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Thread: Weber backfire issues

  1. #1
    Member
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    Weber backfire issues

    My 72 T (webers) is having backfire issues. After checking the usual vacuum,wires,fuel plugs points timing, etc, I am thinking the fuel pump may not be producing the necessary pressure. currently has a purolator pro 42S which I have no idea if it is adequate.Any help would be appreciated.

  2. #2
    If it runs well while accelerating but back fires at steady state or idle, pull the idle jets and clean.

  3. #3
    Use a pressure gauge and check the pressure at the carburetor, should be 3 to 4 psi.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  4. #4
    Quote Originally Posted by patjolson View Post
    My 72 T (webers) is having backfire issues. After checking the usual vacuum,wires,fuel plugs points timing, etc, I am thinking the fuel pump may not be producing the necessary pressure. currently has a purolator pro 42S which I have no idea if it is adequate.Any help would be appreciated.
    In real terms pressure is not an issue as all a carburettor needs to operate correctly is sufficient fuel flow. Pressure is only generated by the fuel pump when flow is restricted and this can be caused by blockages in fuel lines or issues with the float valves as well as pump malfunctions.

    A Purolator 42S should develop a maximum pressure of around 3.5 psi and at this pressure flow will be Zero gph.

    The rated flow of 28 gph is only available at a nominal pressure of Zero psi. In practice a very small pressure is needed to open the positive shut off valves in the pump so maximum flow would tend to occur at 0.1-0.2 psi.

    If your pump does develop around 3.5 psi then it is most likely to be working correctly.

    If the pump doesn't deliver enough fuel then you will tend to experience fuel starvation at high speed and this will sound more like a misfire than a backfire.

    I would tend to look at the distributor and make sure that the mechanical advance/retard is operating correctly and not sticking.

  5. #5
    Xavier Petit-Jean-Boret Xavier PJB's Avatar
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    Too poor mixture maybe ?

    BTW, I have experienced 3 earlies with Webers (IDA and IDS) and they do/did backfire - especially when driving hard and releasing throttle, but work(ed) fine.
    They are/were tuned by the reputable and well known Crubilé.
    Last edited by Xavier PJB; 12-09-2019 at 06:18 AM.
    O-G 26 - Early911S 2407
    Current Stable:
    Porsche 911 L 1968 Bahama Yellow - Porsche 911 E 1973 Ruby Red - Porsche 924 Martini 1977 - VW T2 Devon 1979.
    Porsche Panamera Sporturismo E-hybrid

  6. #6
    My experience with PMO's on my 3.0 is..... When the engine suddenly starts to backfire/misfire, it is usually plugged idle jets. It is a quick 10 min job to pull them, blow them out with compressed air and reinstall.
    Tom Butler
    1973 RSR Clone
    1970 911E Restoration in Progress

  7. #7
    Oil Cooled Heart Bullethead's Avatar
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    Quote Originally Posted by tom1394racing View Post
    My experience is..... usually plugged idle jets. It is a quick 10 min job to pull them, blow them out with compressed air and reinstall.
    This. I have several cars with Webers and clogged idle jets are nearly always the culprit, thankfully it's an easy fix. Had this issue with my 356 yesterday in fact.
    Russ

    ESR # 1537

    '62 356S Notchback Hotrod
    '67 S Das Geburtstagsgeschenk
    '68 T Targa Sporto
    '70 914/6 GT

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