Weak Vacuum & Pulse in Idle - Team Camaro Tech
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post #1 of 17 (permalink) Old Jun 24th, 14, 09:59 AM Thread Starter
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Weak Vacuum & Pulse in Idle

Hi All,

I have another engine question. My relatively stock 350 is only pulling about 10" of vacuum at idle. I have sprayed brake cleaner everywhere and besides the usual pulse in the idle that I seem to have the RPM does not change one bit so the Intake\Carb\ entire outside of the engine is sealed tight.

So like I mentioned my motor has a slight pulse that is not quite regular. She idles about 800RPM but every once in a while dips to about 600 for about a second and then right back up to 800. The odd thing is that it is not consistent, like every 4 seconds it does it it seems to be fairly random but frequent if that makes sense.

I have checked just about everything I can think of to check but I am a backyard mechanic at best so what am I missing? Is my motor just getting old and tired?
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post #2 of 17 (permalink) Old Jun 24th, 14, 10:05 AM
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Re: Weak Vacuum & Pulse in Idle

Does the car have power brakes and if so, have you checked the check valve on the vacuum booster?

Have you checked the PCV valve?

Both are items that can cause vacuum leaks and effect performance.

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post #3 of 17 (permalink) Old Jun 24th, 14, 10:09 AM Thread Starter
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Re: Weak Vacuum & Pulse in Idle

No Power breaks and I just checked the PCV, when I put a piece of paper under it it gets sucked onto the valve with a good amount of force.

Last edited by jeremywrags; Jun 24th, 14 at 10:23 AM.
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post #4 of 17 (permalink) Old Jun 24th, 14, 10:42 AM
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Re: Weak Vacuum & Pulse in Idle

Quote:
Originally Posted by jeremywrags View Post
No Power breaks and I just checked the PCV, when I put a piece of paper under it it gets sucked onto the valve with a good amount of force.
Where is your ignition timing set to (initial)?

Are you running a vacuum advance canister?

If yes to previous question, is it connected to ported or manifold vacuum?

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #5 of 17 (permalink) Old Jun 24th, 14, 12:01 PM Thread Starter
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Re: Weak Vacuum & Pulse in Idle

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Originally Posted by NH69Z28RS View Post
Where is your ignition timing set to (initial)?

Are you running a vacuum advance canister?

If yes to previous question, is it connected to ported or manifold vacuum?
The timing is right about 11 deg and I am running a vacuum advance and it is hooked up to the ported vacuum on the Carb.
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post #6 of 17 (permalink) Old Jun 24th, 14, 12:59 PM
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Re: Weak Vacuum & Pulse in Idle

Quote:
Originally Posted by jeremywrags View Post
The timing is right about 11 deg and I am running a vacuum advance and it is hooked up to the ported vacuum on the Carb.
Okay, unless you have an EGR valve and are trying to pass some emissions regulations, you should have the vacuum advance connected to manifold vacuum. I know what the MSD instructions say, ignore them and try moving it to manifold vacuum and then try it and report back?

Also, check the timing at idle once you connect it to the manifold vacuum and see how far the timing advances when at idle on manifold vacuum, that number will be useful to us?

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #7 of 17 (permalink) Old Jun 24th, 14, 01:10 PM
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Re: Weak Vacuum & Pulse in Idle

If your idle mixture screws are not set correct you can see 2-3 inches increase there and if your valve lash is too tight you might be loosing another 2 inches there...

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post #8 of 17 (permalink) Old Jun 24th, 14, 01:56 PM
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Re: Weak Vacuum & Pulse in Idle

I agree with the consensus.
Vacuum - get your base line timing set to factory first, then using a vacuum gauge adjust your idle mixture for highest vacuum. Then tweak for performance as you like.

Pulse - Could also be a fuel pump or bowl or float issue. Could also be a coil failure.


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post #9 of 17 (permalink) Old Jun 24th, 14, 02:14 PM
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Re: Weak Vacuum & Pulse in Idle

Ar the idle and mixture screws 1 1/2 turns out +/- 1/2 turn?
If not the secondaries are most properly set to far open and thats where the 'vaccuum 'leak is.

The PVC, "test with a bit of paper" that doesn't mean its clean and working properly.. take it off and rinse / clean it out in some kero or petrol...

Is the EGR hooked up?

And give the VA onm the dizzy a good suck, see if it holds a vacuum.. the diaphamn inside may have sucked a kumura

My Spelling is not incorrect...it is creative

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post #10 of 17 (permalink) Old Jun 24th, 14, 02:37 PM
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Re: Weak Vacuum & Pulse in Idle

Quote:
Originally Posted by Steptoe View Post
the diaphamn inside may have sucked a kumura
What exactly is a Kumura?

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post #11 of 17 (permalink) Old Jun 24th, 14, 03:31 PM
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Re: Weak Vacuum & Pulse in Idle

Quote:
Originally Posted by cv2065 View Post
What exactly is a Kumura?
It's something a diaphamn sucks on in NZ... LOL!!! or it could be a sweat potato in NZ or both!!!

...Dennis

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post #12 of 17 (permalink) Old Jun 24th, 14, 09:01 PM
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Re: Weak Vacuum & Pulse in Idle

HOLD ON, BEFORE you go further with full manifold vacuum for the vacuum advance, you MUST make a stop for the advance, so it doesn't over time the engine on full manifold vacuum, NO MATTER THE CANISTER USED, STOCK GM OR ADJUSTABLE.

If you don't do the stop, expect issues with the idle timing, as much as 40 degrees of it.

Simple pics and instructions on how to do it two different ways are available, just ask for the ones for the type of distributor you are using, and they will be provided. Send a real world email, and I'll send 'em for you.

[email protected]

Don't use ported vacuum for the vacuum advance on that engine, and don't go full manifold vacuum before you do one of the stops. This also has to be done BEFORE any and all carb adjustments and/or changes.

The 11 degrees initial are OK, 12 better, and the engine wants to IDLE @ 20 to 22 deg/BTDC. Obviously, the initial should NOT be set to 20 to 22 degrees, so, full manifold vacuum added timing is the right way to go, to supplement the idle timing, NOT ported vacuum sourcing. So, the stop must be built to give 10 crankshaft degrees, instructions will come with the pics.
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post #13 of 17 (permalink) Old Jun 24th, 14, 09:16 PM
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Re: Weak Vacuum & Pulse in Idle

Quote:
Originally Posted by DjD View Post
It's something a diaphamn sucks on in NZ... LOL!!! or it could be a sweat potato in NZ or both!!!
local slang... google it

My Spelling is not incorrect...it is creative

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post #14 of 17 (permalink) Old Jun 24th, 14, 09:31 PM
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Re: Weak Vacuum & Pulse in Idle

Quote:
HOLD ON, BEFORE you go further with full manifold vacuum for the vacuum advance, you MUST make a stop for the advance, so it doesn't over time the engine on full manifold vacuum, NO MATTER THE CANISTER USED, STOCK GM OR ADJUSTABLE.

If you don't do the stop, expect issues with the idle timing, as much as 40 degrees of it.

Simple pics and instructions on how to do it two different ways are available, just ask for the ones for the type of distributor you are using, and they will be provided. Send a real world email, and I'll send 'em for you.

[email protected]

Don't use ported vacuum for the vacuum advance on that engine, and don't go full manifold vacuum before you do one of the stops. This also has to be done BEFORE any and all carb adjustments and/or changes.

The 11 degrees initial are OK, 12 better, and the engine wants to IDLE @ 20 to 22 deg/BTDC. Obviously, the initial should NOT be set to 20 to 22 degrees, so, full manifold vacuum added timing is the right way to go, to supplement the idle timing, NOT ported vacuum sourcing. So, the stop must be built to give 10 crankshaft degrees, instructions will come with the pics.
u really should read up up on some older posts ....
eg the difference between a egr and non EGR Dizzies
And how much idle advance a street engine really does require...20 degs thats getting up into dedicated track cam
And as to stops , once again check out older posts u will learn when they are required , what they adjust, and its not just the degs

And take avbit of time out and go thru the Ac delco electrical manual, u will find the number of combos in dizzies run into the 1000s, including VAs so to blanket state
Quote:
HOLD ON, BEFORE you go further with full manifold vacuum for the vacuum advance, you MUST make a stop for the advance, so it doesn't over time the engine on full manifold vacuum, NO MATTER THE CANISTER USED, STOCK GM OR ADJUSTABLE.
is a load of rubish
keep in mind many of us have been dialing in engines moding dizzies for many decades now, going back to good old pionts era....and still do those even today.

My Spelling is not incorrect...it is creative

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post #15 of 17 (permalink) Old Jun 25th, 14, 09:38 AM
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Re: Weak Vacuum & Pulse in Idle

Hey, 'steptoe", I DO DISTRIBUTORS FOR A LIVING, YOU DON'T, and, I wrote quite a lot of the tech on distributor curves, emissions vs non-emissions, so, belt up and stay quiet.

Stop confusing people with totally inaccurate "info" that is not fact, not the way it works, and not sensible.

And, Mr. Wit and Knowledge, most people are NOT aware that the number on the stock vacuum advance is for a unit that has a rubber stop bush over the pull pin, which DOES NOT COME WITH A REPLACEMENT UNIT. Why didn't you tell that to everyone, Mr. Ignition? BECAUSE YOU HAVE NO CLUE.

What I have done with the physical stops, is to make them from metals, so they cannot rot, fall off, be forgotten, they are there, working just like the rubber stop GM use/uses from the get-go, and will not fail. You know so much, why didn't YOU come up with any of that, nor the fixes, nor, the stops??? BECAUSE YOU HAVE NO CLUE.

And, YES, pre-emissions engines do like between 20 and 24 degrees IDLE timing, but, they don't like INITIAL timing set that high, hard to start, so, the vacuum advance supplements the difference from INITIAL to IDLE timing. Full manifold vacuum supplemented added timing degrees are the way to get to the optimum IDLE timing, added to the INITIAL, AFTER the engine starts. THAT'S THE WAY IT WORKS, MR. WONDERFUL.

Now, go browbeat someone else, I know what I am doing, and what I recommend, works, go force your BS on someone else that also has no clue.

Enough is enough, stop your bogus "info".

A disclaimer, NOBODY, NOT ME, NOBODY is forcing anyone to do it my way, steptoe's way, any way, not even the correct way, do it any way you want, but, don't flame me for knowing how to do it right, and posting same, I am not wrong, and steadfastly refuse to post incorrect info, EVER.

Moderators, I, and no-one else here needs this to go on, and I am all for posting what works, some here don't feel that way, and cause me to have to defend my every post, and, I am fed up with it, do something positive, please, NOBODY needs any of this stuff going on, from anyone, including me. When called an Outright LIAR< I WILL DEFEND MYSELF.......FAIRLY WARNED. iT SHOULD NOT HAVE TO BE THIS WAY. DO SOMETHING.
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