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Thread: Help needed diagnosing Weber 40 IDA 3C problem

  1. #1

    Help needed diagnosing Weber 40 IDA 3C problem

    Hi Folks -

    FWIW, except for Weber carbs, ported heads and cams with more aggressive acceleration ramps, the engine in my 914/6 is essentially built to ‘73 Carerra RS specs.

    The engine ran fine, but suddenly began missing under operating conditions less than approximately 75% throttle. However it still pulls smoothly and strongly at WOT to redline.

    Checked the idle jets. All are clear.

    Driven only 369 miles since PPI and extensive servicing, including carb sync and idle & mixture adjustment by preeminent Porsche shop.

    What should I check next? (Beside checking and possibly upgrading fuel & air filtration already on my radar)

    Any help greatly appreciated.
    Last edited by Bob Schaefer; 08-16-2022 at 11:17 AM.
    Bob Schaefer

    67 911S (Canary / Lemon yellow sunroof coupe w/100l tank on COA & Kardex 1 of 1 produced??)



    70 914/6 (2.7l RS spec engine, but searching for 6404915 original)

  2. #2
    Senior Member
    Join Date
    Aug 2020
    Location
    Shawnee, OK
    Posts
    113
    How long did the engine run fine before the problems started?
    My guess is you do not have a fuel flow problem as it works OK at full throttle.
    What is your ignition system?
    Steve

  3. #3
    Quote Originally Posted by 1969911T View Post
    How long did the engine run fine before the problems started?
    My guess is you do not have a fuel flow problem as it works OK at full throttle.
    What is your ignition system?
    Steve
    Hi Steve. Thanks for your reply.

    Just guessing, but it probably ran fine for (most?) of the first 300 of the 369 miles I’ve driven it. At about 200 miles, the engine started missing after I was caught in backed up traffic approaching a road construction site. New plugs appeared to resolve that 1st problem for about another 100 miles. I don’t know, maybe I misdiagnosed the problem. Could an idle jet have been restricted and then magically cleared when I changed the plugs? Seems a remote coincidence.

    I agree 100% wrt no fuel flow restriction under WOT conditions. Am I correct concluding from your statement there’s no other point in the idle circuit likely to become restricted beside the idle jets?

    The distributor ignition has a Pertronix and a Classic Retrofit 6 pin CDI. Timing was checked during the service before I took delivery of the car.
    Last edited by Bob Schaefer; 08-18-2022 at 06:35 AM.
    Bob Schaefer

    67 911S (Canary / Lemon yellow sunroof coupe w/100l tank on COA & Kardex 1 of 1 produced??)



    70 914/6 (2.7l RS spec engine, but searching for 6404915 original)

  4. #4
    Senior Member
    Join Date
    Nov 2003
    Location
    Santa Monica CA
    Posts
    1,881
    I am no expert on Weber carbs but when I saw CLASSIC RETOFIT 6 PIN CDI ; I thought that is not a factory set up. Is there a way to copy exactly what the factory did for a 73 RS engine? Or are those parts No longer available to achieve CDI status ? CDI was done to fix the engine getting sick while stuck in traffic as I recall back in the 66 and 67 911S days. People were told to blow out the engine to clear carbon build up back in the day.
    1. Chris-Early S Registry#205
    2. '70 911S Tangerine
    3. '68 911L Euro Ossi Blue

  5. #5
    Remove the spark plugs and look at them. What was in previously and what did you put in? Are they all the same? Are they black and sooty? I guess you just bought this car with this engine, and it ran fine for the previous owner? What ignition system do you have? A 6 pin is normally used with a SC engine.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  6. #6
    Remove emulsion tubes & check them for cleanliness.

    Check that idle jet holders are the correct type for your Webers, Weber IDAs built before 1969 did not use o-rings and all later Webers did use o-rings. Using later style jet carriers in early Webers will not properly seat the idle jets which can lead to rich low speed circuit which, by the way, can stay active through 5000 RPM on partial throttle. It is easy to install idle jets into early Webers using later jet holders, the jet may not be seated into the holder at the time of installation which will allow it to seat but it can then lose contact with the seat and create a rich condition.

    Another issue can be a bit of debris can partially clog the idle air correction jets which will richen fuel delivery. It is possible that this issue can be a little intermittent. Photo shows idle air correction jet location near the stud, the jet in the image has been modified to be tunable, OEM jets are pressed into top of carb body. A little bit of debris can bypass air cleaner seals if not perfect and find its way to this jet which sucks air into the fuel delivery for the idle circuit.

    Name:  air correctors installed.JPG
Views: 179
Size:  27.6 KB

    The next photo shows difference between non-o-ring and o-ring idle jet carriers; upper carrier uses o-ring in IDTP Webers and IDA Webers with alpha-numeric serial numbers such as 5F or 2H. Early jet carrier (non-o-ring) is the lower one and may be used in all Webers.
    Attached Images Attached Images  
    Paul Abbott
    Early S Member #18
    Weber service specialist
    www.PerformanceOriented.com
    info@PerformanceOriented.com
    530.520.5816

  7. #7
    Thanks Raspy - I don’t know what would be necessary to duplicate an RS ignition. I do have an MSD CDI I could try if that might help.

    Thanks Ed -
    “What was in previously and what did you put in?” The car arrived with WR5DC+ plugs. That’s what I replaced them with the 1st time I changed them. The 2nd change I put in a step hotter, WR6DC+ plugs.

    “Are they all the same? Are they black and sooty?” There was some variation between plugs. Some a little sooty; some more normal. But not every plug really sooted up.

    “I guess you just bought this car with this engine, and it ran fine for the previous owner?” Yes to both. No reported drivability issues by the previous owner or the individual who performed the PPI and service prior to my taking delivery. All accounts of performance and drivability were quite to the contrary. BTW: reported PPI leakdown values were all 10+/-%.

    “What ignition system do you have? A 6 pin is normally used with a SC engine.” Could you please give some guidance as to what other info I can provide beyond what I stated above re a Classic retrofit 6 pin CDI?
    Bob Schaefer

    67 911S (Canary / Lemon yellow sunroof coupe w/100l tank on COA & Kardex 1 of 1 produced??)



    70 914/6 (2.7l RS spec engine, but searching for 6404915 original)

  8. #8
    Quote Originally Posted by 1QuickS View Post
    Remove emulsion tubes & check them for cleanliness.

    Check that idle jet holders are the correct type for your Webers, Weber IDAs built before 1969 did not use o-rings and all later Webers did use o-rings. Using later style jet carriers in early Webers will not properly seat the idle jets which can lead to rich low speed circuit which, by the way, can stay active through 5000 RPM on partial throttle. It is easy to install idle jets into early Webers using later jet holders, the jet may not be seated into the holder at the time of installation which will allow it to seat but it can then lose contact with the seat and create a rich condition.

    Another issue can be a bit of debris can partially clog the idle air correction jets which will richen fuel delivery. It is possible that this issue can be a little intermittent. Photo shows idle air correction jet location near the stud, the jet in the image has been modified to be tunable, OEM jets are pressed into top of carb body. A little bit of debris can bypass air cleaner seals if not perfect and find its way to this jet which sucks air into the fuel delivery for the idle circuit.

    The next photo shows difference between non-o-ring and o-ring idle jet carriers; upper carrier uses o-ring in IDTP Webers and IDA Webers with alpha-numeric serial numbers such as 5F or 2H. Early jet carrier (non-o-ring) is the lower one and may be used in all Webers.
    Excellent! Thanks Paul. My jet carriers are the O-ring type and the carb serials are 4B and 4C. Nobody’s mentioned this as a potential problem, but the o-rings feel a little aged / hard. And appear a little flatened at their cross sectional OD’s. Not a problem unless there’s external leakage, right?

    I’m not sure when I’ll have a chance to check the emulsion tubes and idle air correction jets. I’ll report back after I check them.
    Attached Images Attached Images  
    Last edited by Bob Schaefer; 08-16-2022 at 06:16 PM.
    Bob Schaefer

    67 911S (Canary / Lemon yellow sunroof coupe w/100l tank on COA & Kardex 1 of 1 produced??)



    70 914/6 (2.7l RS spec engine, but searching for 6404915 original)

  9. #9
    Senior Member
    Join Date
    Aug 2020
    Location
    Shawnee, OK
    Posts
    113
    I would try the MSD, then you will know if it is an ignition or a carb problem
    Steve

  10. #10
    Thanks Steve, I’ll keep that idea in mind for a couple of weeks from now when I’m back where my MSD is.
    Bob Schaefer

    67 911S (Canary / Lemon yellow sunroof coupe w/100l tank on COA & Kardex 1 of 1 produced??)



    70 914/6 (2.7l RS spec engine, but searching for 6404915 original)

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