backfire when engine revs down
Thread Starter
6th Gear
Joined: Sep 2007
Posts: 6
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From: mentor ohio
backfire when engine revs down
has anyone else had problems with engine backfiring when engine rev's down. what has changed was that i replaced the vacuum canister and connected hose to vacuum port on intake manifold. checked timing,ok.runs great until i let engine rev down, at about 25000 rpm's i get a couple pops threw the exhaust. any idea's on what's wrong. engine is new zz4, 4 speed trans. 1974 stingray. i'm new to the forum.
Drifting
Joined: May 2007
Posts: 1,491
Likes: 2
From: North Royaton Ohio
What kind of exhaust are you running? Mine does the same thing. I think it's kind of normal if you have a more open exhaust, as it's just unburned fuel firing in the exhaust. On cars with normal/more restrictive mufflers, it quiets it down so you can't hear it. But when you open up the exhaust, you can hear it. That's why you see flames out the pipes of race cars when they let off before a turn.
At least that's what I think...
At least that's what I think...
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Do you mean that you removed the vacuum cannister? If you did, you've created a situation which the car's design never intended. The vacuum tank is an "accumulator" which helps to stabilize the vacuum level under various manifold conditions. If you did not remove the cannister, make sure there are no leaks or loose hoses. As this just started since your vacuum "work", it is likely the cause....either because the system has changed the operating behavior of the engine, or because there is some improperly connected/installed component.
Drifting
Joined: May 2007
Posts: 1,491
Likes: 2
From: North Royaton Ohio
Do you mean that you removed the vacuum cannister? If you did, you've created a situation which the car's design never intended. The vacuum tank is an "accumulator" which helps to stabilize the vacuum level under various manifold conditions. If you did not remove the cannister, make sure there are no leaks or loose hoses. As this just started since your vacuum "work", it is likely the cause....either because the system has changed the operating behavior of the engine, or because there is some improperly connected/installed component.
I must have skipped the part about the vacuum cansister. But if everything there does check out, then my money's on exhaust. But yes, I'd check to make sure you've got everything hooked up properly and no leaks.
Thread Starter
6th Gear
Joined: Sep 2007
Posts: 6
Likes: 0
From: mentor ohio
What kind of exhaust are you running? Mine does the same thing. I think it's kind of normal if you have a more open exhaust, as it's just unburned fuel firing in the exhaust. On cars with normal/more restrictive mufflers, it quiets it down so you can't hear it. But when you open up the exhaust, you can hear it. That's why you see flames out the pipes of race cars when they let off before a turn.
At least that's what I think...
At least that's what I think...
Thread Starter
6th Gear
Joined: Sep 2007
Posts: 6
Likes: 0
From: mentor ohio
Yes i went back and checked the hoses to make sure they were connected. The hose is new so there should be no holes. The only thing i haven't checked was the line coming from the tank to the canister. I have noticed that i don't smell the gas fumes now when i drive. That is leading me to believe the system is working. What guitarcrazy02 says about the less restrictive exhaust sounds good. But you brought up manifold pressure. Could that be fixed by retuning the idle mixture needles on the carburator.?running a 670 holley street avenger
Instructor
Joined: Sep 2007
Posts: 109
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From: Woodstock Ga
My 81 does the same thing. If i let off the gas and coast down a hill or downshift to second it will backfire. From what ive heard its pretty normal. Everything on my car checks out so I hope so! Im running hooker headers with stage 2 flowmasters.. so when im comming home at night coasting down my hill the neighbors love it!! haha...
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Advanced
Joined: Sep 2007
Posts: 69
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From: Greenville WI
I know my '69 has a vacuum problem and it performs the same way. Letting off the throttle in the mid/high end of the RPM band causes some minor popping/backfiring as it winds down. I also have a slight gas fume smell....and a loud hissing noise when I turn the car off.
I had no idea the vacuum system had this much of an impact!!! I was just ticked, because my wiper door doesn't open. With a vacuum problem, is it safe to be running the car? Could we be causing any damage?
I had no idea the vacuum system had this much of an impact!!! I was just ticked, because my wiper door doesn't open. With a vacuum problem, is it safe to be running the car? Could we be causing any damage?
Burning Brakes
Joined: Jan 2007
Posts: 1,164
Likes: 1
From: Port Clinton Ohio
St. Jude Donor '08-'09-'10-'11-'12-'13
My crusie control quit working last week. It's an old aftermarket unit so I figured it was dead. When I poped the hood, there was a big hole in the vacuum line going to it. I cut off the last 3/4" of the line where the hole was, plugged it back in, and it's good as new.
Team Owner
Joined: Jul 2004
Posts: 40,338
Likes: 786
From: Redondo Beach, California
When I rev my ZZ4 up (say 3500 rpm) and suddenly let the throttle close to it's idle position, I get an occassional back fire or two as the rpm drops. I don't have any mufflers on the car right now, just headers. The vacuum advance is connected into the intake manifold. I've using a tach drive HEI distributor, but I removed the weights, vacuum can, and electronics that came with the ZZ$'s HEI and installed them in my tach drive HEI.
I've always thought this might be a slight timing or carb jetting problem. I think it's a common problem, I've watch Formula 1 cars sometimes backfire as the drop rpms after a downshift.
I've always thought this might be a slight timing or carb jetting problem. I think it's a common problem, I've watch Formula 1 cars sometimes backfire as the drop rpms after a downshift.
Instructor
Joined: Sep 2007
Posts: 109
Likes: 0
From: Woodstock Ga
Another weird thing that I guess I just realized is also a vacuum problem is when Im driving with the wipers on and I hit the gas hard the wipers just stop where ever they are for about 10 seconds and then resume. Is this just the car giving all its vaccum to the engine? or something I should be worried about?
Drifting
Joined: May 2007
Posts: 1,491
Likes: 2
From: North Royaton Ohio
I don't think wipers are controlled by vacuum, are they? I could be wrong. I know wiper doors on early C3's are controlled by vacuum, but I don't believe the wipers themsleves are.
I seriously think it's your exhaust. Mine never made that popping sound till I put the new exhaust on. Like I said, it's just unburned fuel igniting.
I seriously think it's your exhaust. Mine never made that popping sound till I put the new exhaust on. Like I said, it's just unburned fuel igniting.
Team Owner
Joined: Jul 1999
Posts: 26,545
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From: Huntersville NC
