Backfire through exhaust
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Instructor
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Backfire through exhaust
I am having some trouble figuring out why my car is backfiring through the exhaust. It only happens above 4000 RPMs when the secondarys are open. i am running a 355 small block chevy with a 650 Speed Demon. I have installed brand new plugs this year , and a new distributor last year. I just recently noticed the problem. The car has been sitting since last year. I just put on Hooker side pipes with STS baffles, but it did it before this with the true dual exhaust so I know the problem isn't the new exhaust. The float levels on the carb are set to the Lower line next to the sight glass. The jetting is the same as when it came from Demon. The plugs are a light tan in color. I have checked wire connections, and vacuum lines. i would appreciate any ideas, Thanks.
I should also mention that this backfire is on acceleration above 4000 rpm.
I should also mention that this backfire is on acceleration above 4000 rpm.
Last edited by IRON WILL; Jun 6, 2005 at 07:12 AM.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
I would look at the ignition first. I recently had problems with my 74 that resulted in backfiring and shutting down while driving. After going through all the systems I decided it was the Pertronix unit in the distributor. I pulled the unit and replaced it with a Crane XR1 and everything went back to normal. You say you replaced the distributor last year. If you still have the old unit I would swap it to see if the problem disappears or stays. That will guide you toward your next step. Good luck.
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Instructor
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I'll check the other distributor and maybe swap the modual. But it won't do it if I don't mat the pedal right to the floor. meaning if the secondarys don't open it doesn't backfire through the exhaust, But as soon as I mat it and the secondarys open up above 4000 it pops through the exhaust. Thats why I think it is more of a carb problem than an ignition problem. But I will look into it anyways.
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
After reading this again I'm wondering if there could be a problem with the vacuum advance in the distributor. I had a car that experienced back firing because the vacuum can was faulty. Make sure it's functioning correctly and allowing movement of the distributor plate with no binding. It may be as simple a fix as this.
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I took the carb apart last night to clean it and check the settings. I found a piece of metal or aluminum in the secondary bowl. It was about the size and shape of a chocolate chip. Very light too. I am thinking that it was getting sucked into one of the jets. I never have had this carb apart yet and have never seen this piece of metal in any other Demon carb i have had apart. I am going to drive the car again tonight to see if that was the problem.
Pro
Joined: May 2001
Posts: 524
Likes: 59
From: watchung N.J.
I had a similar problem with mine I chased for 3 mos. It would backfire through the exhaust under hard acceleration only above 4000 rpms and the problem turned out to be that the air cleaner lid was too close to the top of the carb. ( due to hood clearance problems) It was actually sucking the lid down closer to the carb causing a rich cond. similar to a choke. So try running without the air cleaner and see if goes away.
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Thread Starter
Instructor
Joined: Mar 2003
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I wish it was as simple as the air cleaner. I have a regular base air cleaner with a 14x3in element plenty of room in that. I did check the plug wires last night after checking some distributor components. I found one that was open with the voltmeter and another that had alot of resistance. I am going to replace the wires and see what happens, they should be replaced anyways. If that doesn't fix it I will try to richen up the mixture. Thanks for the help.
