exhaust backfiring
#1
Racer
Thread Starter
exhaust backfiring
What is making my exhaust backfire as it is throttling down. It has sidepipes and when I start slowing down when driving it will backfire. I always thought this was due to timing but changing it doesn't have any effect. Thanks Larry
#2
You could have an exhaust leak and when let off the gas air is sucked into the exhaust and mixes with the hot unburned fuel and causes it to explode.
#4
Pro
I like that sound. The C6R race cars pop very loudly on deceleration in gear. My DeLorean does it, and because of the short exhaust on a rear engines car, sometimes flames are visible. My 79 Vette also does it.
#5
Team Owner
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Hi tl,
I believe a backfire through the exhaust while decelerating can sometimes be caused by how the engine's timing is set.
Perhaps you might want to check that?
Regards,
Alan
I believe a backfire through the exhaust while decelerating can sometimes be caused by how the engine's timing is set.
Perhaps you might want to check that?
Regards,
Alan
#6
Racer
Thread Starter
That's what I always was led to believe but it doesn't seem to change the problem by changing the timing. Which would be more apt to cause the problem? Too advanced or too retarded?
#8
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I was just reading last night you maybe running a little lean. Im trying to diagnose a carb issue right now as well
#9
Le Mans Master
#11
Drifting
Had same symptoms, found solution in post by Lars, i have it still. Is it considered ok to post a picture of his post? Not sure of forum etiquitte or spelling of etiquitte either
#12
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#13
Racer
Thread Starter
The backfire is just small pops that are just annoying. The dist. is a stock GM HEI with initial timing around 10 degrees with total timimg around 36 degrees. Didn't notice the problem until I put the sidepipes on. I'll try opening up the adjustment screws a little.Thanks everyone for all the advice. Larry
Last edited by truckerlary; 04-22-2017 at 05:58 PM.
#14
Racer
Thread Starter
#15
Melting Slicks
It's usually a combination of things that cause after fire popping in the exhaust. Lean idle mixture, retarded ignition timing and exhaust leaks.
Make sure your vacuum advance is working properly. If it is not working, it will contribute to the after fire popping.
Set your idle mixture screws with a vacuum gauge. Start by turning them all the way in and lightly seating them. Turn them out 1 1/2 turns to start out with. Now start the car and fully warm it up. Hook the vacuum gauge to manifold vacuum. Start turning each in a little at a time. If the reading on the gauge goes up, keep turning them in until the reading starts to drop, then turn them back out until you find the highest reading on the gauge. If the reading goes down when you first start to turn the idle mixture screws in, just go the other way and start turning them out until you find the highest reading. Make sure you turn both mixture screws the same amount on both sides.
Now check very closely for exhaust leaks. Just a small leak will contribute to popping. Fix any leak you find.
If you do all of the above, it should eliminate or at least minimize the after fire popping.
Mike
Make sure your vacuum advance is working properly. If it is not working, it will contribute to the after fire popping.
Set your idle mixture screws with a vacuum gauge. Start by turning them all the way in and lightly seating them. Turn them out 1 1/2 turns to start out with. Now start the car and fully warm it up. Hook the vacuum gauge to manifold vacuum. Start turning each in a little at a time. If the reading on the gauge goes up, keep turning them in until the reading starts to drop, then turn them back out until you find the highest reading on the gauge. If the reading goes down when you first start to turn the idle mixture screws in, just go the other way and start turning them out until you find the highest reading. Make sure you turn both mixture screws the same amount on both sides.
Now check very closely for exhaust leaks. Just a small leak will contribute to popping. Fix any leak you find.
If you do all of the above, it should eliminate or at least minimize the after fire popping.
Mike
Last edited by v2racing; 04-22-2017 at 06:24 PM.
#16
Instructor
I know I'm probably alone in this camp but if the above advice doesn't work for you. ...try this.... Set baseline mixture as stated above with mixture adjustment backed off from seated 1 1/2 turns. Street test with engine at normal temp....if it still pops on deceleration turn each mixture adjustment clockwise(leaner) 1/4 turn and street test again. Repeat as needed but you still need decent idle when finished. If you have to crank mixture in a lot you may have internal fuel leaks in the carb. My take on this is on deceleration with throttle plates closed, vacuum goes high drawing in fuel from the idle ports, raw fuel looking for ignition....red glowing carbon deposits in exhaust system. I've had luck with this a few times. Good luck.
#17
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Bottom line is this: Afterfire (popping in the exhaust) is almost always caused by a lean condition. Not by timing or a rich condition. Richen it up.
Lars
#18
Melting Slicks
Leaks can cause decel pop also.
Mike
Last edited by v2racing; 04-24-2017 at 01:01 AM.