 car is backfiring, decel valve? |
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| 914 RZ-1 |
 Feb 15 2016, 03:33 PM
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#1
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Porsche Padawan  Group: Members Posts: 683 Joined: 17-December 14 From: Santa Clarita, CA Member No.: 18,230 Region Association: Southern California  |
My car is backfiring. I am currently running Techron thru it to clean the fuel injectors, but I thought I'd ask what else could be causing this.
1. Is the decal valve on a '72 1.7L supposed to just pass air thru at idle, or is there supposed to be a vacuum? 2. How strong in the decal vacuum supposed to be? |
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| r_towle |
Feb 15 2016, 03:41 PM
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#2
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Custom Member Group: Members Posts: 24,577 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States 
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muffler leak?
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| TheCabinetmaker |
Feb 15 2016, 03:43 PM
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#3
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I drive my car everyday Group: Members Posts: 8,300 Joined: 8-May 03 From: Tulsa, Ok. Member No.: 666 |
2nd that.
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| Larmo63 |
Feb 15 2016, 03:49 PM
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#4
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Advanced Member Group: Members Posts: 4,264 Joined: 3-March 14 From: San Clemente, Ca Member No.: 17,068 Region Association: Southern California |
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| 76-914 |
Feb 15 2016, 04:25 PM
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#5
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Repeat Offender & Resident Subaru Antagonist Group: Members Posts: 13,504 Joined: 23-January 09 From: Temecula, CA Member No.: 9,964 Region Association: Southern California
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You could disconnect the decel valve and probably would not notice. Just a very small and muted popping when cold. (IMG:style_emoticons/default/beerchug.gif)
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| Rand |
Feb 15 2016, 04:30 PM
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#6
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Cross Member Group: Members Posts: 7,409 Joined: 8-February 05 From: OR Member No.: 3,573 Region Association: None |
Check the color of your spark. When one of my 914s started backfiring, the problem was a weak ground between the advance plates in the distributor (one end of the wire broke loose). This created a weak spark condition that made for some nasty backfires. The clue to my answer was a weak yellow spark.
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| era vulgaris |
Feb 15 2016, 04:56 PM
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#7
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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 |
As others have mentioned it could be several causes.
However, my old '72 1.7 used to backfire like a rifle when I'd let off the gas over 3K rpm. This was after I'd installed and correctly plumbed all new vacuum hoses, and fixed all vac leaks, cleaned electrical grounds, installed new injectors, etc. For me the fix was the decel valve. I didn't really do any kind of scientific adjustment. I just turned the adjuster guess-and-check style until the backfiring stopped happening. But it did cure the backfiring issue I was having. |
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| 914_teener |
Feb 15 2016, 05:04 PM
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#8
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914 Guru Group: Members Posts: 5,198 Joined: 31-August 08 From: So. Cal Member No.: 9,489 Region Association: Southern California |
Check out Brad Anders site for theory of operation:
http://members.rennlist.com/pbanders/DJetParts.htm When I have seen them fail the result is a high idle. I vote for exhaust leak and mixture. |
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| Dave_Darling |
Feb 15 2016, 08:25 PM
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#9
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914 Idiot Group: Members Posts: 14,990 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California
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My car did that when one of the intake valve seats was dropping... I hope that's not the cause!
--DD |
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| dangrouche |
Feb 15 2016, 10:30 PM
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#10
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dangrouche Group: Members Posts: 550 Joined: 1-May 04 From: San Francisco Bay Area Member No.: 2,012 Region Association: None |
I vote that its a muffler leak; I have a Bischoff muffler that had a few extra holes hat backfired regularly on my 2.0 djet. When I got a replacement Bischoff that was basically brand new, the backfires ceased. The backfires only occurred on deceleration only.
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| porschetub |
Feb 16 2016, 12:30 AM
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#11
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Advanced Member Group: Members Posts: 4,701 Joined: 25-July 15 From: New Zealand Member No.: 18,995 Region Association: None |
QUOTE(914 RZ-1 @ Feb 16 2016, 10:33 AM)  My car is backfiring. I am currently running Techron thru it to clean the fuel injectors, but I thought I'd ask what else could be causing this. 1. Is the decal valve on a '72 1.7L supposed to just pass air thru at idle, or is there supposed to be a vacuum? 2. How strong in the decal vacuum supposed to be? Timing is it right? |
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| HappyTrails |
Feb 23 2016, 09:46 AM
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#12
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Newbie Group: Members Posts: 10 Joined: 4-November 15 From: California central coast Member No.: 19,328 Region Association: None |
I came upon this post while looking for suggestions on the same issue and found and fixed my exhaust leak. Problem solved.
Thanks to the power of the forum! (IMG:style_emoticons/default/signal914.jpg) |
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| 914 RZ-1 |
Apr 20 2016, 03:37 PM
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#13
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Porsche Padawan Group: Members Posts: 683 Joined: 17-December 14 From: Santa Clarita, CA Member No.: 18,230 Region Association: Southern California |
Problem seems to be solved!
It was loose bolts at the heads. I tightened these up/replaced a few and the backfiring stopped. I tapped them with a hammer to get them to seat and tightened the bolts by hand. Drove it 200 miles this past weekend and had no issues. Thanks to all. (IMG:style_emoticons/default/beerchug.gif) |
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