Low Manifold Vacuum [Archive] - Chevelle Tech

: Low Manifold Vacuum

Mar 10th, 10, 4:52 PM
Hey guys,

I am trying to tune my carb because it falls flat on it's face when I stomp on it. I hooked up a vacuum guage and it reads that I have about 6 in of vacuum. It seems not to move a bunch when I turn the idle adjust screws. I only have two vacuum lines hooked up on it. One to the brake booster and the other to a pcv valve. I can't remember my cam specs right now but is nothing too radical. Any ideas?

Mar 10th, 10, 9:50 PM
I think anything below 15 in. will give you a really weak idle. Check for vacuum leaks!! Plug your brake booster line first and see- then your PVC. Something -Maybe intake manifold leaks-check with carb cleaner,spray where the runners meet the heads, if it chokes like a dog that's it! Possibly carb plate/gasket or carb plugs?

Mar 10th, 10, 10:14 PM

Mar 10th, 10, 10:43 PM
Sounds like way not enough timing advance.

Verify that your timing is about 36 BTDC at 4000 RPM with the vac adv disconnected from the dist. Optimum might be a couple of degrees away from that, but it should be a good starting point anyway. It goes without saying, verify your timing mark FIRST, that 0 on the mark = EXACTLY TDC; if the mark is off, then nothing you ever do with a light will ever be accurate.

When you "stomp it", your vacuum goes to 0, regardless of what it was right before that. Low idle vacuum doesn't cause the problem you describe. However, low idle vaccum and wimpy throttle response might both share the same cause.

Mar 10th, 10, 11:18 PM
After setting my timing correctly and fuel mixture to highest vacum reading my 454 stumbled bad still off idle. Changed the primary carb squirter from a .025 to a .035 and all is good now.

Mar 10th, 10, 11:44 PM
Got to be a vacuum leak. When I changed my intake, My vacuum went from 9 to 15 in. at idle

Mar 11th, 10, 12:28 AM
1st some questions:

What are cam specs?

You said you cant remember cam specs well look them up or find cam card because we need that cam info so wek now approx what idle/low engine speed vacuum should be along with motor size is too which you also didnt include.

What size motor?

What is base timing set at & at what idle rpm?

What is total timing & at what rpm is it all in?


Moving on:

Doesnt have to be vac leak as stated abve with low int vacuum.

If base timing is overly retarded with perf cam that alone will make idle /low engine speed intake vacuum considerably lower then it should be.

Ensure you have approx 18 deg base timing with perf cam & vac adv hooked to fullintake vac all the time.

Set base timig with idle at approx 600 rpm to ensure mech adv isnt partially activated at idle upsetting base timing adj.

Look for approx 36-38 deg total timing in by approx 2600-2800rpms,but the already suggested 4k rpm is way to late/lazy for total to be comming in IMHO.

BTW,in general cam profile-size-duaration etc vs motor size has a large affect on idle/low engine speed vacuum along with if base with perf cam is too retarded too so keep that in mind when dealing with issues like this.

Now if you ensure base/total timing are correct and if running vac adv to full ntake vac if running one along with idle mixture/idle speed set correctly and idle vac is still low for cam vs motor size then look for poss carb/intake/ vacuum leake/s at that point but enusre base /total timing is right 1st with vac adv to full int vacuum for most street perf motor's unless its real hot without enough vac at idle to run vac adv.


Mar 11th, 10, 1:27 AM
^^ what they said about timing.

Mar 16th, 10, 9:01 PM
Okay, I found my cam card. I have a Lunati Voodo 268 installed in the engine. It is a 350 with a MSD distributor (non-vacuum advance) and a 6AL box.

I am sitting at 18 degrees advance at 800 rpm. I still only have about 6 in of vaccum. When I advance the timing it goes up but only reaches 12 in at about 50 degrees of advance! So, I backed it back off.

I checked all of my vacuum lines and no leaks that I can see. I sprayed carb cleaner between the intake and heads and no difference.

Mar 16th, 10, 9:38 PM
I wouldn't think it would even idle at all with 50 deg. 12 in/hg is about ballpark for that cam I would imagine. You NEED a vacuum advance to boost your idle advance and cruise performance. I think this is where most of your problem is. This will bring your vacuum up as well.

Mar 17th, 10, 4:15 PM
So a vacuum advance distributor should bring up my idle vacuum right?

Mar 17th, 10, 7:22 PM
Sounds to me like you have a mis-matched timing mark setup, with the crank damper timing mark at 12:00 at TDC and the timing tab showing 0 at 1:30. That'll put your "light" about 35 out of whack, such that when the marks show 0, you're ACTUALLY at 35 retarded.

Just one more reason why I don't pay any attention whatsoever to the ABSOLUTE number that a "light" shows; and instead, put the timing where the engine says it wants it. Listen to the engine: it will tell you what it wants. Give it what it wants and it runs better. Sounds like kind of a no-brainer (which is precisely why I, having no brain, fit right into it) but it's amazing how many people try to out-think the obvious. If it RUNS better, it IS better. Period.

What damper do you have, and what timing mark? Specifically, is your timing tab aftermarket?

Mar 17th, 10, 7:56 PM
What RB69SS396Conv said is true, and yes I think a vacuum advance distributor would greatly improve your idle quality and vacuum as well as seat of the pants street performance.

77 cruiser
Mar 17th, 10, 8:31 PM
Did you degree the cam? Sounds like it could be retarded, the cam that is.

Mar 19th, 10, 12:33 AM
^^^what they said about vac advance. (Worth verifying the timing mark too.)

Mar 19th, 10, 2:40 AM
the first step is to hook up the vacuum advance to full manifold vacuum.
next, you should "baseline" the carb settings. make sure the choke is all the way open and that the fast idle cam is disengaged.
back the idle speed screw out until it no longer makes contact with the throttle arm- then turn it in about 1 turn past when it makes contact. this gets the throttle blades to cover most of the transfer slot so that the carb is actually on the idle circuit and the mixture screws can do something.
set the idle screw to 1.5 turns out.
start it up- if it starts but then runs rough or dies, then use the gas pedal to keep it running until it warms up a bit and somewhat idles on it's own.
now you are ready to start playing with the mixture and idle speed screws. you might have to bump up the idle speed screw to keep it running- but don't go too far, or you will get back out of the idle circuit and the mixture screws won't work.
you just gotta keep dancing between the idle mixture screws- turn them no more than 1/2 turn at a time, and try to keep them both the same- and the idle speed screw. you want the idle speed screw to be out as far as possible while stil maintaining a livable idle speed where it doesn't die when you put it in gear and good vacuum.