Vacuum Reading / carb adjustments - Team Camaro Tech
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post #1 of 11 (permalink) Old Jan 5th, 19, 01:25 PM Thread Starter
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Dale
 
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Vacuum Reading / carb adjustments

I have a 67 RS/SS 350 all stock. Engine runs smooth but rich so I thought the carb mixer screws need adjustment. I connected a vacuum gauge to the transmission port located on the intake that's shared with the bake booster and I'm reading between 5-10 as the needle bounces back and forth. If I move the screws no change in readings and I don't have any carb or manifold leaks. Motor dies when I cover the carb with my hands. I believe I should have vacuum in the 20 range any thougts?
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post #2 of 11 (permalink) Old Jan 5th, 19, 01:41 PM
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Re: Vacuum Reading / carb adjustments

What carb ?

68 Camaro~LSx ~all motor
1.54 60'--6.95 @ 98.45 660'--10.96 @ 121.53
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post #3 of 11 (permalink) Old Jan 5th, 19, 01:57 PM
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Re: Vacuum Reading / carb adjustments

Also find a smaller vacuum port as that large of a pipe will show every pulse in the manifold (why the needle is fluctuating so wildly). A smaller (eighth inch tube compared to a three eighth inch tube) will average out those pulses for a steadier needle and easier read of your vacuum in the manifold.

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post #4 of 11 (permalink) Old Jan 5th, 19, 03:09 PM Thread Starter
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Re: Vacuum Reading / carb adjustments

Carb is a Rochester Quad (was rebuilt) and as for vacuum ports I don't have any other options on the manifold.
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post #5 of 11 (permalink) Old Jan 5th, 19, 03:49 PM
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Re: Vacuum Reading / carb adjustments

Quadrajet…..I'm no help there.

68 Camaro~LSx ~all motor
1.54 60'--6.95 @ 98.45 660'--10.96 @ 121.53
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post #6 of 11 (permalink) Old Jan 5th, 19, 08:21 PM
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Re: Vacuum Reading / carb adjustments

If you are using the tranny vac side that, at least on mine, is the smaller 1/8" size vs the other side of the "T" fitting feeding the brake booster. If your tranny vac side is as large a port as brake boster use reducer fitting into the vac gauge

Typical low vac would be a leak somewhere but you noted you don't have a vac leak. I use a can of spray starter fluid to see if idle increases around manifold/carb

FWIW my rebuilt 327 has about 19". Camed motors would have less. Had same vacuum with the Rodchester 2 brl as it does now with the Edelbrock 600 I put on

IDK if the Quad, for whatever reason, inherently knocks vacuum down to 5-10 (not seeing how it would without a big vac leak) your vacuum seems quite low to me

What is your timing at idle?
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post #7 of 11 (permalink) Old Jan 6th, 19, 07:22 AM Thread Starter
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Re: Vacuum Reading / carb adjustments

Timing is 4* I played with the timing but didn't see any change.
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post #8 of 11 (permalink) Old Jan 6th, 19, 08:23 AM
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Re: Vacuum Reading / carb adjustments

Quote:
Originally Posted by dc7432 View Post
Timing is 4* I played with the timing but didn't see any change.
wow...4 degrees at idle

Have you tried bringing it up to 12-15 at idle?

I recently did a 2 brl to 4 brl swap. I installed Timing mark tape on damper and set idle timing starting at 7 degrees advance. I used the unmetered vac port of new carb for distributor advance. Adjusted idle RPM to around 750 and got good (19 in) high steady vacuum. What I found though is "total" advance only was 23-25 at 2500 rpm. The vacuum advance itself was about 16-18 degrees max. Motor ran well but felt, as I was advised by several SBC owners, 34-35 would yield more performance.

It was suggested I just set timing for "Total" advance not using any vacuum and plugging vac to distributor and letting "idle" advance be what it may. It is about 16 now and vacuum is steady at 19 as it was 15 and needle slightly bouncing when I just set timing at idle only using distributor vac advance.


If you have a timing light you might want to play with the timing so you get much more vacuum than you have. You will need to adjust idle RPM (it will likely increase) as you increase timing. Once you have timing and RPM good then adjust idle lean mix on carb
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post #9 of 11 (permalink) Old Jan 12th, 19, 04:16 PM Thread Starter
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Re: Vacuum Reading / carb adjustments

Oops 14 degree forgot the "1". I may try moving the distributed a tooth as I can get any more out of the timing.
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post #10 of 11 (permalink) Old Jan 12th, 19, 05:17 PM
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Re: Vacuum Reading / carb adjustments

14 degrees of timing at idle is normal

You can just loosen distributor and advance or retard some just to see if either direction adds more vacuum. Typically a slight advance does add some and also increases idle RPM so if that is the case you just adjust idle down some if vacuum did increase with a little more timing.
Once you get idle/vacuum good you then adjust lean idle mix one side at a time
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post #11 of 11 (permalink) Old Jan 14th, 19, 02:36 PM
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Re: Vacuum Reading / carb adjustments

There seems to be some ambiguity here. Here's my example. Relatively mild 327.
Base timing 14 degrees, vac can disconnected and line plugged. RPM around 600.
With can connected, 26-28 degrees at idle. RPM around 900.
Vac gauge reading, vac can connected, at 900 rpm, ~ 18 in. steady.

Don't go by the vac gauge reading with the vac advance disconnected. Late timing gives lower vac readings.
https://www.onallcylinders.com/2016/...auge-readings/
Adjust your timing, then carb and you should be able to get 18-20 in on a stock-cammed, no vac leaking engine.

'67 rs - ordered new by my Grandfather
327 L30, K-K, Deluxe int, tach & gauges, 12 bolt posi, 4 speed.

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