Backfire / Afterfire issue
Thread Starter
Heel & Toe
Joined: Aug 2019
Posts: 22
Likes: 2
Backfire / Afterfire issue
Hello everyone.
Chiming in with an issue I can't seem to wrap my head around:
..
Thx
A
Chiming in with an issue I can't seem to wrap my head around:
..
77 C3 L82. I recently had my carb rebuilt by Lars due to some sub-optimal performance issues (Quadrajet). Once I got the carb back from Lars, I installed it and the vette ran better than ever. I decided to then have my mechanic fix an intake manifold (oil) leak that I have been meaning to take care of. Got the vette back from him, and ever since then, I get a backfire / afterfire upon deceleration. Very loud booms / bangs coming from both of the side pipes once I let off the accelerator. I decided to try and diagnose the issue myself. Here is the list thus far:
..
..
- Timing. Mechanic confirmed that my timing is fine when I called him about the issue.
- Vacuum leak: I went around and checked all of the vacuum hoses for leaks using smoke. I also checked carb / manifold seating with a can of carb cleaner. To the best of my ability, I couldn't find a single leak anywhere.
- Fuel / Air mixture: I theorized that the mechanic must have accidentally or purposefully adjusted the F/A mixture on the carb during the manifold repair, therefore I used a vacuum gauge and reset the mixture. I also tried to richen it a bit, then lean it a bit though trial and error. No matter the F/A mixture, I still get backfire.
- Carburetor: The carb was just rebuild by Lars, therefore I rule out any carburetor issues at this junction.
Thx
A
Last edited by Adam Lawrence; Aug 25, 2020 at 02:51 AM. Reason: mistake
Race Director
Joined: Mar 2017
Posts: 11,947
Likes: 4,506
From: Virginia
Afterfire or backfire?
If it's afterfire, my 80 started doing this when I accidentally hooked up the vacuum advance to ported vacuum. I bet the mechanic who told you timing was fine did this, too (since that's "correct"). This was REALLY loud through the side pipes. Hook it up to manifold vacuum, and see if that fixes it.
Better yet, email @lars for his timing instructions, and follow them. You can also follow his instructions to set the idle mixture valves. You'll need a vacuum gauge, a good timing light, and possibly a carb adjustment screwdriver, but you'll save thousands over taking it to a mechanic.
If it's afterfire, my 80 started doing this when I accidentally hooked up the vacuum advance to ported vacuum. I bet the mechanic who told you timing was fine did this, too (since that's "correct"). This was REALLY loud through the side pipes. Hook it up to manifold vacuum, and see if that fixes it.
Better yet, email @lars for his timing instructions, and follow them. You can also follow his instructions to set the idle mixture valves. You'll need a vacuum gauge, a good timing light, and possibly a carb adjustment screwdriver, but you'll save thousands over taking it to a mechanic.
Thread Starter
Heel & Toe
Joined: Aug 2019
Posts: 22
Likes: 2
Afterfire.
I tried hooking up the vacuum advance to manifold vacuum (it was hooked up to ported since I bought it). That maybe helped a little. I am still getting afterfire, on deceleration, particularly in lower RPMs per each gear (seems to be ok above 3500). The afterfire seems less loud than before, maybe less frequent but that's just my base assessment.
I received the timing docs from Lars. Going to dive into that soon and post back.
Thanks for your tip @Bikespace
I tried hooking up the vacuum advance to manifold vacuum (it was hooked up to ported since I bought it). That maybe helped a little. I am still getting afterfire, on deceleration, particularly in lower RPMs per each gear (seems to be ok above 3500). The afterfire seems less loud than before, maybe less frequent but that's just my base assessment.
I received the timing docs from Lars. Going to dive into that soon and post back.
Thanks for your tip @Bikespace
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Afterfire and backfire are usually caused by a lean condition - your "mechanic" has likely induced a vacuum leak. Timing will not induce the afterfire/backfire, unless it's off by a MASSIVE amount, which is possible since he did remove and re-install the distributor. Always check and correct timing before doing anything else. If you don't have them, e-mail me for my timing papers. You need to set up for 36 degrees total.
Lars
Lars
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
