Backfire on decelleration after new exhaust
11-06-06, 03:03 PM
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Backfire on decelleration after new exhaust
Hi folks,
I had a custom exhaust whipped up, 2 1/4" pipe, High Flow Cat, High Flow Muffler. Car really seems to have pepped up, but I am now getting some annoying backfire on decelleration. I'm guessing has to do with less backpressure, or running too rich (or both...or maybe neither...been at least 15 yrs since i owned a carburated vehicle).
Any common ideas?
tia
I had a custom exhaust whipped up, 2 1/4" pipe, High Flow Cat, High Flow Muffler. Car really seems to have pepped up, but I am now getting some annoying backfire on decelleration. I'm guessing has to do with less backpressure, or running too rich (or both...or maybe neither...been at least 15 yrs since i owned a carburated vehicle).
Any common ideas?
tia
11-06-06, 04:32 PM
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Got the same thing now going from stock exhaust to headers. I can fairly reliably get a backfire downshifting into 2nd or 3rd. Helps keep tailgaters away. I watched some kid following too closely the other day as his eyes lit up and he backed off real fast after the backfire. I think it's useful.
11-06-06, 04:42 PM
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I have a RB header and presilencer with stock pipe/muffler following that. On my SP 13B, I can hear the car burbling/backfiring on decelleration, but I do not get flames. Once I finish my exhaust, though, I know it will toss some nice ones.
11-06-06, 05:00 PM
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I could shoot very nice fireballs out of my stock '79 SA. I always figured it was probably a combination of running rich and some minor exhaust leak (mixing a little bit of air with the unburned hyrocarbons in the exhaust). I too found it useful, and am considering running a plug and coil in the muffler of my 85... That's a big nono around here, though, whereas driving a shitbox that backfires incessantly will barely even draw attention, lol.
11-06-06, 08:43 PM
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Originally posted by Ovid213
I always figured it was probably a combination of running rich and some minor exhaust leak (mixing a little bit of air with the unburned hyrocarbons in the exhaust).
I always figured it was probably a combination of running rich and some minor exhaust leak (mixing a little bit of air with the unburned hyrocarbons in the exhaust).
Last edited by mar3; 11-06-06 at 08:45 PM.
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11-06-06, 11:32 PM
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go for flames...my streetport gets them like hiccups...everyonce in a while...I take advantage of it and scare off tailgators, welcome to the love for the 7
11-07-06, 12:22 AM
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Definitely start checking for that exhaust leak. I have the RB system, and no backfiring at all. On a properly set up system, you should not be experiencing this issue. Timing will also contribute to backfiring if not set correctly. Good luck. 
11-07-06, 12:04 PM
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Originally Posted by Kentetsu
Definitely start checking for that exhaust leak. I have the RB system, and no backfiring at all. On a properly set up system, you should not be experiencing this issue. Timing will also contribute to backfiring if not set correctly. Good luck.
11-07-06, 12:27 PM
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Timing can be off, carb running pig-rich like a '79 and a sealed aftermarket exhaust with a functioning cat will still not go 'pop' after letting up on the gas pedal...it is the introduction of oxygen to the high temp mix as it's exiting the engine that causes the backfire...and I've lost an '79 engine that didn't smoke to backfire...
11-09-06, 08:54 AM
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I run headers and a presilencer and get backfires decelerating from 4th. Sometimes they are little other times it can be quite loud. A crew of my boys where going to a meet and when we got there they said that I scared the **** out of some old lady when we where taking a turn. They said it was priceless! I did not see her expression, but hey!
11-09-06, 09:36 PM
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A new, free flowing exhaust is going to allow more flow through the carb.
Your carb is jetted and tuned to run for the exhaust you had, presumably stock. But with less restrictive flow through the carb, it's going to want more fuel.
What's happening is you are running just a bit lean in the upper end now, and the exhaust is getting hotter than the restrictive one did. When you let off the throttle, the valves shut, but you still have suction acting on the carb from the rotors, especially if the tranny's still engaged. This sucks in more fuel from the idle circuit than most people realize, and when this fuel came into contact with your old exhaust, nothing happened. But when it comes into contact with your hot exhaust, POP!
I have an aluminum flywheel, but I downshift all the time. My carb is jetted pig rich in the middle, lean up top, and I shift @ 8K all the time. So when I wind her out in third in a straight doing 90, I heat the exhaust real nice. When the curve and twisties comes up and break down to 5.5K and slam it into 2nd to make the hairpin, it pops like an M80.
The dash-pot is a little valve with a piston that a tongue on the primary throttle shaft makes contact with just during the last few mm of closing the throttle. The valve is actually filled with oil, and has a baffle inside with a series of holes in it. As the piston is pushed in, the oil has to go through the holes, and so the piston closes slowly.
It's adjustable, and it is also overlooked when people rebuild their carbs and then try to adjust the idle. They sometimes forget that the dash-pot may be keeping the carb from idling low enough because it has not been properly adjusted.
I don't run a dash-pot. I like my engine to be as responsive as possible in every way. But then, I'm not looking for my car to be civil in any way.
I can see the reflection of the flash from the backfire in my rearview at night, and it has drawn the local Barney Fife's attention twice, now.
John, your just plain mean.
Your carb is jetted and tuned to run for the exhaust you had, presumably stock. But with less restrictive flow through the carb, it's going to want more fuel.
What's happening is you are running just a bit lean in the upper end now, and the exhaust is getting hotter than the restrictive one did. When you let off the throttle, the valves shut, but you still have suction acting on the carb from the rotors, especially if the tranny's still engaged. This sucks in more fuel from the idle circuit than most people realize, and when this fuel came into contact with your old exhaust, nothing happened. But when it comes into contact with your hot exhaust, POP!
I have an aluminum flywheel, but I downshift all the time. My carb is jetted pig rich in the middle, lean up top, and I shift @ 8K all the time. So when I wind her out in third in a straight doing 90, I heat the exhaust real nice. When the curve and twisties comes up and break down to 5.5K and slam it into 2nd to make the hairpin, it pops like an M80.
The dash-pot is a little valve with a piston that a tongue on the primary throttle shaft makes contact with just during the last few mm of closing the throttle. The valve is actually filled with oil, and has a baffle inside with a series of holes in it. As the piston is pushed in, the oil has to go through the holes, and so the piston closes slowly.
It's adjustable, and it is also overlooked when people rebuild their carbs and then try to adjust the idle. They sometimes forget that the dash-pot may be keeping the carb from idling low enough because it has not been properly adjusted.
I don't run a dash-pot. I like my engine to be as responsive as possible in every way. But then, I'm not looking for my car to be civil in any way.
I can see the reflection of the flash from the backfire in my rearview at night, and it has drawn the local Barney Fife's attention twice, now.
John, your just plain mean.
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