Vacuum gauge readings- buzzy needle - Team Camaro Tech
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post #1 of 9 (permalink) Old Nov 20th, 14, 04:00 PM Thread Starter
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Vacuum gauge readings- buzzy needle

SBC 350 with 9.25 compression.
ported 041 heads with 2.02 and 1.6 valves fresh valve job and seats, 63cc combust. chambers.
600 vac sec Holley
Eddy Perf RPM intake.
Cheapy Jegs 1 5/8 long tube headers.
Comp HE268H cam with 1.6 rockers.
218 @ .050 and .454 lift with 1.5 rockers, 1.6 is .483 lift and 220 @.050 duration. 110 lsa with 106 int c/l. (4 deg adv ground in)

My vac gauge has an extreme rapid flutter on the needle at idle reading between 10 and 12 in/hg in drive. Has a good choppy idle. In park its about 18 in/hg and still has the 2 deg rapid flutter on it.

The old stock 275 L30 cam had the gauge needle steady as could be.

The question is, is it because ita on a 110 lsa and 220 dur?
Normal?
Checked for intake leaks, and valves are adjusted spot on. Just chicken checking...

- John
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'67 V8 Camaro - 12637, 797-Z (Parchment), M-M (Royal Plum), 2M, 3K, 5Y
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post #2 of 9 (permalink) Old Nov 20th, 14, 04:46 PM
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Re: Vacuum gauge readings- buzzy needle

the vac, unless a radical racing cam, should be fairly steady...you may have a cam lobe issue (as I did) or a sticking valve...

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post #3 of 9 (permalink) Old Nov 20th, 14, 04:48 PM
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Anne
 
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Re: Vacuum gauge readings- buzzy needle

I would think this is normal.

You have some valve overlap that will cause pulsing, especially at idle.

But a 268 cam should have 12 to 14" vacuum, not 10-12. Do you have your vacuum advance hooked up to manifold vacuum ?

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post #4 of 9 (permalink) Old Nov 20th, 14, 05:12 PM
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Re: Vacuum gauge readings- buzzy needle

110 LSA is going to be a rougher idle than a 112. Smoother at 115.
Mines 112 and at idle the vacuum fluctuates between 9-10".
I'm using a B28 VA can connected to full manifold vacuum with a
limiter plate I made to get a better idle.
16* initial, 36* total. Timing at idle with VA connected is limited to 20.5*.

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post #5 of 9 (permalink) Old Nov 20th, 14, 05:19 PM Thread Starter
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Re: Vacuum gauge readings- buzzy needle

Quote:
Originally Posted by Lawrence454 View Post
I would think this is normal.

You have some valve overlap that will cause pulsing, especially at idle.

But a 268 cam should have 12 to 14" vacuum, not 10-12. Do you have your vacuum advance hooked up to manifold vacuum ?
Yes. Full manifold vac.
I do have the idle turned down a bit in drive, (500) but in park the needle is still a blur

mmm, yeah could be more like 12-14 in drive.....

- John
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post #6 of 9 (permalink) Old Nov 21st, 14, 04:42 AM
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Re: Vacuum gauge readings- buzzy needle

As said, wider LSA, smoother idle, steadier needle due to intake valve timing.
As engine increases RPM's, vacuum will increase and steady out due to shorter time between valve events, meter movement cannot react fast enough, so the movement is dampened, or averaged out.

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post #7 of 9 (permalink) Old Nov 21st, 14, 07:06 AM
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Re: Vacuum gauge readings- buzzy needle

Some interesting reading.
https://www.centuryperformance.com/forum/forum.php

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post #8 of 9 (permalink) Old Nov 21st, 14, 07:40 AM
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I have seen this caused by weak valve springs. How old are they? And when the valve job was done, did the springs get thrown on, our measured and shimmed to spec? This is often overlooked and can have significant impacts on performance.
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post #9 of 9 (permalink) Old Nov 21st, 14, 08:58 AM Thread Starter
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Re: Vacuum gauge readings- buzzy needle

Quote:
Originally Posted by TheSmithFamily View Post
I have seen this caused by weak valve springs. How old are they? And when the valve job was done, did the springs get thrown on, our measured and shimmed to spec? This is often overlooked and can have significant impacts on performance.
Brand new. Shimmed.


Reelknots:
That was good reading.

I feel better now, with everybodys above posts.
Thanks for the info.

- John
Misfit tortured soul. Lone wolf. Unicorn.
'67 V8 Camaro - 12637, 797-Z (Parchment), M-M (Royal Plum), 2M, 3K, 5Y
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