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Old 05-29-2008, 06:51 AM   #1
Von Rigg Fink
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Default vacuum gauge needle vibrates 1"HG

here is the back ground.
SBC 327 Rochester quadrajet. (stock spread bore). Edlebrock perfomer intake
HEi ignition. comp cam 12-242-2 if you need more info let me know

while at Idle engine has about 14 to 15 inches Hg. vacuum
the needle vibrates somewhere between 13 and 14 Hg consitantly..like a buzz so fast that the needle is a blurr.

at acceleration the needle keeps the same blurr. and while holding the RPM at say 2000 or 2200 or any other elevated RPM the needle continues to keep that blurr or vibration.
also to add to the equation at the elevated RPM there is a uneavenness to the engine, like a stumble.

Tonight i plan on changing the spark plugs, and the carb base gasket to see if this solves or changes anything.

anyone out there ever have this type of senario before? what was your diagnosis? what changed things. should i worry about the rapid needle vibration its only 1" and focus more on the engine stumble?

yes i have gone thru and adjusted and re adjusted the valves , it doesnt remove the 1" needle fluxuation.
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Last edited by Von Rigg Fink; 05-29-2008 at 07:29 AM.
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Old 05-29-2008, 07:45 AM   #2
325w
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Default Re: vacuum gauge needle vibrates 1"HG

Try another gauge. Maybe a vac leak.
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Old 05-29-2008, 08:03 AM   #3
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Default Re: vacuum gauge needle vibrates 1"HG

That's a sign of an engine miss that could be caused by many things.

Read this
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Old 05-29-2008, 08:07 AM   #4
plym_46
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Default Re: vacuum gauge needle vibrates 1"HG

Just posted a link to a site with a bunch of vacuum gauge diagnosis illustrations. It's on the driving by vacuum thread.
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Old 05-29-2008, 08:14 AM   #5
thunderbirdesq
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Default Re: vacuum gauge needle vibrates 1"HG

Valve guides a little sloppy? What's the engine rebuild history?
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Old 05-29-2008, 08:18 AM   #6
hotrod1940
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Default Re: vacuum gauge needle vibrates 1"HG

In the bottom of the drawer there is a folded up piece of paper that has a series of little pictures that show what the different readings mean.
It is under the Playboy magazine.
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Old 05-29-2008, 08:23 AM   #7
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Default Re: vacuum gauge needle vibrates 1"HG

considering the size of your cam, it sounds pretty normal to me. the cam is spec'd to have about 15.5" vac at 800 RPM, so perhaps you can tune idle a bit.

my cam is expected to have slightly less vac than yours (13-14"). the nature of valve overlap gives funky vacuum reading.

if you are concerned about the condition of your motor, i think a proper leak-down test will tell you quite a bit more information.

with a normal cam that pulls nearly 20" at idle, the vacuum gauge would be steady. this data is off of a newly built motor with a hyd-roller cam expected to get 13-14" vacuum, idles at 750 RPM:

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Old 05-29-2008, 09:38 AM   #8
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Default Re: vacuum gauge needle vibrates 1"HG

Most vac gauges I've had used a 1/8 pipe fitting to screw into the manifold/carb, this had a small hole [less than 1/16] in it. That serves to dampen the pulses,without it I would get he same flutter. Not much to worry about.
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Old 05-29-2008, 09:45 AM   #9
Danimal
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Default Re: vacuum gauge needle vibrates 1"HG

Heads are supposed to have been newly gone through. He and I both got sets from the same guy. Looks like Tommy's like leans more towards ignition with the 1" bounce. Heads would tend to be higher, 3-4".

We'd like to get this sorted out so he can drive to Billetproof Detroit in just over a week.
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Old 05-29-2008, 10:10 AM   #10
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Default Re: vacuum gauge needle vibrates 1"HG

Quote:
Originally Posted by tommy View Post
That's a sign of an engine miss that could be caused by many things.

Read this
Yes good site.
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Old 05-29-2008, 10:19 AM   #11
Von Rigg Fink
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Default Re: vacuum gauge needle vibrates 1"HG

im leaning toward the ignition issue..im changing the plugs tonight and the carb base spacer and gaskets tomorrow , each change i am going to test.
i do feel that some of it could be valve overlap and possibly normal for this cam and head set up the valves are 1.94s..1.60's and the vac gauge is new, and it does have that tiny hole in the inlet im feeding it with a 1/4" hose off of my manifold. really the engine seems to run fine..may need a bit of tweeking (timing& mixture). the only thing that is bothersome is the studdering at RPM.
and that could be plugs or carb tuneing and timing all playing with each other.
I will go thru a sytematic elimination of the possibilities so i can pin point what was causing the stumble and or the needle buzz.
If i dont get the needle buzz to disapear but the engine feels healthy i will atribute it to the valve and cam combination.
oh and no the engine wasnt completely rebuilt. rod and mains were re done, cam and lifters and heads were redone. piston bores looked good and didnt have much ridge on the top so I elected to stay out of re-ringing or new pistons.
the vac. readings are not indicating any problems with the lower end..ie piston ring wear etc..new carb, new manifold.
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Old 05-29-2008, 10:59 AM   #12
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Default Re: vacuum gauge needle vibrates 1"HG

Pretty good diognastics here:

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Old 05-29-2008, 11:09 AM   #13
Von Rigg Fink
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Default Re: vacuum gauge needle vibrates 1"HG

yeah nice chart...but it doesnt have the one that shows what my gauge is showing..i have that chart too..thats why when i ran across the reading im getting i was a bit stumped.
im going with the valve overlap thinking..because the engine runs good, and the stumble is unrelated, and i will find what is causing the stumble.
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Old 05-29-2008, 11:27 AM   #14
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Default Re: vacuum gauge needle vibrates 1"HG

I think yours falls into the loose valve guides/burnt valves area...

Is that cam hyd or mech? If mech, check valve lash...
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Old 05-29-2008, 11:34 AM   #15
Von Rigg Fink
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Default Re: vacuum gauge needle vibrates 1"HG

its hyd. lifters..could be loose guides..definetly not burnt..yet. they are new. I hope the dude that did the valve job didnt scrimp, on doing it right.

Flat ernie, what do you think about the possibility of the "valve overlap" issue that was discussed? is it a possibility?
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Old 05-29-2008, 11:37 AM   #16
Von Rigg Fink
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Default Re: vacuum gauge needle vibrates 1"HG

im still going after the ignition system first, than the carb, than if none of that cleans it up..than i will suspect the guides
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Old 05-29-2008, 12:22 PM   #17
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Default Re: vacuum gauge needle vibrates 1"HG

If I understand it correctly the fluctuation that you are seeing is the difference between one cylinder and the other 7. A hot cam usually results in a lower vacuum reading than a stock cam but it should still be relatively stable at that lower reading. I was taught that the bouncing is a result of one cylinder not producing the same amount of vacuum as the others. Others may disagree.
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Old 05-30-2008, 07:19 AM   #18
Von Rigg Fink
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Default Re: vacuum gauge needle vibrates 1"HG

ok i got a little more to go on..
I pulled my plugs last night..i dont know how i fucked this one up, but my plug gap was only at 32 to 35 th...mmm that aint right for an HEI ignition..should be around like 45 right? and yes im talking .045..any way.
as i see one chart does show having some erratic needle movement if the plug gaps are too tight..That will be rectified this weekend and also a new carb base gasket. The low vac. is normal for this cam per manufacture. so its still possible to even out that needle flutter. i guess we will see.

yes i verified the plug gap...R44TS should be gapped at 0.045 for use with HEI ignitions
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Last edited by Von Rigg Fink; 05-30-2008 at 07:27 AM.
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Old 05-30-2008, 07:38 AM   #19
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Default Re: vacuum gauge needle vibrates 1"HG

how much does your RPM vary while idling?

lumpier cams have lumpier idle RPM. remember, without load vac will increase with RPM. watch your vac gauge go up as you lightly rev your motor.
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Old 05-30-2008, 07:42 AM   #20
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Default Re: vacuum gauge needle vibrates 1"HG

Okay, now I gotta watch. I have never heard of that being the problem (except when a plug got bumped and was only gapped to a few thou, and different from the rest).

Keep us posted.
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