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  #46 (permalink)  
Old 01-20-2010, 10:19 AM
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monzter,
this link has just some examples of what a vac gauge in a car will tell you immediately,,,click on each "scenario"

http://www.secondchancegarage.com/public/186.cfm

my favorite "diagnostic" example from my car turned out to be:
a plug wire that was just barely loose on the plug,,,,ign is strong enough that was was firing but with less spark joules,, just that was enough to make the needle "quiver" accelerating...
quiver in this case from not equal power from all cylinders...

a motor is a "vacuum pump" so how well it sucks in A/F (Hg readings) directly correlates to how much power it can produce...
by comparing no load and load Hg readings you can determine how hard is the motor working under load (and how much fuel is feed per cubic foot of air for mpg) ...

"back in the day" vac gauge was a dash mount oem option on some cars....
they were marked similar to the one in the video,,,green for mpg, orange for poor mpg,,,red for power etc...
excellent "watch dog" to help you control that size 12 right foot...
and "diagnostics" to boot...
my daughter-in-law "had" a lead foot and crummy mpg on a good motor until I installed and explained how to use a vac gauge...

the one in blue's video is about $40?,,,use it just for better mpg and it will pay for itself in a very short time...

very very roughly,,,at hwy cruise speed,,,1Hg higher is worth roughly 2mpg and very common for just 2-3mph difference to change the Hg...

Blue does have a adjustable vacuum advance so hopefully this thread will get to that step,,,, to tune total timing b+c+v for best hwy Hg since he is driving 25miles per day...

open plenum = the single piston moving down on the intake stroke is trying to pull a vacuum from the entire intake cubic area and both carb primaries...
dual plane = pulls from only half the total area and only one carb primary=higher hg

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Last edited by red65mustang; 01-20-2010 at 10:29 AM.
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  #47 (permalink)  
Old 01-20-2010, 10:55 AM
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Blue,
just hopefully for better understanding,,, attached is a correct graph representation for the particular accell adjustable vac adv unit on my car....
(different model unit=different graph)

show's you the changes per 3/32 hex full revolution....
same as cent,,,basically everything is adjustable,,,start point/total/*per hg

slim chance but possibly the mechanic turned yours to full off??? is why it is not working now....

he wrongly assumed you were on manifold vacuum for setting the idle base so what else did he do???

edit note: the graph is in dist degrees,,,crank degrees is x 2...
8 turns on my unit adds 28 crank degrees...
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Last edited by red65mustang; 01-20-2010 at 11:01 AM.
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  #48 (permalink)  
Old 01-20-2010, 11:15 AM
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Quote:
but new pick up coil
Take the cap off and make sure the vac advance is connected to the pick up coil. You have to disconnect it to replace the coil. It's possible that the clip broke or fell off, and the vac adv is disconnected.
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  #49 (permalink)  
Old 01-20-2010, 01:13 PM
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freak,
good call,,,likely contributing the other "while driving" performance changes he mentioned...
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  #50 (permalink)  
Old 01-20-2010, 02:57 PM
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Last edited by cobalt327; 01-20-2010 at 08:01 PM. Reason: my mistake
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  #51 (permalink)  
Old 01-20-2010, 04:59 PM
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Quote:
Originally Posted by red65mustang
monzter,
Blue does have a adjustable vacuum advance so hopefully this thread will get to that step,,,, to tune total timing b+c+v for best hwy Hg since he is driving 25miles per day...
You bet! I did not have time to work on it after work. It drove home nice though! However, I did fill the tank today for the first time since runing a full tank since the work and my adjustment.

Before I was getting anywhere between 12-14 mpg pending my foot! Today showed 13.5 mpg. My 25 mile (50 round trip) is on a two lane country road with a few 45 mph zones, one stop sign and one signal light. I usually drive either 50 or 65 mph with a modest 60mph. It HATES 55!!!!!!!!!! It also loves to pass slow pokes or teens in Camaros! I have a good feeling it will get 15mph on any steady 60-65mph trip! Oh, the Jeeps HATES headwinds too it even hates "high pressure" weather from the west!. Kills mpg and mph! Even with the wind not blowing, a "high pressure" moving through the area tends to drop my vacuum by 2-3Hg because of the system movement even though the "wind" is not blowing, when heading into the pressure!

I will try more adjustments tomorrow afternoon!
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  #52 (permalink)  
Old 01-20-2010, 05:58 PM
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Quote:
Originally Posted by red65mustang
monzter,
this link has just some examples of what a vac gauge in a car will tell you immediately,,,click on each "scenario"

http://www.secondchancegarage.com/public/186.cfm

my favorite "diagnostic" example from my car turned out to be:
a plug wire that was just barely loose on the plug,,,,ign is strong enough that was was firing but with less spark joules,, just that was enough to make the needle "quiver" accelerating...
quiver in this case from not equal power from all cylinders...

a motor is a "vacuum pump" so how well it sucks in A/F (Hg readings) directly correlates to how much power it can produce...
by comparing no load and load Hg readings you can determine how hard is the motor working under load (and how much fuel is feed per cubic foot of air for mpg) ...

"back in the day" vac gauge was a dash mount oem option on some cars....
they were marked similar to the one in the video,,,green for mpg, orange for poor mpg,,,red for power etc...
excellent "watch dog" to help you control that size 12 right foot...
and "diagnostics" to boot...
my daughter-in-law "had" a lead foot and crummy mpg on a good motor until I installed and explained how to use a vac gauge...

the one in blue's video is about $40?,,,use it just for better mpg and it will pay for itself in a very short time...

very very roughly,,,at hwy cruise speed,,,1Hg higher is worth roughly 2mpg and very common for just 2-3mph difference to change the Hg...

Blue does have a adjustable vacuum advance so hopefully this thread will get to that step,,,, to tune total timing b+c+v for best hwy Hg since he is driving 25miles per day...

open plenum = the single piston moving down on the intake stroke is trying to pull a vacuum from the entire intake cubic area and both carb primaries...
dual plane = pulls from only half the total area and only one carb primary=higher hg

WOW Thanks for the info Red, I appreciate it a lot, The function of a vacuum gauge has got me totally speechless , didn't know I could diagnose so many things, with it, Grate to know.

Thanks !

You guy's rock
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  #53 (permalink)  
Old 01-20-2010, 06:19 PM
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Quote:
Originally Posted by red65mustang
monzter,
this link has just some examples of what a vac gauge in a car will tell you immediately,,,click on each "scenario"

http://www.secondchancegarage.com/public/186.cfm
Scenario #3 is what I get at idle but only 13Hg
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  #54 (permalink)  
Old 01-20-2010, 07:15 PM
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Quote:
Originally Posted by red65mustang
monzter,
the one in blue's video is about $40?,,,use it just for better mpg and it will pay for itself in a very short time...
Oh, JC Whitey catalog #732L-04, page 107...$29.99.. .normaly $43.99
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  #55 (permalink)  
Old 01-21-2010, 06:55 AM
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Blue,
scenario 3 confirms the motor has a performance cam...

at idle the stock cam makes plenty of HP/TQ (18-20Hg) to overcome the motor counter rotational resistance....

a performance cam is yuch for idle power output so it needs more idle rpms (rotational inertia and A/F speed)...

bump the idle up to 750-800 if you want a dead steady needle but that bounce shown at idle (only) isn't really hurting anything,,,it's reflecting uneven cylinder fill=yuch for power balance=yuch for total power....
(note on that comp cams link,,,bigger the cam=the more idle rpms needed)

"Autogage" vac gauge right?
I have the same one in my car (black face/white script match's my oem dash instruments perfect)...
"BUT" it did not go into the car until I did compare readings results to my never been abused or bumped WIKA brand vac guage testing on the car with both... (calibrated and reads to a .5Hg display for industrial instrumentation)....

it only takes one drop onto a concrete floor to mess up the calibration,,,could be in-transit damage,,,could be JC Whitney buys "seconds" to sell so cheap,,,who knows....
I'm hoping yours "is" broken,,,evidenced by the screwy light load and idle readings!!!
because the next source "suspect" to see those idle and hwy readings results is the torque convertor and tranny!!!
(or dragging brakes!!!)

got another oem car handy/available? hook the gauge up with just a long piece of hose and test drive at 50/60/70 and post results

Last edited by red65mustang; 01-21-2010 at 07:03 AM.
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  #56 (permalink)  
Old 01-21-2010, 07:57 AM
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Blue,
I am enjoying this thread,,,your jeep is doing really funky things based on the Hg readings...
(forcing my feeble brain to overheat)

high pressure is more oxygen atoms per cubic foot air density so Hg should go up not down...
(same as airplane altitude)
but high pressure air is also usually much lower humidity and that can reduce power just a tad....

I have never seen just air high pressure change the cruise Hg because basically it would take a extremely (!!!) low cruise rpms motor power TQ output to weight ratio to cause a 2-3hg change...
(or a screwed up tranny putting a huge additional load on the motor so there is no power left at the wheels)
typical car is only using 60-80 of 200hp available at 2k to maintain speed =18hg cruise=not sensitive to air pressure ....

OR it is a BIG cam on a stock converter and your cruise rpms is below the minumum cam operating power band=yuch power output at only 2k cruise on mechanical timing only (without having more timing help from the non functioning vac adv timing added!!!)

cobalt,,that thread you deleted did add to this thread,,,so darn often just different words chosen does help understanding,,,American english is the worst for being ambiguous.....

monzter,
your welcome...
LOL,,,God I hope his gauge is busted,,,fair chance this thread will cover everything a correct reading vac gauge can tell you by the time it is done,,,Blue keeps adding "new" screwy readings results to it...

Last edited by red65mustang; 01-21-2010 at 08:13 AM.
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  #57 (permalink)  
Old 01-21-2010, 09:32 AM
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cobalt,,,
I just noticed for the very first time you are in Atlanta....

any chance of you just evaluating the Jeep for no more guessing...

where the heck is Hahira,,,I traveled GA alot for business and it doesn't ring a bell at all,,,,dang CRS sucks
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  #58 (permalink)  
Old 01-21-2010, 10:01 AM
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Quote:
Originally Posted by red65mustang
cobalt,,,
I just noticed for the very first time you are in Atlanta....

any chance of you just evaluating the Jeep for no more guessing...

where the heck is Hahira,,,I traveled GA alot for business and it doesn't ring a bell at all,,,,dang CRS sucks
He's nearly to Valdosta, pretty good ways.
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  #59 (permalink)  
Old 01-21-2010, 04:46 PM
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lol, thanks guys! I think its me that busted, not the vacuum gauge.

I think I am incorrect on Scenario 3. That is more in rhythm with the cam. Mine is a bit slower, shows only about 1/2Hg deflection but can be erratic, but not always, in that deflection. Not always the same distance in needle movement!

What I need to do is make a nice video for you guys!!! Its been raining here since yesterday so I have not done much to it. I want to go out and at least check the vacuum lines on the distributor and the Allen scew setting.

Last edited by Blue84JeepCJ7; 01-21-2010 at 05:18 PM.
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  #60 (permalink)  
Old 01-21-2010, 05:26 PM
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Ok, here is a bad video of the vacuum gauge at idle. It is on 13 1/2-14Hg. Temp is still 175 deg. The needle starts out at 13 1/2, jumps to 13 3/4 then back to 13.

http://www.youtube.com/watch?v=2tOVu6ytnJc
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