71 LS5 not accelerating
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 1,139
Likes: 82
71 LS5 not accelerating
Specs:
4 speed trans.
71 LS5 pretty much stock other than a mild roller cam.
Dist overhauled, timing, dwell set.
Clean stock plugs and new plug harness.
Carb just back from Lars and installed.
Stock exhaust
The engine was built about nine years ago by a very reputable builder and ran very well on the dyno 398hp and 470 torque.
This car only has a few road miles since frame on restoration.
My issue is the engine is flat and doesn’t pick up rpm as it should. The carb was overhauled by Lars about eight years ago so I recently sent it back after finding a fair amount of debris in the filter at the carb fuel inlet. After today’s test drive it’s still not picking up rpm as it should. It has a new fuel tank and it looks clear. It has a fairly new fuel pump on the car but I haven’t verified fuel pressure and flow. I’m not sure what direction to go next.
KC
4 speed trans.
71 LS5 pretty much stock other than a mild roller cam.
Dist overhauled, timing, dwell set.
Clean stock plugs and new plug harness.
Carb just back from Lars and installed.
Stock exhaust
The engine was built about nine years ago by a very reputable builder and ran very well on the dyno 398hp and 470 torque.
This car only has a few road miles since frame on restoration.
My issue is the engine is flat and doesn’t pick up rpm as it should. The carb was overhauled by Lars about eight years ago so I recently sent it back after finding a fair amount of debris in the filter at the carb fuel inlet. After today’s test drive it’s still not picking up rpm as it should. It has a new fuel tank and it looks clear. It has a fairly new fuel pump on the car but I haven’t verified fuel pressure and flow. I’m not sure what direction to go next.
KC
Last edited by grumman41; Mar 5, 2024 at 09:10 PM.
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 1,139
Likes: 82
It’s a 4 speed trans. What is the symptoms of a failed or failing fuel pump? I made an effort to use non ethanol fuel but may have had some in it. I thought fuel pumps usually fail and the engine quits, not runs poorly?
Race Director
Joined: Nov 2006
Posts: 13,301
Likes: 7,876
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Have you put a timing light on it and verified the ignition timing is advancing with RPM?
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Need a better description of what is happening.
a bad fuel pump will cause gas too, run out, during high revs/loads and not deliver what engine needs.
I am trying to under stand your issue.
a bad fuel pump will cause gas too, run out, during high revs/loads and not deliver what engine needs.
I am trying to under stand your issue.
Race Director
Joined: Nov 2006
Posts: 13,301
Likes: 7,876
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
OK, that was vague. Your ign timing is hitting what degrees at what maximum RPM? Your vacuum advance is tipping at what level of vacuum and is all in at how many inches of vacuum? Your engine idle vacuum is what?
As in. 18 degrees initial. 36 degress all in at 2,900 RPM. vac can tips at 7 inches, all in at 12 inches.12 degrees total vacuum advance. Engine idle vacuum is 14.5 inches at 850 RPM.
This would be helpful for helping you.
As in. 18 degrees initial. 36 degress all in at 2,900 RPM. vac can tips at 7 inches, all in at 12 inches.12 degrees total vacuum advance. Engine idle vacuum is 14.5 inches at 850 RPM.
This would be helpful for helping you.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
5 Best & 5 Worst Corvette Daily Drivers
Joe Kucinski
The Headlights of Every Corvette Generation Explained
Joe Kucinski
5 Best & 5 Most Overrated Corvette Track Packages of All Time!
Joe Kucinski
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
Race Director
Joined: Nov 2006
Posts: 13,301
Likes: 7,876
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
What is the timing at now? Does it increase with RPM and end up at 36+ degrees? If not....that's a problem.....
Short of a restricted fuel flow....there is really nothing else that can cause what you are describing.....
EDIT: Does this car still have the heat riser? And is it open?
Jebby
Short of a restricted fuel flow....there is really nothing else that can cause what you are describing.....
EDIT: Does this car still have the heat riser? And is it open?
Jebby
Last edited by Jebbysan; Mar 6, 2024 at 08:12 AM.
Thread Starter
Burning Brakes
Joined: Jul 2013
Posts: 1,139
Likes: 82
This does have the original heat riser(not verified opening correct), 36 degrees of timing all in by 2500rpm.
4 Vettes, I haven’t checked idle vacuum or vacuum when the advance comes in.
Yes, I have followed Lars papers for setting timing.
4 Vettes, I haven’t checked idle vacuum or vacuum when the advance comes in.
Yes, I have followed Lars papers for setting timing.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Pretty easy to tell if it is shut when cold......a massive amount comes out of the left pipe and very little out of the right.......after warm up both pipes should be even.....if not, that is 100% what it is.....
Jebby
Jebby
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Race Director
Joined: Mar 2017
Posts: 11,975
Likes: 4,524
From: Virginia