Back fire on decel., Where to look? |
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Back fire on decel., Where to look? |
KaptKaos |
Apr 12 2014, 09:57 PM
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#1
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Family Group: Members Posts: 4,009 Joined: 23-April 03 From: Near Wausau Member No.: 607 Region Association: Upper MidWest |
I am getting a back fire on deceleration. I can replicate the back fire fairly reliably, and avoid it as well. I am hoping this may help provide some hints as to where to look.
The car is a 1.8 running 40idf carbs, with an unknown spec carb cam. It has been running great for nearly the last two years until this started. Ignition is a SVDA dizzy with a pertronix. I have cleaned all plugs. They were all in nice shape, without carbon build up. I have tightened all intake and exhaust nuts/bolts. I have examined the exhaust for leaks or holes, and have found none. Valves adjusted about 1,000 miles ago. If I take my foot off the gas, and leave the car in gear allowing it to slow, I will get a back fire almost every time. If I take my foot off the gas, put the car into neutral and use the brakes to slow the car, I will not get a back fire. Assuming that there are no leaks, and the the timing is good, where should I look? How do I tell where the problem may be. |
brant |
Apr 12 2014, 10:33 PM
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#2
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914 Wizard Group: Members Posts: 11,639 Joined: 30-December 02 From: Colorado Member No.: 47 Region Association: Rocky Mountains |
sounds like an exhaust leak...
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KaptKaos |
Apr 12 2014, 10:51 PM
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#3
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Family Group: Members Posts: 4,009 Joined: 23-April 03 From: Near Wausau Member No.: 607 Region Association: Upper MidWest |
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Bob L. |
Apr 12 2014, 11:10 PM
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#4
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Senior Member Group: Members Posts: 848 Joined: 7-August 11 From: Austin TX Member No.: 13,411 Region Association: Southwest Region |
Similar problem here. I keep tightening 3 & 4 but they wont stay. Is it OK to torque them when hot?
I'm thinking locking nuts, like "Stage8" |
Elliot Cannon |
Apr 12 2014, 11:10 PM
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#5
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914 Guru Group: Retired Members Posts: 8,487 Joined: 29-December 06 From: Paso Robles Ca. (Central coast) Member No.: 7,407 Region Association: None |
Timing? Sometimes you can have an exhaust leak and not be able to tell right away. If you look under the car while the engine is running at idle and have someone block the exhaust with a rag or piece of cardboard or something you might be able to find something. It isn't always easily identified.
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KaptKaos |
Apr 12 2014, 11:13 PM
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#6
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Family Group: Members Posts: 4,009 Joined: 23-April 03 From: Near Wausau Member No.: 607 Region Association: Upper MidWest |
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euro911 |
Apr 13 2014, 01:56 AM
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#7
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Retired & living the dream. God help me if I wake up! Group: Members Posts: 8,851 Joined: 2-December 06 From: So.Cal. & No.AZ (USA) Member No.: 7,300 Region Association: Southern California |
Timing? Sometimes you can have an exhaust leak and not be able to tell right away. If you look under the car while the engine is running at idle and have someone block the exhaust with a rag or piece of cardboard or something you might be able to find something. It isn't always easily identified. All one really needs is a stethoscope or a length of hose to find where an exhaust is leaking. Having a lift is a plus (IMG:style_emoticons/default/shades.gif) |
monkeyboy |
Apr 15 2014, 08:43 AM
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#8
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Senior Member Group: Members Posts: 808 Joined: 8-June 08 From: Los Angeles, Ca Member No.: 9,147 Region Association: None |
Backfires on decel can also be lean idle jets, or possibly a clogged one.
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era vulgaris |
Apr 15 2014, 09:09 AM
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#9
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J is for Genius Group: Members Posts: 982 Joined: 10-November 13 From: Raleigh, NC Member No.: 16,629 Region Association: South East States |
I'm not sure if the 1.8 deceleration valve is plumbed the same as on the 1.7, but when I got my car it would consistently backfire when I let off the throttle above 3000rpm and stayed in gear. Likewise, if I simply went into neutral and braked, it didn't backfire.
The problem on my car was that some idiot (scotty boy?) had unplugged the front-most vacuum hose of the deceleration valve, that plugs into the plenum. Then left that hose sitting disconnected on top of the engine, and plugged the port on the plenum. Once I reconnected the hose the problem went away. I'm sure your hose is probably still connected, but it's possible you might have a leak around that hose. Edit: scratch that! Nevermind, just noticed you said you're running carbs. |
euro911 |
Apr 15 2014, 12:19 PM
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#10
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Retired & living the dream. God help me if I wake up! Group: Members Posts: 8,851 Joined: 2-December 06 From: So.Cal. & No.AZ (USA) Member No.: 7,300 Region Association: Southern California |
Joe's running carbs. It's more than likely exhaust related ... it has a history of exhaust leaks (IMG:style_emoticons/default/poke.gif)
Bring it to shop class tonight (IMG:style_emoticons/default/popcorn[1].gif) |
r_towle |
Apr 15 2014, 01:54 PM
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#11
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Custom Member Group: Members Posts: 24,591 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
take a small piece of 2*4 and after the car is running, lay on the floor and hold the piece of wood flat on the end of the tailpipe.
Seal the tailpipe so nothing can escape. If you have a leak, you will have a much easier time finding it using this method. rich |
KaptKaos |
Apr 16 2014, 02:49 PM
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#12
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Family Group: Members Posts: 4,009 Joined: 23-April 03 From: Near Wausau Member No.: 607 Region Association: Upper MidWest |
take a small piece of 2*4 and after the car is running, lay on the floor and hold the piece of wood flat on the end of the tailpipe. Seal the tailpipe so nothing can escape. If you have a leak, you will have a much easier time finding it using this method. rich I'll try that Rich. Thanks for the tip. |
KaptKaos |
Apr 16 2014, 02:49 PM
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#13
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Family Group: Members Posts: 4,009 Joined: 23-April 03 From: Near Wausau Member No.: 607 Region Association: Upper MidWest |
Joe's running carbs. It's more than likely exhaust related ... it has a history of exhaust leaks (IMG:style_emoticons/default/poke.gif) Bring it to shop class tonight (IMG:style_emoticons/default/popcorn[1].gif) Shop was cancelled till next week. Did you get the message? |
euro911 |
Apr 16 2014, 04:17 PM
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#14
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Retired & living the dream. God help me if I wake up! Group: Members Posts: 8,851 Joined: 2-December 06 From: So.Cal. & No.AZ (USA) Member No.: 7,300 Region Association: Southern California |
No. I didn't make it anyhow ... stayed at the shop to crate up a motor (IMG:style_emoticons/default/smash.gif)
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