Engine knock created after shutdown backfire
Thread Starter
Instructor
Joined: Mar 2017
Posts: 111
Likes: 7
Engine knock created after shutdown backfire
the other day I took a quick ride and when I parked the car and shut it down the engine ran on for what seemed long and did a hard pop out the exhaust. I feel like it reversed rotation. This is not normal for this to happen. The carb was rebuilt by Lars and timing set per his paper. I have a Mallory unilite without vac adv. and prior to this engine was running great. Now the engine sounds like drums in a tin can or loud knocking at all rpms. Someone told me timing could have jumped. Is this a condition that could happen. Any other ideas what to check? Thanks in advance for your help.
Advanced
Joined: Mar 2018
Posts: 83
Likes: 14
From: Maine
The person who told you it jumped timing is most likely correct. The backfire probably caused your timing chain to jump a couple of teeth. I wouldn't run it until you figure out whether the timing marks are aligned on the cam and crank gear. Loud knocking is not a good thing hopefully you haven't broken anything.
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 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
A jumped timing chain is like Big-Foot. I have heard of them, but have yet to see one or even a decent picture of one verifying it.
There would have to be an enormous amount of slop in the chain for that to happen. And for the chain to not strike the timing cover or oil pan is hard to believe.
There would have to be an enormous amount of slop in the chain for that to happen. And for the chain to not strike the timing cover or oil pan is hard to believe.
Last edited by HeadsU.P.; Sep 7, 2018 at 03:53 PM.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Edit (re read) Seen 1 or 2 that jumped a tooth, most broke. It is possible for sure though .
Most were stock nylon geared pieces, They could last seemingly forever or 40kmiles.
Would be nice to see a true spring loaded tensioner. Couple here and there that arent so hot attempts.
How hard could it be even if a hole needed to be drilled/tapped. GOFAST you around?
Most were stock nylon geared pieces, They could last seemingly forever or 40kmiles.
Would be nice to see a true spring loaded tensioner. Couple here and there that arent so hot attempts.
How hard could it be even if a hole needed to be drilled/tapped. GOFAST you around?
Last edited by cv67; Sep 6, 2018 at 10:39 PM.
Le Mans Master
Joined: Nov 2004
Posts: 5,253
Likes: 768
From: Waterford WI
A jumped timing chain is like Big-Foot. I have heard of them, but have yet to see one or even a decent picture of one verifying it.
There would have to be an enormous amount of slop in the chain for that to happen and not strike the timing cover or oil pan is hard to believe.
There would have to be an enormous amount of slop in the chain for that to happen and not strike the timing cover or oil pan is hard to believe.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
I missed a shift in my 65 GTO. Wiped out every nylon timing gear tooth. Turned around rolled it down the long hill put it in 2nd gear try to get it to start. I heard it bang through the carb heard it bang it through the exhaust felt it catch then it was dead then it was banging through the carb then it was banging through the exhaust then it would run for a second then it was dead. So yeah these things jump time. I bent all 16 valves. I was 17 at the time and had no idea what I was doing. So I bought a 66 GTO and pulled the 389 out of it. Apparently somebody had stuck at 326 in my GTO already so I didn't miss it much.
Last edited by derekderek; Sep 7, 2018 at 08:05 AM.
Racer
Joined: May 2018
Posts: 297
Likes: 43
its also possibly an intermittent spark issue wither a plug or wire is loose or damaged and not always burning the fuel, this can easily cause the detonation/backfire sound if you have a cat as it tends to collect vapor but as op said it is more probable that its timing related.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Well as usual the forum has formed a posse lynch mob and want to rip the engine out before diagnosing the issues. The O.P. states:
"sounds like drums in a tin can or loud knocking at all RPMs."
If the timing chain truly jumped a tooth or two it would retard the valve and ignition timing, not advance it. So how does one hear drums in a tin can and loud knocking if all the timing is retarded? Any slack in a chain would result in the cylinder firing AFTER the compression stroke if not on the exhaust stroke.
I would be surprised if it ran at all. And if it did, the exhaust manifolds or headers would be cherry red.
So put your tools and away for a minute and think about diagnostics first.
"sounds like drums in a tin can or loud knocking at all RPMs."
If the timing chain truly jumped a tooth or two it would retard the valve and ignition timing, not advance it. So how does one hear drums in a tin can and loud knocking if all the timing is retarded? Any slack in a chain would result in the cylinder firing AFTER the compression stroke if not on the exhaust stroke.
I would be surprised if it ran at all. And if it did, the exhaust manifolds or headers would be cherry red.
So put your tools and away for a minute and think about diagnostics first.
Team Owner
Joined: Mar 2002
Posts: 24,160
Likes: 78
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Okay, get yourself a piece of heater hose. It makes a pretty good stethoscope. Fire the engine and hold one end of hose to your ear. Run the other end across the tops of both valve covers and then go down underneath and running across the oil pan rail lip. Both sides. And maybe take the water pump off see what it sounds like at the front of the engine. In fact, come to think of it, take all the belts off and start it up. That wull rule out any water pump alternator power steering noises. And you might be able to get in there with the pump on and listen to the timing chain cuz you're not going to have a fan blade trying to take your fingers off. if automatic transmission, it could also be a cracked starter wheel making the torque converter Bang all over the place.
Last edited by derekderek; Sep 7, 2018 at 06:00 PM.