67 Backfire
Thread Starter
Drifting
Joined: Aug 2004
Posts: 1,313
Likes: 512
From: Carmel CA
67 Backfire
My 67 427/400 has developed a strange backfire problem. I drive the car to a local car show 5 miles from my house on Thursday nights. On a typical night, it sits for about 3 hours before I start it to go home. For the last two Thursdays, it has backfired through the side exhaust when I go to start it for the trip home. The backfire is a big one! I drive the car a lot, and this is the only time that I get the problem. I thought that timing would cause a backfire through the carb, but I checked it anyway. It was set at the spec. My next thought is that some gas is finding it's way into the exhaust while the car is sitting. The carbs seem to be fine and the gaskets are new. Has anyone seen this before?
Thanks,
Doc
Thanks,
Doc
Melting Slicks
Joined: Jun 2012
Posts: 3,115
Likes: 196
From: Wilton Ca.
Doc,
First thing, before starting the car next time remove the air cleaner and look down into the carb throats to see if there's a bunch of gas on the manifold. If so, something is going on with the carb like a stuck float bowl needle. If not, then it may be an ignition issue. One or more cylinders aren't firing at times. They build up unburnt gas then fire off and..........BOOM!
Does it have to turn over a few times more than usual just before the backfire? If so, (and there's no sign of gas on the manifold) I'd be looking at an ignition issue such as the the coil or loose connections. Also could be a bad plug/plugs or wires. Good luck.
Gary
First thing, before starting the car next time remove the air cleaner and look down into the carb throats to see if there's a bunch of gas on the manifold. If so, something is going on with the carb like a stuck float bowl needle. If not, then it may be an ignition issue. One or more cylinders aren't firing at times. They build up unburnt gas then fire off and..........BOOM!
Does it have to turn over a few times more than usual just before the backfire? If so, (and there's no sign of gas on the manifold) I'd be looking at an ignition issue such as the the coil or loose connections. Also could be a bad plug/plugs or wires. Good luck.
Gary
Last edited by Gary's '66; Jul 10, 2016 at 04:56 PM.
Race Director
Joined: Jan 2002
Posts: 15,342
Likes: 2,717
From: Close to DC
C2 of the Year Finalist - Modified 2020
It possible your center carb is leaking some internally over that three hours. That couples with your timing being right on the edge could cause the backfire.
Last edited by Bluestripe67; Jul 10, 2016 at 04:49 PM.
Burning Brakes
Joined: Mar 2006
Posts: 1,102
Likes: 171
From: Colorado Springs, CO
I was having a similar issue with my 57, it would occasionally backfire on cold start-up, and sometimes after sitting for a few hours. Also I was having an intermittent misfire when cruising. Turned out to be a bad coil. Good luck.
Le Mans Master
Joined: Feb 2004
Posts: 7,356
Likes: 870
From: Norcal CA
2018 C1 of Year Finalist
Melting Slicks
Joined: Jul 2004
Posts: 2,865
Likes: 195
From: Maryland
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
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
Thread Starter
Drifting
Joined: Aug 2004
Posts: 1,313
Likes: 512
From: Carmel CA
I checked all of my connections and they are good. The engine was rebuilt less than 500 miles ago and it has electronic ignition. I tried to duplicate the conditions, but I could not get it to backfire. It must know when people are looking.
Doc
Doc
Last edited by watson; Jul 11, 2016 at 03:22 PM.
Melting Slicks
Joined: Jun 2012
Posts: 3,115
Likes: 196
From: Wilton Ca.
Gary
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,270
Be careful!
The OP's situation is afterfire, not backfire.
Remove the air cleaner cover after shutdown and check to see if any fuel is dribbling out the the booster venturi nozzles. If so it could just be a float level adjustment, but a leaky needle and seat is also a possibility.
Duke
Last edited by SWCDuke; Jul 11, 2016 at 04:26 PM.