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Thread: Exhasut leak causing backfiring

  1. 08-26-2014, 11:53 AM #1
    lkadams
    lkadams is offline
    912->911 conversion
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    Nov 2004
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    Calgary, Alberta, Canada
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    Exhasut leak causing backfiring

    Oh collectve wisdom....

    Over the winter, I had the engine (69 911E motor with Webers) out of my car (69 912). I sent my Webers to Kurt at Partsklassic for a beautiful rebuild. Bought new Dansk heat exchangers. Re-installed the carbs and manifolds with all new gaskets. Installed the heat exchangers with all new head studs (well, 11/12 - couldn't get one to budge), new hardware, new gaskets.

    However despite all that, I am plagued still by my demon - a deceleration backfire loud enough to send Suge Knight running (too soon?) and I'm sure set my neigbours cursing (of course, I live down a big hill, so I'm poppin and bangin the whole way down the hill). I know I likely have a pinhole exhaust leak somewhere. I suspect that one of the flanges on my exhaust port is a little wonky. It doesn't backfire constantly on decel, but pops a bit and lets out an occasional BANG!

    Before I commit an atrocity like gooping up the gaskets with form-a-gasket in hopes of sealing the leak, is there any good methods for trying to figure out where the leak is? I'vetried laying under the car wih a chunk of tubing stuck in my ear and listening, but to no avail (aside from minor heat-stroke due to being that close to the heat exchangers).

    I want to get on with the final tuning of the carbs, but this backfire is the bane of my existence. I had such high hopes that all this work would solve the problem, so you can imagine my frustration.

    Thanks,
    Keith (slightly deaf in left ear now)
    Keith Adams
    Calgary, Alberta, Canada
    Early 911S Registry #906 | PCA member IG: @912R
    1969 Blutorange 912R - 912 to 911 conversion
    1969 Mercedes 280 SE (W111) Coupe
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  2. 08-26-2014, 01:21 PM #2
    Silverbullit
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    Perhaps try lying (hoist?) under the running motor with a spray bottle of slightly soapy water. Any exhaust leak will reveal itself with bubbles.
    Brian
    S Reg #1032

    "I measured twice, cut three times, and it's still too short!"
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  3. 08-26-2014, 01:32 PM #3
    gsjohnson
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    With misadjusted carbs, an exhaust leak may not be the culprit. Do you actually hear the exhaust leak?
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  4. 08-26-2014, 03:56 PM #4
    Fishcop
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    I spray a little Aerostart (ether) around the manifolds, one at a time - you soon hear a change in revs if there is an induction leak. Doing the same around the exhaust and flanges will probably illicit a pop when you're close to the right spot.
    John Forcier
    EarlyS #1987
    1968 911 Race Car "Grun Hilda"
    1969 S/T interpretation "Blau Healer"
    Restoration Saga
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