Cali,68,L-79

Drove home yesterday and noticed that when I'm going at a steady cruise at anything under 2800 RPM the engine jumps or jitters like it's getting shots of gas, not constant. Now the distributer is set via LARS rules at 26 degrees advance at 2500RPM and i forget the max advance but LARS says it's okay. over 2800 RPM there is no issues and when it comes to WOT the car drives like a raped ape. I believe the problem is the Torquer-2 manifold. being a single plane intake with an advertised powerband of 3000-6500RPM I think it does not develop a good steady fuel flow below 3000RPM and that's why I have this chugging symptom. Carb is Eldebrock 600 just that I adjusted per LARS same time dist was so I think it has nothing to do with carb or distributer.

Thoughts.

marshrat99

Could be a lot of things. But unless you've recently changed something, the first thing I'd check would be plugs, wires and points if you've got'em. :smash:

The Dude

Primary jetting a number or two too small?

SpyderD

Too much vacuum advance?

SLine78

According to Lar's paper on vacuum advance units:

"A GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition."

You may need to try a adjustable vacuum advance unit or look at yours and determine how much advance it is allowing and then replace it with one that produces a little less advance.

Mike

Brettmc

Lean surging?

Brett :thumbs:

Fevre

Unplug your vac adv and see if that 'cures' it, if it does you need to get an adj vac adv unit if you want to retain it.

killain

How about the fuel filter?

Cali,68,L-79

So your saying my timings right on and should not mess with but it's the vacuum advance that's adding too much advance combined with mechanical.
Until I'm able to order a adjustable cannister I wonder if I should decrease the advance by 3 degrees and see what that yields.

American Boy

A quick test with the vacuum advance disconnected would tell you if it's the cause.

I had a similar problem during the tunning of my carb and it was my cruise area jetting that was too lean. The rod were inserted too deep in the jets.

Stephan

Cali,68,L-79

Yep just came back from a trip and it's the vac advance. i'm ordering an adjustible cannister but for now I'm doing this.

put in one size larger spring (1 spring) retime the car to achieve 36 degrees advance by 2500RPM and theoretically the springs will not achieve 54 degrees total before redline which will temporarily acheive the same result as an adjustible cannister would Just until the new cannister arrives from Summit
ASSUME NO ADJUSTMENT OF DISTRIBUTER BODY
Here's what I got so far w/ vac/ advance disconnected. With mixed silver and black spring.

23 degrees at 961RPM
36 degrees at 2540RPM
39 degrees at 4000RPM and will go no higher.
w/ vac advance connected 65 degees at 3800RPM!!!

Now with 2 silver springs and vac advance disconnected
19 degrees at 950RPM
32 degrees at 2670RPM
38 degrees at 4300 RPM and will go no higher
w/ vac advance connected 62 degrees at 3200RPM!!

LARS PLEASE HELP!!!

:banghead:


[Modified by Cali,68,L-79, 8:40 PM 3/31/2004]

Cali,68,L-79

OKAY I'LL TAKE YOUR ADVICE AND ORDER THIS.



Vendor: Crane Cams, Inc.
Product Line: Crane Vacuum Advance Kit
Material: Steel
Finish: Zinc Plated

Get the advance you want

GM: All Delco point type distributors, steel, zinc plated, adjustable, vacuum advance kit


This Crane adjustable vacuum advance kit lets you tailor your ignition to the advance you want. The adjustability provided by these kits permit you to run the maximum amount of ignition advance throughout the rpm range, without incurring detonation. Once installed, you can quickly compensate for changes in fuel quality and altitude. Each kit comes with a vacuum diaphragm, allen wrench, advance weight springs and instructions.

Sold as a kit.