Help engine stumbles and almost stalls on sudden acceleration
Thread Starter
Instructor
Joined: Nov 2023
Posts: 164
Likes: 32
From: East Tennessee
Help engine stumbles and almost stalls on sudden acceleration
See response #39 for summarized update and progress
I am having an issue on my 1980 corvette. I haven't owned the car long and I'm still working on getting the engine to run well.
The car starts and idles great. You can gently drive around fine, but whenever you stab the throttle, the engine completely fails to accelerate and misses/stumbles hard until rpms are above approx 3000.
This is the same while revving in neutral It revs great with excellent response, but if you floor it, it will just hesitate and stumble til it catches itself and gets above 3000 rpm
I rebuilt the carb and changed the filter with no change. Accelerator pump confirmed working.
New plugs and different plug wires make no difference.
There are no vacuum leaks and the engine is set for ported vacuum with 8 degrees of initial timing. Pulls a steady 20 inHg at an idle.
Both mechanical and vac advance are confirmed working.
Any help would be appreciated.
I am having an issue on my 1980 corvette. I haven't owned the car long and I'm still working on getting the engine to run well.
The car starts and idles great. You can gently drive around fine, but whenever you stab the throttle, the engine completely fails to accelerate and misses/stumbles hard until rpms are above approx 3000.
This is the same while revving in neutral It revs great with excellent response, but if you floor it, it will just hesitate and stumble til it catches itself and gets above 3000 rpm
I rebuilt the carb and changed the filter with no change. Accelerator pump confirmed working.
New plugs and different plug wires make no difference.
There are no vacuum leaks and the engine is set for ported vacuum with 8 degrees of initial timing. Pulls a steady 20 inHg at an idle.
Both mechanical and vac advance are confirmed working.
Any help would be appreciated.
Last edited by OneRedVette; Dec 31, 2023 at 06:27 PM.
Race Director
Joined: Mar 2017
Posts: 11,960
Likes: 4,516
From: Virginia
Email @lars for his timing papers and Q-Jet papers. Timing is 99% of the problem.
Buy Cliff's book, and read through this thread before attempting another carb rebuild.
https://www.corvetteforum.com/forums...980-q-jet.html
Last edited by Bikespace; Dec 17, 2023 at 11:00 AM.
Race Director
Joined: May 2006
Posts: 18,755
Likes: 2,589
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,235
Likes: 9,376
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Switch to manifold vacuum and 16 base timing....read those papers linked
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
The secondary air valve spring tension is from unloaded and spring tang touching......3/4 turn then lock....the lock screw is small and faces downward.....this can be done on the car in about 5 minutes.......if the accelerator pump is infact good...then you have the "quadrabog" all the idiots talked about in the 80's.......which is 100% controlled by the air valve tension and the secondary pulloff release rate. I have fixed the "qudrabog" without ever having to remove the carb from the car......
Your timing is retarded as mentioned.......
Jebby
Your timing is retarded as mentioned.......
Jebby
Le Mans Master
Joined: Aug 2006
Posts: 7,431
Likes: 1,561
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Thread Starter
Instructor
Joined: Nov 2023
Posts: 164
Likes: 32
From: East Tennessee
I got a chance to mess with it. I tried several settings of ported and manifold vac, but ported at 14 ran the best. Unfortunately I don't have a dial-back timing light, so I am unable to tell what my total timing is. Its the original HEI distributor so I would assume 20 degrees of mechanical advance for a total of 34. It runs great on the primaries, but still breaks up on the secondaries.
I went to adjust the air valve spring, but the set screw is stripped, so I'll be working on that tomorrow.
I went to adjust the air valve spring, but the set screw is stripped, so I'll be working on that tomorrow.
Last edited by OneRedVette; Dec 22, 2023 at 06:02 PM.
Race Director
Joined: Mar 2017
Posts: 11,960
Likes: 4,516
From: Virginia
I got a chance to mess with it. I tried several settings of ported and manifold vac, but ported at 14 ran the best. Unfortunately I don't have a dial-back timing light, so I am unable to tell what my total timing is. Its the original HEI distributor so I would assume 20 degrees of mechanical advance for a total of 34. Its runs great on the primaries, but still breaks up on the secondaries.
I went to adjust the air valve spring, but the set screw is stripped, so I'll be working on that tomorrow.
I went to adjust the air valve spring, but the set screw is stripped, so I'll be working on that tomorrow.
Or, if you only have $9-, buy some MSD timing tape. Stop guessing. It's good enough for Freiburger.
Race Director
Joined: May 2006
Posts: 18,755
Likes: 2,589
I got a chance to mess with it. I tried several settings of ported and manifold vac, but ported at 14 ran the best. Unfortunately I don't have a dial-back timing light, so I am unable to tell what my total timing is. Its the original HEI distributor so I would assume 20 degrees of mechanical advance for a total of 34. Its runs great on the primaries, but still breaks up on the secondaries.
I went to adjust the air valve spring, but the set screw is stripped, so I'll be working on that tomorrow.
I went to adjust the air valve spring, but the set screw is stripped, so I'll be working on that tomorrow.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,235
Likes: 9,376
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
get the carb rebuilt. The fact that you can accelerate normal and its fine but if you mat it it stumbles means its a fuel issue not keeping up with the air intake or that your timing isnt advancing fast enough. Have you checked your vacuum advance works...try manifold vacuum