87 octane - NOT from Arco.
>
> But I'd like to improve the drive-ability, STOP the "backfiring",
> and not damage the engine.
>
> Any thoughts or guidance?
>
> Thanks
>
> Lynn in Vancouver, WA

The most common cause for backfiring on deceleration is idle mixture too
lean. That could be a vacuum leak with this vintage. They really do have
too many vacuum lines, don't they? If that is not it, then short of a new
or rebuilt carburetor, I don't know how you get a proper idle mixture. With
older models you simply back off the idle mixture screw, until the engine is
getting a rich enough mixture. With the smog controlled models, there is
usually something to prevent that.

Mike and Bill have some good ideas too. Just don't take either of those
guys too seriously. They are pretty old and grizzled, both of them. ;^)

Earle

> wrote in message
oups.com...
> My 83 CJ7 with a stock (so far as I know) 258, T176, D300, 33's,
> 4.11's, Detroits, etc and "full" factory emissions crap meaning
> computer, pulse air and all those vacuum lines etc (I have one more WA
> state emissions test to pass before it "ages out" and it no longer
> needs to be tested) is having a mild to severe "backfiring problem"
> on deceleration. LOTS of popping and noises out the exhaust when I
> decelerate or downshift.
>
> The timing is set at "about 12 degrees" before TDC (which
> Chilton's suggests is about right - (pretty sure it's a California
> model). The timing mark sorta "bounces around" when I check it
> witht the light (distributor vacuum line disconnected and plugged) -
> possibly due to some wear in the distributor or ??. I have another
> distributor to swap in to "test" the wear theory, but haven't had
> the time to do so yet.
>
> Anyway - if I advance the timing further which significantly improves
> the driveability and idle, I start to get engine pinging (a little
> difficult to hear over the tires) - so I don't really want to leave
> it too far advanced. I'm running 87 octane - NOT from Arco.
>
> But I'd like to improve the drive-ability, STOP the "backfiring",
> and not damage the engine.
>
> Any thoughts or guidance?
>
> Thanks
>
> Lynn in Vancouver, WA
>



--
Posted via a free Usenet account from http://www.teranews.com

Hey there old timer, who you callin' old?

One sign of a vacuum leak is oil puking into the air filter. If that is
also happening along with a crappy idle, the OP should post back.

The carb 'can' be adjusted for mix, but it is supposed to stay factory
set so the computer does the mix. It 'can' be set back factory if
needed. That is another fix.

If the engine won't idle under 1100 or so rpm, then there is another fix
for that. (plugged idle tubes)

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

Earle Horton wrote:
>
> The most common cause for backfiring on deceleration is idle mixture too
> lean. That could be a vacuum leak with this vintage. They really do have
> too many vacuum lines, don't they? If that is not it, then short of a new
> or rebuilt carburetor, I don't know how you get a proper idle mixture. With
> older models you simply back off the idle mixture screw, until the engine is
> getting a rich enough mixture. With the smog controlled models, there is
> usually something to prevent that.
>
> Mike and Bill have some good ideas too. Just don't take either of those
> guys too seriously. They are pretty old and grizzled, both of them. ;^)
>
> Earle
>
> > wrote in message
> oups.com...
> > My 83 CJ7 with a stock (so far as I know) 258, T176, D300, 33's,
> > 4.11's, Detroits, etc and "full" factory emissions crap meaning
> > computer, pulse air and all those vacuum lines etc (I have one more WA
> > state emissions test to pass before it "ages out" and it no longer
> > needs to be tested) is having a mild to severe "backfiring problem"
> > on deceleration. LOTS of popping and noises out the exhaust when I
> > decelerate or downshift.
> >
> > The timing is set at "about 12 degrees" before TDC (which
> > Chilton's suggests is about right - (pretty sure it's a California
> > model). The timing mark sorta "bounces around" when I check it
> > witht the light (distributor vacuum line disconnected and plugged) -
> > possibly due to some wear in the distributor or ??. I have another
> > distributor to swap in to "test" the wear theory, but haven't had
> > the time to do so yet.
> >
> > Anyway - if I advance the timing further which significantly improves
> > the driveability and idle, I start to get engine pinging (a little
> > difficult to hear over the tires) - so I don't really want to leave
> > it too far advanced. I'm running 87 octane - NOT from Arco.
> >
> > But I'd like to improve the drive-ability, STOP the "backfiring",
> > and not damage the engine.
> >
> > Any thoughts or guidance?
> >
> > Thanks
> >
> > Lynn in Vancouver, WA
> >
>
> --
> Posted via a free Usenet account from http://www.teranews.com

On Tue, 13 Jun 2006 08:42:40 -0600, "Earle Horton"
> wrote:

>The most common cause for backfiring on deceleration is idle mixture too
>lean. That could be a vacuum leak with this vintage. They really do have
>too many vacuum lines, don't they? If that is not it, then short of a new
>or rebuilt carburetor, I don't know how you get a proper idle mixture. With
>older models you simply back off the idle mixture screw, until the engine is
>getting a rich enough mixture. With the smog controlled models, there is
>usually something to prevent that.

Plastic caps that are easily removed with a pocket/exacto knife or the
tip of a soldering iron (both are standard shop practices).

Dan
>
>Mike and Bill have some good ideas too. Just don't take either of those
>guys too seriously. They are pretty old and grizzled, both of them. ;^)
>
>Earle

"Hootowl" > wrote in message
...
> On Tue, 13 Jun 2006 08:42:40 -0600, "Earle Horton"
> > wrote:
>
> >The most common cause for backfiring on deceleration is idle mixture too
> >lean. That could be a vacuum leak with this vintage. They really do
> >have too many vacuum lines, don't they? If that is not it, then short
> >of a new or rebuilt carburetor, I don't know how you get a proper
> >idle mixture. With older models you simply back off the idle mixture
> >screw, until the engine is getting a rich enough mixture. With the smog
> >controlled models, there is
> >usually something to prevent that.
>
> Plastic caps that are easily removed with a pocket/exacto knife or the
> tip of a soldering iron (both are standard shop practices).
>
Quite. If you are going to do that, it is a good idea to record the number
of turns out for the factory setting. Just in case you later decide, that
the problem was something else.

Earle

Nope - no puking of oil into the air cleaner.
I'll do the various leak checks that several of you have suggested.
Not sure if there's any "plastic caps" left anywhere on the carb that
may be hiding mixture adjusting screws. Approx where on the carb will
they be located?

Lots of great advice - even from the couple old guys (says Lynn with
the full head of white hair!!)

Thanks!


Earle Horton wrote:
> "Hootowl" > wrote in message
> ...
> > On Tue, 13 Jun 2006 08:42:40 -0600, "Earle Horton"
> > > wrote:
> >
> > >The most common cause for backfiring on deceleration is idle mixture too
> > >lean. That could be a vacuum leak with this vintage. They really do
> > >have too many vacuum lines, don't they? If that is not it, then short
> > >of a new or rebuilt carburetor, I don't know how you get a proper
> > >idle mixture. With older models you simply back off the idle mixture
> > >screw, until the engine is getting a rich enough mixture. With the smog
> > >controlled models, there is
> > >usually something to prevent that.
> >
> > Plastic caps that are easily removed with a pocket/exacto knife or the
> > tip of a soldering iron (both are standard shop practices).
> >
> Quite. If you are going to do that, it is a good idea to record the number
> of turns out for the factory setting. Just in case you later decide, that
> the problem was something else.
>
> Earle

Try looking somewhere low, about the level of the throttle plate. You will
see a plastic cover, over an idle mixture screw. Like Hootowl says, you can
pop, cut or melt it off, depending on tools available and what works best.
In the late seventies, early eighties we used to take these off all the
time. But we found out, that if this was necessary, there was usually
another cause. Sometimes, we just didn't like the factory setting. I guess
changing it qualifies as "emissions control tampering".

Although you can tune the idle mixture "by ear" or with a tach., it is
helpful to have some kind of exhaust gas analyzer for the proceedings.
Since you probably don't have one, I would recommend, that if adjusting the
idle mixture helps, do it just enough to eliminate the backfire on
deceleration.

Earle

> wrote in message
ups.com...
> Nope - no puking of oil into the air cleaner.
> I'll do the various leak checks that several of you have suggested.
> Not sure if there's any "plastic caps" left anywhere on the carb that
> may be hiding mixture adjusting screws. Approx where on the carb will
> they be located?
>
> Lots of great advice - even from the couple old guys (says Lynn with
> the full head of white hair!!)
>
> Thanks!
>
>
> Earle Horton wrote:
> > "Hootowl" > wrote in message
> > ...
> > > On Tue, 13 Jun 2006 08:42:40 -0600, "Earle Horton"
> > > > wrote:
> > >
> > > >The most common cause for backfiring on deceleration is idle mixture
too
> > > >lean. That could be a vacuum leak with this vintage. They really do
> > > >have too many vacuum lines, don't they? If that is not it, then
short
> > > >of a new or rebuilt carburetor, I don't know how you get a proper
> > > >idle mixture. With older models you simply back off the idle mixture
> > > >screw, until the engine is getting a rich enough mixture. With the
smog
> > > >controlled models, there is
> > > >usually something to prevent that.
> > >
> > > Plastic caps that are easily removed with a pocket/exacto knife or the
> > > tip of a soldering iron (both are standard shop practices).
> > >
> > Quite. If you are going to do that, it is a good idea to record the
number
> > of turns out for the factory setting. Just in case you later decide,
that
> > the problem was something else.
> >
> > Earle
>



--
Posted via a free Usenet account from http://www.teranews.com

If the idle mix screws haven't been messed with recently, they are not
the issue. I would touch them only as a last resort. There are two of
them in the front of the base plate.

You are far more likely to have a loose manifold or a carboned up EGR
valve. The carb itself is known to come loose at the base plate too!

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
Jan/06 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)

" wrote:
>
> Nope - no puking of oil into the air cleaner.
> I'll do the various leak checks that several of you have suggested.
> Not sure if there's any "plastic caps" left anywhere on the carb that
> may be hiding mixture adjusting screws. Approx where on the carb will
> they be located?
>
> Lots of great advice - even from the couple old guys (says Lynn with
> the full head of white hair!!)
>
> Thanks!
>
> Earle Horton wrote:
> > "Hootowl" > wrote in message
> > ...
> > > On Tue, 13 Jun 2006 08:42:40 -0600, "Earle Horton"
> > > > wrote:
> > >
> > > >The most common cause for backfiring on deceleration is idle mixture too
> > > >lean. That could be a vacuum leak with this vintage. They really do
> > > >have too many vacuum lines, don't they? If that is not it, then short
> > > >of a new or rebuilt carburetor, I don't know how you get a proper
> > > >idle mixture. With older models you simply back off the idle mixture
> > > >screw, until the engine is getting a rich enough mixture. With the smog
> > > >controlled models, there is
> > > >usually something to prevent that.
> > >
> > > Plastic caps that are easily removed with a pocket/exacto knife or the
> > > tip of a soldering iron (both are standard shop practices).
> > >
> > Quite. If you are going to do that, it is a good idea to record the number
> > of turns out for the factory setting. Just in case you later decide, that
> > the problem was something else.
> >
> > Earle