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Posted by: 914 RZ-1 Feb 15 2016, 03:33 PM

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?

Posted by: r_towle Feb 15 2016, 03:41 PM

muffler leak?

Posted by: The Cabinetmaker Feb 15 2016, 03:43 PM

2nd that.

Posted by: Larmo63 Feb 15 2016, 03:49 PM

Posted by: 76-914 Feb 15 2016, 04:25 PM

You could disconnect the decel valve and probably would not notice. Just a very small and muted popping when cold.

Posted by: Rand Feb 15 2016, 04:30 PM

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.

Posted by: era vulgaris Feb 15 2016, 04:56 PM

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.

Posted by: 914_teener Feb 15 2016, 05:04 PM

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.

Posted by: Dave_Darling Feb 15 2016, 08:25 PM

My car did that when one of the intake valve seats was dropping... I hope that's not the cause!

--DD

Posted by: dangrouche Feb 15 2016, 10:30 PM

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.

Posted by: porschetub Feb 16 2016, 12:30 AM

QUOTE(914 RZ-1 @ Feb 16 2016, 10:33 AM)

Posted by: HappyTrails Feb 23 2016, 09:46 AM

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!

Posted by: 914 RZ-1 Apr 20 2016, 03:37 PM

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.