Low engine vacume

Mopar1 · 05-23-2002, 06:21 PM

I measured engine vacume today and the gauge read 8. will a high lift cam (.572 or so ) cause this. Engine only has 4000 miles on it

exgmguy · 05-23-2002, 06:37 PM

That is way too low. Maybe you have mis-adjusted valves, or just a major vacuum leak.

PKRWUD · 05-24-2002, 02:22 AM

Cams with alot of duration have lower vacuum readings, but not 8". You should be 12" or so. What does the needle do at idle? is it steady, does it bounce, does it drift? If you were to come to an unexpected panic stop, would the engine stall?

Take care,
-Chris

Mopar1 · 05-24-2002, 05:56 AM

Its wanders a litlle bit and on a panic stop it does not stall. the air pump had been removed. The injectors are kind of old. I dont think its a major intake leak, it doesnt drive like it has 1

PKRWUD · 05-24-2002, 06:13 AM

If it would not stall on a panic stop, then you do not have a vacuum leak, or at least not one large enough to effect the manifold vacuum at idle.

Okay then, 4 questions for you:

1) What compression pistons are you running?

2) What is your ignition timing set to?

3) Is your cam installed straight up, or did you advance/retard the cam when you installed it?

4) Have you done a compression test yet, and if so, what were the numbers?

Take care,
-Chris

Mopar1 · 05-24-2002, 08:53 AM

Have no idea on pistons or cam advance/retard did not build the motor.

Havent done a comperssion check

and timing was 10* but reset at 12*
when its idling there is an intermitant shake to the motor

PKRWUD · 05-24-2002, 09:03 AM

Make sure spark plug wires #5 & #6 are not touching AT ALL from the cap to the plugs, and see if that helps the miss.

Run a wet and dry compression test, dry first. Make sure you remove all 8 plugs before you start, and block open the throttle. Post your results

Take care,
-Chris

Mopar1 · 05-24-2002, 11:25 AM

ill do the check this weekend, thanx and ill post

PKRWUD · 05-24-2002, 11:40 AM

Just so that you know what I'm looking for, the 3 main causes of extremely low manifold vacuum, other than a leak, are retarded ignition timing, retarded valve timing, or low compression.

Take care,
-Chris

05-23-2002, 06:21 PM	#1
Mopar1 Registered Member Join Date: Mar 2002 Location: Lakewood, Wa Posts: 161	Low engine vacume I measured engine vacume today and the gauge read 8. will a high lift cam (.572 or so ) cause this. Engine only has 4000 miles on it __________________ '77 Aspen R/T: 318 w/273 heads, 4spd, Hurst Shifter, 600 holley, edelbrock intake, 8 3/4 axle, 3.23 sure-grip, dual 3" exhaust, MP cam 276 / 276 duration 490 / 490 mech solid lifter, Super Stock Springs (15.65@88 3900lbs) '91 Mustang LX: .030 Bored, Typhoon Intake, 4.10's, Lentech VB, BBK 70mm TB, FMS A/C elim bracket, 19lbs inject, off road H-pipe, 75mm pro-m MAF, FRP unequal shorty, stormin normin hood. 3420 Lbs w/ 220 (me) and 13 gal gas. '02 SS Camaro

05-23-2002, 06:37 PM	#2
exgmguy Registered Member Join Date: Feb 1999 Location: Utica, Michigan Posts: 2,631	That is way too low. Maybe you have mis-adjusted valves, or just a major vacuum leak. __________________ 1988 Mustang GT stock bottom 302, C4 10.9 @ 128 with 8 psi of Novi 1000 boost!----STOLEN 1989 Mustang notchback Warmed up 306 with a Procharger.

05-24-2002, 02:22 AM	#3
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Cams with alot of duration have lower vacuum readings, but not 8". You should be 12" or so. What does the needle do at idle? is it steady, does it bounce, does it drift? If you were to come to an unexpected panic stop, would the engine stall? Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

05-24-2002, 05:56 AM	#4
Mopar1 Registered Member Join Date: Mar 2002 Location: Lakewood, Wa Posts: 161	Its wanders a litlle bit and on a panic stop it does not stall. the air pump had been removed. The injectors are kind of old. I dont think its a major intake leak, it doesnt drive like it has 1 __________________ '77 Aspen R/T: 318 w/273 heads, 4spd, Hurst Shifter, 600 holley, edelbrock intake, 8 3/4 axle, 3.23 sure-grip, dual 3" exhaust, MP cam 276 / 276 duration 490 / 490 mech solid lifter, Super Stock Springs (15.65@88 3900lbs) '91 Mustang LX: .030 Bored, Typhoon Intake, 4.10's, Lentech VB, BBK 70mm TB, FMS A/C elim bracket, 19lbs inject, off road H-pipe, 75mm pro-m MAF, FRP unequal shorty, stormin normin hood. 3420 Lbs w/ 220 (me) and 13 gal gas. '02 SS Camaro

05-24-2002, 09:03 AM	#7
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Make sure spark plug wires #5 & #6 are not touching AT ALL from the cap to the plugs, and see if that helps the miss. Run a wet and dry compression test, dry first. Make sure you remove all 8 plugs before you start, and block open the throttle. Post your results Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

05-24-2002, 06:13 AM	#5
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	If it would not stall on a panic stop, then you do not have a vacuum leak, or at least not one large enough to effect the manifold vacuum at idle. Okay then, 4 questions for you: 1) What compression pistons are you running? 2) What is your ignition timing set to? 3) Is your cam installed straight up, or did you advance/retard the cam when you installed it? 4) Have you done a compression test yet, and if so, what were the numbers? Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

05-24-2002, 08:53 AM	#6
Mopar1 Registered Member Join Date: Mar 2002 Location: Lakewood, Wa Posts: 161	Have no idea on pistons or cam advance/retard did not build the motor. Havent done a comperssion check and timing was 10* but reset at 12* when its idling there is an intermitant shake to the motor __________________ '77 Aspen R/T: 318 w/273 heads, 4spd, Hurst Shifter, 600 holley, edelbrock intake, 8 3/4 axle, 3.23 sure-grip, dual 3" exhaust, MP cam 276 / 276 duration 490 / 490 mech solid lifter, Super Stock Springs (15.65@88 3900lbs) '91 Mustang LX: .030 Bored, Typhoon Intake, 4.10's, Lentech VB, BBK 70mm TB, FMS A/C elim bracket, 19lbs inject, off road H-pipe, 75mm pro-m MAF, FRP unequal shorty, stormin normin hood. 3420 Lbs w/ 220 (me) and 13 gal gas. '02 SS Camaro

05-24-2002, 11:25 AM	#8
Mopar1 Registered Member Join Date: Mar 2002 Location: Lakewood, Wa Posts: 161	ill do the check this weekend, thanx and ill post __________________ '77 Aspen R/T: 318 w/273 heads, 4spd, Hurst Shifter, 600 holley, edelbrock intake, 8 3/4 axle, 3.23 sure-grip, dual 3" exhaust, MP cam 276 / 276 duration 490 / 490 mech solid lifter, Super Stock Springs (15.65@88 3900lbs) '91 Mustang LX: .030 Bored, Typhoon Intake, 4.10's, Lentech VB, BBK 70mm TB, FMS A/C elim bracket, 19lbs inject, off road H-pipe, 75mm pro-m MAF, FRP unequal shorty, stormin normin hood. 3420 Lbs w/ 220 (me) and 13 gal gas. '02 SS Camaro

05-24-2002, 11:40 AM	#9
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Just so that you know what I'm looking for, the 3 main causes of extremely low manifold vacuum, other than a leak, are retarded ignition timing, retarded valve timing, or low compression. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

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