Vacuum question ?
Thread Starter
Burning Brakes
Joined: Feb 2004
Posts: 758
Likes: 14
From: Ville de la Baie Quebec. A winner is just a loser who tried again.
Vacuum question ?
I just purchased a vacuum gauge, and did a quick check on the manifold vacuum, My concern is at idle, form what i've been reading it is normal that the vacuum level is slightly lower because of my agressive cam, but is it normal that the needle jumps (flickers) one or two points when idling. when I give it gas, it steadies out.. This vacuum thing is all new to me, but doe's not seem very complicated, its these little things that only experience can answer. I'v checked the old posts, and even checked a hot rod forum (oups 
),, but nothing to help me.. 
),, but nothing to help me..
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
If it's a slow fluctuation, it's probably some instability in the carb. Idle mixture adjustment, leaking boosters, bad gaskets...
If it bounces, then that's a bigger issue as it would likely be in the valve train.
If it bounces, then that's a bigger issue as it would likely be in the valve train.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by viperjesse
Vacuum leak somewhere???
all those flickers mean something....did yours come with a sheet of paper that relates what the needle is doing to a list of problem areas?
Le Mans Master
Joined: Jun 2002
Posts: 8,339
Likes: 24
From: I can walk to MA
If you are running a high duration cam, and the idle is slow and
lumpy, then you are likely seeing the pressure pulses as each
cylinder draws mixture in. This coupled with some valve overlap
will show the intake vaccum being reduced with overlap to the
exhaust. This would be more pronounced at idle, when the pulses
have time to propagate back to the vacuum port. At higher RPM,
the dynamics of mixture flowing through the manifold will smooth
out any variation.
I'd say you are OK, unless there are other symptoms.
lumpy, then you are likely seeing the pressure pulses as each
cylinder draws mixture in. This coupled with some valve overlap
will show the intake vaccum being reduced with overlap to the
exhaust. This would be more pronounced at idle, when the pulses
have time to propagate back to the vacuum port. At higher RPM,
the dynamics of mixture flowing through the manifold will smooth
out any variation.
I'd say you are OK, unless there are other symptoms.
Drifting
Joined: Oct 2004
Posts: 1,496
Likes: 1
From: Lafayette Louisiana
Originally Posted by NHvette
If you are running a high duration cam, and the idle is slow and
lumpy, then you are likely seeing the pressure pulses as each
cylinder draws mixture in. This coupled with some valve overlap
will show the intake vaccum being reduced with overlap to the
exhaust. This would be more pronounced at idle, when the pulses
have time to propagate back to the vacuum port. At higher RPM,
the dynamics of mixture flowing through the manifold will smooth
out any variation.
I'd say you are OK, unless there are other symptoms.
lumpy, then you are likely seeing the pressure pulses as each
cylinder draws mixture in. This coupled with some valve overlap
will show the intake vaccum being reduced with overlap to the
exhaust. This would be more pronounced at idle, when the pulses
have time to propagate back to the vacuum port. At higher RPM,
the dynamics of mixture flowing through the manifold will smooth
out any variation.
I'd say you are OK, unless there are other symptoms.
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Thread Starter
Burning Brakes
Joined: Feb 2004
Posts: 758
Likes: 14
From: Ville de la Baie Quebec. A winner is just a loser who tried again.
The motor is quite new, I mean it was rebuilt last year and I only put about 3ooo miles on it, it was a total overhaul, so I would be very surprise if it were bigger problems, but then, all is possible. The gauge did come with a paper for trouble shooting, and it is pointing out to valve timing or ignition timing, it also refers to a possible leak or bad compression, I dout the later. The flicker is very constant and minor in my oppinion,,. Is there a way to pinpoint a leak on the manifold ? I recall reading something way back but can't find it.
Melting Slicks
Joined: Sep 2004
Posts: 2,655
Likes: 19
Yes the symptoms can be valve train, ie sticking valves, etc. But my 68 small block has a BIG cam, headers, sidepipes, aftermarket intake, and a very lumpy 600rpm idle and I see the same fast flickeing. Everything checks out OK.
Gary
Gary
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
Sounds completely normal. A car with a bit of a cam will bounce the needle around. I've had cams aggressive enough that the needle bounces so bad you really can't get a vacuum reading unless a restrictor oriface is placed in the gauge line to smooth out the pulses. If you have a lumpy idle, you will have a bouncing needle at idle. You don't have any problem.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
heres those diagnostic instructions:
http://img.photobucket.com/albums/v4...s/00000847.jpg
http://img.photobucket.com/albums/v4...s/00000846.jpg
http://img.photobucket.com/albums/v4...s/00000847.jpg
http://img.photobucket.com/albums/v4...s/00000846.jpg
Pro
Joined: Mar 2001
Posts: 633
Likes: 1
From: Hamilton, ON, Canada
I wouldn't be concerned unless there's another problem such as bogging or hesitatation, etc. I've got a Comp Cams 292H in a 350 running at 6.5" vac at idle. Needle bounces at least 0.5" but smooths out at higher rpm and engine pulls smoothly & quickly from idle. The pulsations are likely due to pressure pulses from the valve overlap. This may suggest that the idle rpm is a bit on the low side. If it concerns you, try increasing idle rpm a bit. Pulsations should decrease or disappear.
Thread Starter
Burning Brakes
Joined: Feb 2004
Posts: 758
Likes: 14
From: Ville de la Baie Quebec. A winner is just a loser who tried again.
Sounds like mine, I realy don't have other symptoms, the engine runs rreal nice and pulls hard. I want to play around with the carb calibration, do a little experimenting, got a calibration kit from edelbrock. Hope to widen my knowledge and maybe improve the performance. This is why it is important to be sure that all these little things are correct..... thanks all !!!!
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
in all of my years of restorations i consider the vacuum gauge one of the best diagnostic tools...if you learn to use it and read it properly you will become a very good engine tuner....good luck
