engine jumping during steady driving
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
engine jumping during steady driving
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.
Thoughts.
Melting Slicks
Joined: Aug 2003
Posts: 2,135
Likes: 2
From: Dayton Oh
Re: engine jumping during steady driving (Cali,68,L-79)
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:
Racer
Joined: Apr 2001
Posts: 395
Likes: 0
From: Shoreline WA
Re: engine jumping during steady driving (Cali,68,L-79)
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
"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
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Re: engine jumping during steady driving (killain)
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.
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.
Pro
Joined: Jun 2001
Posts: 672
Likes: 0
From: Quebec
Re: engine jumping during steady driving (Cali,68,L-79)
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
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
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Re: engine jumping during steady driving (American Boy)
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]
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]
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Re: engine jumping during steady driving (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.
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.
