Help with backfire thru carb
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
Help with backfire thru carb
I have a 1981 C3 with the computer or electronic carb removed. It has been replaced with Edelbrock AVS, intake, distributor.
I have an issue where if I slowly raise the rpms, the engine does ok. As soon as I hit the trottle for a rapid increase, it backfires thru the carb.
I replaced the accelorator pump in the carb. Made sure the squirters were receiving ample flow. I checked the timing.
I set timing at 4 degrees BTD.
When I raised the rpms slowly to avoid the backfires, I could only get about 25 degrees ALL IN at 2K rpms........
Could it be a broken intake or exhaust valve?
All new spark plugs.......idles fine, at 600 rpm, starts right up with just a turn of the key.
Where do I go next to try to isolate the issue???????
Thanks,
Baffled AZ in AZ.
I have an issue where if I slowly raise the rpms, the engine does ok. As soon as I hit the trottle for a rapid increase, it backfires thru the carb.
I replaced the accelorator pump in the carb. Made sure the squirters were receiving ample flow. I checked the timing.
I set timing at 4 degrees BTD.
When I raised the rpms slowly to avoid the backfires, I could only get about 25 degrees ALL IN at 2K rpms........
Could it be a broken intake or exhaust valve?
All new spark plugs.......idles fine, at 600 rpm, starts right up with just a turn of the key.
Where do I go next to try to isolate the issue???????
Thanks,
Baffled AZ in AZ.
Drifting
Joined: Jul 1999
Posts: 1,542
Likes: 43
From: Plano TX
Timing should be 12 btc for a baseline with vacuum advance not connected and line plugged .
What distributor do you have? HEI or other. If an ignition problem, this is typical of an HEI ignition module failing especially after the engine gets to temperature and distributor gets hot.
What distributor do you have? HEI or other. If an ignition problem, this is typical of an HEI ignition module failing especially after the engine gets to temperature and distributor gets hot.
Melting Slicks
Joined: Apr 2021
Posts: 2,714
Likes: 2,162
From: Minnesota
Assuming your 4 at idle and 25 at 2K is with the vacuum advance unplugged you have a 21 degree swing of mechanical advance. Meaning to get a proper spec of 36 total you’ll want to set your timing at idle to 15, again, with the VA unplugged. 2K is also far too low to have all of your mechanical timing all in. I would add heavier springs until it’s all in at 2,800-3,000 RPM. Unless you have 11:1+ of compression you should not be running into pre ignition issues with this setup.
Last edited by Piersonpie; Apr 16, 2026 at 01:58 PM.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
Assuming your 4 at idle and 25 at 2K is with the vacuum advance unplugged you have a 21 degree swing of mechanical advance. Meaning to get a proper spec of 36 total you’ll want to set your timing at idle to 15, again, with the VA unplugged. 2K is also far too low to have all of your mechanical timing all in. I would add heavier springs until it’s all in at 2,800-3,000 RPM. Unless you have 11:1+ of compression you should not be running into pre ignition issues with this setup.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
Timing should be 12 btc for a baseline with vacuum advance not connected and line plugged .
What distributor do you have? HEI or other. If an ignition problem, this is typical of an HEI ignition module failing especially after the engine gets to temperature and distributor gets hot.
What distributor do you have? HEI or other. If an ignition problem, this is typical of an HEI ignition module failing especially after the engine gets to temperature and distributor gets hot.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
I have been having the vacuum advanced plugged......when I check or set timing. I've tried the 15 degrees BTD to try to get to the 35 all in and it seems as the rpms
go above 2000 to 3000 the carb popping begins........
I'll pull the HEI module.......and replace........any siggestions on a relaible replacement??????????
go above 2000 to 3000 the carb popping begins........
I'll pull the HEI module.......and replace........any siggestions on a relaible replacement??????????
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Aug 2006
Posts: 7,427
Likes: 1,560
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Race Director
Joined: Mar 2017
Posts: 11,953
Likes: 4,508
From: Virginia
By all means, fix the timing to the current best practice.
However, I doubt 10 degrees of retarded timing is enough to cause a backfire through the carb.
Sounds like a massive vacuum leak, and/or a valve issue.
However, I doubt 10 degrees of retarded timing is enough to cause a backfire through the carb.
Sounds like a massive vacuum leak, and/or a valve issue.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
Drifting
Joined: Jun 2025
Posts: 1,722
Likes: 598
All good suggestions, ALL.
Also, valve(s) Not sealing; via bent or burned or Both.
Additionally, perhaps some valve(s) adjusted too tight.
systematic compression checks; both dry & wet can help rule in-rule out.
Also, valve(s) Not sealing; via bent or burned or Both.
Additionally, perhaps some valve(s) adjusted too tight.
systematic compression checks; both dry & wet can help rule in-rule out.
Last edited by Rebelyell; Apr 17, 2026 at 06:25 PM.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
I considered pulling the valve covers and looking for the issue. At my senior age....working on and under my C3 can be considered proof I'm
ready for the guys in white coats. I've learned to stay within my lane and not create even more problems with mistakes and bad ideas.
Drifting
Joined: Jun 2025
Posts: 1,722
Likes: 598
Maybe time to call in a pro...........find a mechanic whos pay grade is way over my head and skills..
I considered pulling the valve covers and looking for the issue. At my senior age....working on and under my C3 can be considered proof I'm
ready for the guys in white coats. I've learned to stay within my lane and not create even more problems with mistakes and bad ideas.
I considered pulling the valve covers and looking for the issue. At my senior age....working on and under my C3 can be considered proof I'm
ready for the guys in white coats. I've learned to stay within my lane and not create even more problems with mistakes and bad ideas.
And yes sir, I get that senior thing; daily.
Last edited by Rebelyell; Apr 17, 2026 at 09:30 PM.
Thread Starter
Safety Car
Joined: Dec 2004
Posts: 3,742
Likes: 217
From: Arizona AZ
My thoughts on pulling the valve cover was just to inspect for a broken spring. I realize it does not help if it's a burnt or stuck valve. I thought the removal of the valve covers would be a good place to start since the timing has been set and reset and a slight noted but soft miss is a telltale sign of something else. I have all new vacuum lines installed, new rubber, even new plastic connector trees etc.
So, I'll chew on the valve cover removal and seeif it's the short path to what may be a long path to repair. I have nothing to lose removing the valve covers.
So, I'll chew on the valve cover removal and seeif it's the short path to what may be a long path to repair. I have nothing to lose removing the valve covers.
Racer
Joined: Nov 2013
Posts: 252
Likes: 84
From: Uhrichsville Ohio
You can also check the valve lift of each valve with the rocker covers removed. Look at each rocker and see how much it moves. Also lift the rockers to check how loose they are. The lobe will be almost flat to cause your symptoms. JP