Low vacuum output
 

Have installed a rebuilt 2.8L which doesnt develop enough vacuum to
adequately control the carb/emissions stuff, etc.
At high idle it only pulls 14-15" Hg vac. instead of a normal 18-20".
the vac advance to the distrib. is therefore weak and I have had to
manually advance to about 20" BTDC for best (?) HP output and idle rpm.
Due to the low vacuum control, the carburator (Rochester VariJet 2SE) is
corrrespondingly running rich (probably due to bad control of the EGR).
The rich mix plus the inability to advance the dist. correctly has
yielded some 'driveability' probs and poor fuel economy. Have 2 carbs,
.... both carbs perform EXACT same. Current fuel mpg is 16-17.... on
most 2.8s I can usually 'tweak' to 20-22 mpg. Distributer has been
advanced to get maximum vacuum at high idle (about 20 deg BTDC with
centrifugal advance engaged).
Due to rich mix and low vacuum (to the dist advance), I CANT get this
engine to a 'preignition/knock' state (under lugging load) .... can
advance all the way to high speed stumble - all without ANY pre-ignition.
Ive run propane all over the edges of the intake manifold and carb base
without seeing any rpm increase (gasket leakage). Engine idles VERY
smooth and an applied vac gauge on the intake manifold is VERY steady so
Im sure the valve train is properly adjusted (not tight). Dry cylinder
compression is all normal and equal. All vacuum connections/hoses are
integral.
ANY vac hose (either connected to intake manifold or lower throat of
carb - high vac zone of venturi) that is 'pulled' while idling results
in INCREASE or rpm .... instead of 'stumble'.



Any thoughts?

Re: Low vacuum output
 

"Mike Romain" <romainm@sympatico.ca> wrote in message
news:3F2BE721.E960BBD0@sympatico.ca...
> Well, maybe I have been doing it wrong, but I always have checked
> vacuum at normal idle. High idle has the throttle plate open which
> lowers the vacuum...
>
> Meanwhile, low vacuum, running rich, needing to advance the ----
> out of the timing to make it run all screams 'timing chain off a tooth' to
me.
>

Ouch. I have never had one of those V6 timing covers off, but it doesn't
look like much fun.

Earle

> Mike
> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
> 88 Cherokee 235 BFG AT's
>
> RichH wrote:
> >
> > Have installed a rebuilt 2.8L which doesnt develop enough vacuum to
> > adequately control the carb/emissions stuff, etc.
> > At high idle it only pulls 14-15" Hg vac. instead of a normal 18-20".
> > the vac advance to the distrib. is therefore weak and I have had to
> > manually advance to about 20" BTDC for best (?) HP output and idle rpm.
> > Due to the low vacuum control, the carburator (Rochester VariJet 2SE) is
> > corrrespondingly running rich (probably due to bad control of the EGR).
> > The rich mix plus the inability to advance the dist. correctly has
> > yielded some 'driveability' probs and poor fuel economy. Have 2 carbs,
> > ... both carbs perform EXACT same. Current fuel mpg is 16-17.... on
> > most 2.8s I can usually 'tweak' to 20-22 mpg. Distributer has been
> > advanced to get maximum vacuum at high idle (about 20 deg BTDC with
> > centrifugal advance engaged).
> > Due to rich mix and low vacuum (to the dist advance), I CANT get this
> > engine to a 'preignition/knock' state (under lugging load) .... can
> > advance all the way to high speed stumble - all without ANY
pre-ignition.
> > Ive run propane all over the edges of the intake manifold and carb base
> > without seeing any rpm increase (gasket leakage). Engine idles VERY
> > smooth and an applied vac gauge on the intake manifold is VERY steady so
> > Im sure the valve train is properly adjusted (not tight). Dry cylinder
> > compression is all normal and equal. All vacuum connections/hoses are
> > integral.
> > ANY vac hose (either connected to intake manifold or lower throat of
> > carb - high vac zone of venturi) that is 'pulled' while idling results
> > in INCREASE or rpm .... instead of 'stumble'.
> >
> > Any thoughts?

Re: Low vacuum output
 

You should be able to turn that distributor well past normal...to the point
that your idle is way off and it runs funny....if you can't I sure would
consider replacing it. Many people say they don't fail that much and look
elsewhere. Well my distributor looked good...matter of fact the advance
appeared to work as well as the new one outta the box. I dropped the new one in
and problem solved.

Brian
88 YJ

Re: Low vacuum output
 

In message <3F2BC976.4010008@NOSPAM.net>, "RichH" wrote:

>Have installed a rebuilt 2.8L which doesnt develop enough vacuum to
>adequately control the carb/emissions stuff, etc.

Retarded cam timing. Did you put the timing chain on right?

>At high idle it only pulls 14-15" Hg vac. instead of a normal 18-20".
>the vac advance to the distrib. is therefore weak and I have had to
>manually advance to about 20" BTDC for best (?) HP output and idle rpm.
>Due to the low vacuum control, the carburator (Rochester VariJet 2SE) is
>corrrespondingly running rich (probably due to bad control of the EGR).

Bad control of EGR causing rich mixture? Uh, no.

> The rich mix plus the inability to advance the dist. correctly has
>yielded some 'driveability' probs and poor fuel economy. Have 2 carbs,
>... both carbs perform EXACT same. Current fuel mpg is 16-17.... on
>most 2.8s I can usually 'tweak' to 20-22 mpg. Distributer has been
>advanced to get maximum vacuum at high idle (about 20 deg BTDC with
>centrifugal advance engaged).
>Due to rich mix and low vacuum (to the dist advance), I CANT get this
>engine to a 'preignition/knock' state (under lugging load) .... can
>advance all the way to high speed stumble - all without ANY pre-ignition.
>Ive run propane all over the edges of the intake manifold and carb base
>without seeing any rpm increase (gasket leakage). Engine idles VERY
>smooth and an applied vac gauge on the intake manifold is VERY steady so
>Im sure the valve train is properly adjusted (not tight). Dry cylinder
>compression is all normal and equal. All vacuum connections/hoses are
>integral.
>ANY vac hose (either connected to intake manifold or lower throat of
>carb - high vac zone of venturi) that is 'pulled' while idling results
>in INCREASE or rpm .... instead of 'stumble'.
>
>
>
>Any thoughts?