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Thread: Misfiring - 901/05 - 2.0 ltr

  1. #1

    Misfiring - 901/05 - 2.0 ltr

    I've got a problem with my 1966 model that misfires and partly backfires (some light pops through the carburetors) when idling. When you drive the car at normal speed you don't notice it, and the engine seems to pull ok.

    The misfiring occurs at both cylinder banks, and as far as I know at all cylinders. Therefore my first idea was ignition problems. I have renewed the distributor cap and rotor, the breaker points and the spark plugs without any luck. Breaker point gap was adjusted to 0.4 mm. Resistance in the plug wires was equal and good.

    My car was fitted with wrong spark plugs, Bosch WR7DC, which I assume is for a 3.2 Carrera engine. The original W250P21 was gone out of production, and according to my supplier some WR3CS plugs were found in an old dealers list.

    The same plug was specified for the 'S and RS Carrera. This is a much colder plug than I had (and I think a bit colder than it should have been), so I hope they are worth the money. My engine is fitted with the later type heat exchangers and is found to produce 145 bhp compared to the original 130.

    The new plugs had a standard gap of 0.6 mm compared to the factory specification of 0.35 mm for the original plugs. I didn't change this before I had them fitted.

    So, now I'm hoping to get some ideas from you out there. Maybe check of the ignition timing? I believe one should have a perfect idling, no interruption of the pure music.

  2. #2
    Look for air leaking past the throttle shafts creating a vacuum leak. These are the classic symptoms of worn throttle bore/shafts. You can verify this by feeling for excessive play between shaft and housing.

    Another trick is to spray carb cleaner or propane gas (unlit) on the throttle shaft-to-housing area and note idle up changes.

    Throttle shaft bushing repairs are expensive but unavoidable

    Hope this helps,
    Sherwood Lee

  3. #3
    Check the idle fuel adjustment screws, orifices, jets, and progression holes under the bottom of the throttle plates for crud. I previously had 3 901/05 cars and this was a typical problem. The fuel tanks rust a little (or sometimes a lot) and develop small rust particles which then flow past the fuel filter (if it is even there) into the carbs. Ultimately, on all my early Porsches, I've taken out the fuel tank for cleaning, put in new fuel filters, blown out the lines, and completely took apart and cleaned the Webers. After that, they always ran fine - even with worn throttle shaft bushings.

  4. #4
    I agree with the carb statements, it sound like you might be lean at idle due to the reasons stated above. As far as plugs go, I've been using NGK B7ES plugs which is one hotter than the recommended plug, the B8ES, with good results. I'd recommend the B8ES for your application, my engine has some blow-by so I have found I need the hotter plug to keep them from fouling.


  5. #5
    Thanks for your informative answers. My conclusion is that there is no way without dismantling the carbs. This means setting up the carbs afterwards with synchronizing and idle mix adjustment.

    Is there any sound practice for this described anywhere? Or is this difficult to get right without professional help?

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