Bouncing vacuum at idle - Chevelle Tech
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post #1 of 26 (permalink) Old Apr 6th, 20, 2:47 PM Thread Starter
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Bouncing vacuum at idle

Hi all,

Sbc400 with standard iron heads

When i connect my vacuum gauge to my full vacuum port on the carb i get irratic vacuum readings between 13-16Hg.

I already put in new sparkplug + cables.

Last autumn I let a mechanic adjusted the valve lash, some where off a bit but before and after the adjustments still the same readings. The fluctuation is very rapidly.

I also checked compression all are between 7,5 and 8. I guess that on the low side but at least all the same.


I have no leak from heads to intake and from intake to carb. I sprayed a lot of breakcleaner onto the seals but no response on idle rpm

What could be the cause? And what can i check?

Ps. I run 22deg advance at idle with connected vacuum advance (dont know if thats relevant for now)
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post #2 of 26 (permalink) Old Apr 6th, 20, 2:54 PM
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Re: Bouncing vacuum at idle

Usually means uneven compression but you say yours are all the same. Maybe something acting up while it's running like a valve, broken ring,. or head gasket
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post #3 of 26 (permalink) Old Apr 6th, 20, 6:09 PM
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Re: Bouncing vacuum at idle

Which method did the mechanic use to adjust the valves, if it wasn't the E-O, I-C, find someone to get the adjustments right.

What is initial timing???
How much vacuum advance timing at idle with vacuum advance on full manifold vacuum sourcing???

What idle RPM???
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post #4 of 26 (permalink) Old Apr 7th, 20, 1:32 AM Thread Starter
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Quote:
Originally Posted by Dave Ray View Post
Which method did the mechanic use to adjust the valves, if it wasn't the E-O, I-C, find someone to get the adjustments right.

What is initial timing???
How much vacuum advance timing at idle with vacuum advance on full manifold vacuum sourcing???


What idle RPM???

I really dont know how the adjustment was done. What is E-O I-C?

Initial timing is 13 degrees. 9 deg vacuum added at idle connected to full vacuum port. Runs on propane

600rpm idle aprox

Last edited by Bart Fransen; Apr 7th, 20 at 1:49 AM.
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post #5 of 26 (permalink) Old Apr 7th, 20, 8:26 AM
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Re: Bouncing vacuum at idle

All we used to have for engine diagnosis was feeler gauges, vacuum gauge, ears and seat of the pants. I got very used to consulting charts like the ones here. https://www.crankshaftcoalition.com/...gine_diagnosis

The early timing lights were crap. I would always tune to a vacuum gauge in the early 70s. Even after finally getting a nice inductive timing light in 1974 I didnít always trust it to be optimal.

I donít like being manipulated!
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post #6 of 26 (permalink) Old Apr 7th, 20, 8:31 AM
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Re: Bouncing vacuum at idle

What camshaft?

*69 SS 502 EFI /224-228* HR/9.8:1/T56/3.90/ 4000lbs RW 11.5/124mph
*96 Stang 6.0 turbo 10.5/135mph (9.13 ET w/E85 and 16psi) **SOLD**
*2011 Mustang GT premium
*61" Scag Kohler 22hp 7mph
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post #7 of 26 (permalink) Old Apr 7th, 20, 11:01 AM Thread Starter
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Originally Posted by 69-CHVL View Post
What camshaft?
I guess standard once
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post #8 of 26 (permalink) Old Apr 8th, 20, 4:24 PM
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Re: Bouncing vacuum at idle

I know propane can stand both more compression ratio, and ignition timing, I'd go for 16 INITIAL, 10 VACUUM ADVANCE, on yoour full manifold vacuum, and a total of 38 to 40 deg/BTDC.
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post #9 of 26 (permalink) Old Apr 8th, 20, 5:25 PM
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Re: Bouncing vacuum at idle

I'd get rid of the vacuum advance for starters.
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post #10 of 26 (permalink) Old Apr 8th, 20, 6:40 PM
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Hahaha, getting rid of vacuum Advance is a great way to get rid of some free gas mileage, unload some off idle driveability and, self destruct some idle quality !
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post #11 of 26 (permalink) Old Apr 8th, 20, 7:18 PM
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Re: Bouncing vacuum at idle

Quote:
Originally Posted by Reelysalty View Post
Hahaha, getting rid of vacuum Advance is a great way to get rid of some free gas mileage, unload some off idle driveability and, self destruct some idle quality !
Indeed it is!

'71 chevelle, 468, TH400, 12 bolt posi w/3.08's, drinks a lot of 87 octane.
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post #12 of 26 (permalink) Old Apr 9th, 20, 11:30 AM
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Re: Bouncing vacuum at idle

Leave the vacuum advance on, make sure it is connected to full vacuum and not ported vacuum. Connect your gauge to full vacuum and look at this. Listen to Dave Ray
https://www.chevellestuff.net/tech/a...cuum_gauge.htm

Have you ever stopped to think and forgot to start again??
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post #13 of 26 (permalink) Old Apr 10th, 20, 1:15 AM Thread Starter
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Originally Posted by suprdave1 View Post
Leave the vacuum advance on, make sure it is connected to full vacuum and not ported vacuum. Connect your gauge to full vacuum and look at this. Listen to Dave Ray <img src="https://www.chevelles.com/forums/images/smilies/thumbsup.gif" border="0" alt="" title="Thumbsup" class="inlineimg" /><img src="https://www.chevelles.com/forums/images/smilies/thumbsup.gif" border="0" alt="" title="Thumbsup" class="inlineimg" />
https://www.chevellestuff.net/tech/a...cuum_gauge.htm
That means one of my valves are burned....
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post #14 of 26 (permalink) Old Apr 10th, 20, 1:20 AM
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Re: Bouncing vacuum at idle

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Originally Posted by Bart Fransen View Post
That means one of my valves are burned....
Entirely possible.

Randy
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Owning a Chevelle project is like having a giant-size piggy bank that you keep pouring money into....but you can't get it back out.
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post #15 of 26 (permalink) Old Apr 10th, 20, 7:06 AM
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Re: Bouncing vacuum at idle

Quote:
Originally Posted by Bart Fransen View Post

I also checked compression all are between 7,5 and 8. I guess that on the low side but at least all the same.
What does this mean, the "7,5 and 8"? If you have anykind of cam, the vac gauge will bounce a bit.

What is your idle speed?

*69 SS 502 EFI /224-228* HR/9.8:1/T56/3.90/ 4000lbs RW 11.5/124mph
*96 Stang 6.0 turbo 10.5/135mph (9.13 ET w/E85 and 16psi) **SOLD**
*2011 Mustang GT premium
*61" Scag Kohler 22hp 7mph
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