Low Manifold Vacuum

"problem" performance continues in a rebuilt 2.8L GM .....
Engine now has 4000 miles on it and still has relatively low manifold
vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
inches MORE vac. to adequately run all the vacuum controls: Brake
booster, heater controls, dist. vacuum advance, *power piston* (2SE
carb), etc.

I checked and checked for manifold leaks, reset the damn manifold
gaskets three times.... etc.
Im pretty damn sure the problem is the engine 'mechanicals' .
Compression is 130 psig all cylinders
Mechanical valve timing has been degreed, and verified OK.
Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
and by accurate degreeing + timing light. --- both methods equate to
the same EXACT timing advance.
NO leaking vacuum hoses or controls.
If I remove a vac. hose from the manifold port the engine
**increases*** rpm --- which tells me I dont have a leak.
Poor vacuum causes power piston in carb to lift 'early' (for over-rich
mixture) - if I manually hold power piston / metering rod and remove a
manifold vac hose = same rpm increase.

Engine was broken in with 20 weight straight weight non-detergent oil.

Im about ready to dump a half can of cleanser down the carb to scuff
the cylinder walls in desparation and run the valve lifters with ZERO
lash ......

Your sage advice would be appreciated. I need another 2-3 inches of
vacuum to make all the vac. controls, brake booster and power piston-ed
carb. to operate properly. Any advice of why this damn engine is
running 'short' by about 2-3 inches hg. of vacuum and what to do about
it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.

;-)

That's not really low for a SMOG motor at idle. You may see that's
at the edge of the green zone: http://www.billhughes.com/vacuumgauge.jpg
And you have noticed it's up around twenty at the more efficient
operating Revolutions Per Minute. None of the accessories use vacuum,
unless used.
God Bless America, ßill O|||||||O


Rich Hampel wrote:
>
> "problem" performance continues in a rebuilt 2.8L GM .....
> Engine now has 4000 miles on it and still has relatively low manifold
> vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
> inches MORE vac. to adequately run all the vacuum controls: Brake
> booster, heater controls, dist. vacuum advance, *power piston* (2SE
> carb), etc.
>
> I checked and checked for manifold leaks, reset the damn manifold
> gaskets three times.... etc.
> Im pretty damn sure the problem is the engine 'mechanicals' .
> Compression is 130 psig all cylinders
> Mechanical valve timing has been degreed, and verified OK.
> Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
> and by accurate degreeing + timing light. --- both methods equate to
> the same EXACT timing advance.
> NO leaking vacuum hoses or controls.
> If I remove a vac. hose from the manifold port the engine
> **increases*** rpm --- which tells me I dont have a leak.
> Poor vacuum causes power piston in carb to lift 'early' (for over-rich
> mixture) - if I manually hold power piston / metering rod and remove a
> manifold vac hose = same rpm increase.
>
> Engine was broken in with 20 weight straight weight non-detergent oil.
>
> Im about ready to dump a half can of cleanser down the carb to scuff
> the cylinder walls in desparation and run the valve lifters with ZERO
> lash ......
>
> Your sage advice would be appreciated. I need another 2-3 inches of
> vacuum to make all the vac. controls, brake booster and power piston-ed
> carb. to operate properly. Any advice of why this damn engine is
> running 'short' by about 2-3 inches hg. of vacuum and what to do about
> it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.
>
> ;-)

1st up, I'm not familiar with the 2.8, but is it possible you have a leak in
your brake servo ? You say the problem continues in a new engine ?

Dave Milne, Scotland
'91 Grand Wagoneer, '99 TJ

"Rich Hampel" > wrote in message
...

> "problem" performance continues in a rebuilt 2.8L GM .....
> Engine now has 4000 miles on it and still has relatively low manifold
> vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
> inches MORE vac. to adequately run all the vacuum controls: Brake
> booster, heater controls, dist. vacuum advance, *power piston* (2SE
> carb), etc.
>
> I checked and checked for manifold leaks, reset the damn manifold
> gaskets three times.... etc.
> Im pretty damn sure the problem is the engine 'mechanicals' .
> Compression is 130 psig all cylinders
> Mechanical valve timing has been degreed, and verified OK.
> Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
> and by accurate degreeing + timing light. --- both methods equate to
> the same EXACT timing advance.
> NO leaking vacuum hoses or controls.
> If I remove a vac. hose from the manifold port the engine
> **increases*** rpm --- which tells me I dont have a leak.
> Poor vacuum causes power piston in carb to lift 'early' (for over-rich
> mixture) - if I manually hold power piston / metering rod and remove a
> manifold vac hose = same rpm increase.
>
> Engine was broken in with 20 weight straight weight non-detergent oil.
>
> Im about ready to dump a half can of cleanser down the carb to scuff
> the cylinder walls in desparation and run the valve lifters with ZERO
> lash ......
>
> Your sage advice would be appreciated. I need another 2-3 inches of
> vacuum to make all the vac. controls, brake booster and power piston-ed
> carb. to operate properly. Any advice of why this damn engine is
> running 'short' by about 2-3 inches hg. of vacuum and what to do about
> it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.
>
> ;-)

I would guess that your valves are a bit short on time if you don't have
zero lash on them. My book calls for zero.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

Rich Hampel wrote:
>
> "problem" performance continues in a rebuilt 2.8L GM .....
> Engine now has 4000 miles on it and still has relatively low manifold
> vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
> inches MORE vac. to adequately run all the vacuum controls: Brake
> booster, heater controls, dist. vacuum advance, *power piston* (2SE
> carb), etc.
>
> I checked and checked for manifold leaks, reset the damn manifold
> gaskets three times.... etc.
> Im pretty damn sure the problem is the engine 'mechanicals' .
> Compression is 130 psig all cylinders
> Mechanical valve timing has been degreed, and verified OK.
> Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
> and by accurate degreeing + timing light. --- both methods equate to
> the same EXACT timing advance.
> NO leaking vacuum hoses or controls.
> If I remove a vac. hose from the manifold port the engine
> **increases*** rpm --- which tells me I dont have a leak.
> Poor vacuum causes power piston in carb to lift 'early' (for over-rich
> mixture) - if I manually hold power piston / metering rod and remove a
> manifold vac hose = same rpm increase.
>
> Engine was broken in with 20 weight straight weight non-detergent oil.
>
> Im about ready to dump a half can of cleanser down the carb to scuff
> the cylinder walls in desparation and run the valve lifters with ZERO
> lash ......
>
> Your sage advice would be appreciated. I need another 2-3 inches of
> vacuum to make all the vac. controls, brake booster and power piston-ed
> carb. to operate properly. Any advice of why this damn engine is
> running 'short' by about 2-3 inches hg. of vacuum and what to do about
> it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.
>
> ;-)

I've isolated EVERY exterior control and vacuum body including the
brake booster and am pretty much convinced that its an 'engine'
problem.

Im using an uncalibrated vac. gauge.... the the real symptoms are still
the low brake booster output, the inability to pull down the power
piston in the carb, etc....

In article >, Dave
Milne > wrote:

> 1st up, I'm not familiar with the 2.8, but is it possible you have a leak in
> your brake servo ? You say the problem continues in a new engine ?
>
> Dave Milne, Scotland
> '91 Grand Wagoneer, '99 TJ
>
> "Rich Hampel" > wrote in message
> ...
>
> > "problem" performance continues in a rebuilt 2.8L GM .....
> > Engine now has 4000 miles on it and still has relatively low manifold
> > vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
> > inches MORE vac. to adequately run all the vacuum controls: Brake
> > booster, heater controls, dist. vacuum advance, *power piston* (2SE
> > carb), etc.
> >
> > I checked and checked for manifold leaks, reset the damn manifold
> > gaskets three times.... etc.
> > Im pretty damn sure the problem is the engine 'mechanicals' .
> > Compression is 130 psig all cylinders
> > Mechanical valve timing has been degreed, and verified OK.
> > Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
> > and by accurate degreeing + timing light. --- both methods equate to
> > the same EXACT timing advance.
> > NO leaking vacuum hoses or controls.
> > If I remove a vac. hose from the manifold port the engine
> > **increases*** rpm --- which tells me I dont have a leak.
> > Poor vacuum causes power piston in carb to lift 'early' (for over-rich
> > mixture) - if I manually hold power piston / metering rod and remove a
> > manifold vac hose = same rpm increase.
> >
> > Engine was broken in with 20 weight straight weight non-detergent oil.
> >
> > Im about ready to dump a half can of cleanser down the carb to scuff
> > the cylinder walls in desparation and run the valve lifters with ZERO
> > lash ......
> >
> > Your sage advice would be appreciated. I need another 2-3 inches of
> > vacuum to make all the vac. controls, brake booster and power piston-ed
> > carb. to operate properly. Any advice of why this damn engine is
> > running 'short' by about 2-3 inches hg. of vacuum and what to do about
> > it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.
> >
> > ;-)
>
>

At cruise speeds and with the throttle plate essentially closed there
isnt much difference of manifold vacuum than at idle -- just an inch or
two. .... just low vacuum. Ive isolated and locked-out all the vacuum
accessories one by one to prove to myself that these were not the
cause.



In article >, ßill > wrote:

> That's not really low for a SMOG motor at idle. You may see that's
> at the edge of the green zone: http://www.billhughes.com/vacuumgauge.jpg
> And you have noticed it's up around twenty at the more efficient
> operating Revolutions Per Minute. None of the accessories use vacuum,
> unless used.
> God Bless America, ßill O|||||||O
>
>
> Rich Hampel wrote:
> >
> > "problem" performance continues in a rebuilt 2.8L GM .....
> > Engine now has 4000 miles on it and still has relatively low manifold
> > vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
> > inches MORE vac. to adequately run all the vacuum controls: Brake
> > booster, heater controls, dist. vacuum advance, *power piston* (2SE
> > carb), etc.
> >
> > I checked and checked for manifold leaks, reset the damn manifold
> > gaskets three times.... etc.
> > Im pretty damn sure the problem is the engine 'mechanicals' .
> > Compression is 130 psig all cylinders
> > Mechanical valve timing has been degreed, and verified OK.
> > Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
> > and by accurate degreeing + timing light. --- both methods equate to
> > the same EXACT timing advance.
> > NO leaking vacuum hoses or controls.
> > If I remove a vac. hose from the manifold port the engine
> > **increases*** rpm --- which tells me I dont have a leak.
> > Poor vacuum causes power piston in carb to lift 'early' (for over-rich
> > mixture) - if I manually hold power piston / metering rod and remove a
> > manifold vac hose = same rpm increase.
> >
> > Engine was broken in with 20 weight straight weight non-detergent oil.
> >
> > Im about ready to dump a half can of cleanser down the carb to scuff
> > the cylinder walls in desparation and run the valve lifters with ZERO
> > lash ......
> >
> > Your sage advice would be appreciated. I need another 2-3 inches of
> > vacuum to make all the vac. controls, brake booster and power piston-ed
> > carb. to operate properly. Any advice of why this damn engine is
> > running 'short' by about 2-3 inches hg. of vacuum and what to do about
> > it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.
> >
> > ;-)

Then I would calibrate it, get the wife's fingernail polish out and
make a mark on your gauge and call it eighteen.
God Bless America, ßill O|||||||O
http://www.billhughes.com/

Rich Hampel wrote:
>
> I've isolated EVERY exterior control and vacuum body including the
> brake booster and am pretty much convinced that its an 'engine'
> problem.
>
> Im using an uncalibrated vac. gauge.... the the real symptoms are still
> the low brake booster output, the inability to pull down the power
> piston in the carb, etc....

Yeah... my book has zero lash ... but is defined as frictional contact
on the pushrods, then 3/4 turn on the friction nut.

I dont have the means to do a ''pressurized leak down test' at home ...
so Im just guessing about the valves.

In article >, Mike Romain
> wrote:

> I would guess that your valves are a bit short on time if you don't have
> zero lash on them. My book calls for zero.
>
> Mike
> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
> 88 Cherokee 235 BFG AT's
>
> Rich Hampel wrote:
> >
> > "problem" performance continues in a rebuilt 2.8L GM .....
> > Engine now has 4000 miles on it and still has relatively low manifold
> > vacuum. Vacuum is running 14-15 in hg. at idle and should be a few
> > inches MORE vac. to adequately run all the vacuum controls: Brake
> > booster, heater controls, dist. vacuum advance, *power piston* (2SE
> > carb), etc.
> >
> > I checked and checked for manifold leaks, reset the damn manifold
> > gaskets three times.... etc.
> > Im pretty damn sure the problem is the engine 'mechanicals' .
> > Compression is 130 psig all cylinders
> > Mechanical valve timing has been degreed, and verified OK.
> > Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.)
> > and by accurate degreeing + timing light. --- both methods equate to
> > the same EXACT timing advance.
> > NO leaking vacuum hoses or controls.
> > If I remove a vac. hose from the manifold port the engine
> > **increases*** rpm --- which tells me I dont have a leak.
> > Poor vacuum causes power piston in carb to lift 'early' (for over-rich
> > mixture) - if I manually hold power piston / metering rod and remove a
> > manifold vac hose = same rpm increase.
> >
> > Engine was broken in with 20 weight straight weight non-detergent oil.
> >
> > Im about ready to dump a half can of cleanser down the carb to scuff
> > the cylinder walls in desparation and run the valve lifters with ZERO
> > lash ......
> >
> > Your sage advice would be appreciated. I need another 2-3 inches of
> > vacuum to make all the vac. controls, brake booster and power piston-ed
> > carb. to operate properly. Any advice of why this damn engine is
> > running 'short' by about 2-3 inches hg. of vacuum and what to do about
> > it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.
> >
> > ;-)

I can put in a less strong spring under the power piston in the carb;
but, I'll still be operating the brake booster with insufficient
vacuum.

It all comes back to (and no matter what the 'numbers' on the vac gage
are) is that this engine just isnt getting proper vacuum in the intake
manifold and Im stumped on how to increase the vacuum.

Its not the carb as I have set up the carb on a stationary engine
pulling proper vacuum .... and when I move it to the Jeepster it runs
like SH*T because of the low vacuum. Yeah I can re-jet and fiddle with
different thickness metering rods .... but the real problem is LOW
manifold vacuum !!!!!

Ive never been quite so 'stumped' like this before ... therefore the
posting for HELP!

:-)



In article >, ßill > wrote:

> Then I would calibrate it, get the wife's fingernail polish out and
> make a mark on your gauge and call it eighteen.
> God Bless America, ßill O|||||||O
> http://www.billhughes.com/
>
> Rich Hampel wrote:
> >
> > I've isolated EVERY exterior control and vacuum body including the
> > brake booster and am pretty much convinced that its an 'engine'
> > problem.
> >
> > Im using an uncalibrated vac. gauge.... the the real symptoms are still
> > the low brake booster output, the inability to pull down the power
> > piston in the carb, etc....

haha...you beat me to it Bill!

We use compressors at work and were chasing a leak for a week (I can rhyme,
what a time!). In the end it turned out the leak was in the regulator
guage. Didn't soap there! Ah well!



"L.W. ("ßill") Hughes III" > wrote in message
...
> Then I would calibrate it, get the wife's fingernail polish out and
> make a mark on your gauge and call it eighteen.
> God Bless America, ßill O|||||||O
> http://www.billhughes.com/
>
> Rich Hampel wrote:
>>
>> I've isolated EVERY exterior control and vacuum body including the
>> brake booster and am pretty much convinced that its an 'engine'
>> problem.
>>
>> Im using an uncalibrated vac. gauge.... the the real symptoms are still
>> the low brake booster output, the inability to pull down the power
>> piston in the carb, etc....