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Thread: Rebuilt 2.7 engine refuses to work properly

  1. #1
    Senior Member Einar Irgens's Avatar
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    Rebuilt 2.7 engine refuses to work properly

    Last year I bought a rebuilt 2.4 engine with 2.7 litre nicasil cylinders, S cams and ported heads (2.7 RS spec) from an acquaintance who has built several air cooled 911 engines through the years. I myself rebuilt and installed a set of Zenith TIN 40s. I have done this many times before. Selected jets size and venturis after checking several sources. Spark plugs, cables and points are new.

    From the start there were immense running problems that we couldn't figure out, until it appeared that the alternator voltage was 16 volts. Replacing it with a rebuilt unit reduced the problems significantly, but the symptoms are still the same:

    - Exhaust popping on low to medium revs, both while cruising and decelerating.
    - Inconsistent power delivery, very noticeable at load (uphill, acceleration) and medium revs.

    I have checked ignition and fuel components systematically without finding any errors. All cylinders appear to get fuel and sparks at approximately the correct amount. The symptoms are little affected by changes in ignition timing.

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    Can the increased voltage possibly have damaged the CDI module or related components? Both batteries had to be replaced.

    I am very unhappy about the situation. Last summer I hardly got to drive it at all. This summer the weather has been extraordinary, but even cruising in the sun with the top off gives me no pleasure.

    Einar
    Last edited by Einar Irgens; 08-02-2023 at 07:51 PM.

  2. #2
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    I can't speak to the electrical questions, but when I rebuilt my engine, I had a terrible time trying to figure out why one bank was running rich. After a lot of Fuel, Air & Spark investigation, I went back to basics and checked the cam timing. I know I checked it at least 4 times when building the engine, but somehow one bank's cam timing was pretty far off. I believe it was likely a kink in one of the new timing chains that only straightened out under true load, but of course by then the engine was sealed up. Had to pull the engine to redo it, but luckily it wasn't so far off that it wouldn't run, and no damage.

    Long winded way of saying check your cam timing.
    - 1969 911T Ossi Blue #3981

  3. #3
    Senior Member Haasman's Avatar
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    I can hear Ed Mayo's voice here- start with the basics such as The Torch's posting above.

    Keep us updated.
    Haasman

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  4. #4
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    Yes , start with the basics , and as someone else once posted , three thing to tune an MFI engine : an air fuel ratio meter , an AFR meter and an AFR meter . From your running issue description , might be too lean .
    Nevermind , I failed to notice the Zenith TIN 40 description .
    Last edited by Richy; 07-27-2023 at 10:21 PM.

  5. #5
    Yes, basics, and you are taking someone's word about the cams, what if they aren't 'S'? If you don't find any faults I'll suggest taking the engine out so you can have enough access to mount a degree wheel on the pulley and check the opening and closing degrees and see if you really do have 'S' cams.
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  6. #6
    Senior Member Einar Irgens's Avatar
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    I concluded myself that the cams have S profile, and I for sure do not trust me very much, but the procedure seemed logical. Here is how I did it.

    A few years back, I bought an engine with unknown origin and condition. Then I came across another engine kit with new nicasil 2.7 cylinders and 9.something compression ratio, which I also bought. I decided to sell the first one, but by coincidence discovered that the intakes were 36 mm. Perfect for the new project engine. I checked the cam type as follows:

    I marked the pulley on the crankshaft at the angle where S cams are supposed to start opening the intake valve (@38 deg.). I then rotated the pulley while holding a finger on the rocker. When I felt it start moving, the mark was at the expected position. Adequate or not?

    I assume that the engine builder after removing it from the head assembly also read the stamping at the end of the shaft , but I can ask if he actually did. He is, unlike me, a very thorough and patient man, and I am pretty confident that the cam timing was double-checked, but measuring again might be a good idea. Can this be done with sufficient accuracy without taking the engine out?

    Of course the size of some jets might be less than optimal, but I don't suppose that would cause such dramatic symptoms? The exhaust has been checked with an AFR meter in the muffler, and is about right on idle and during driving.

    Einar
    Last edited by Einar Irgens; 07-27-2023 at 12:44 AM.

  7. #7
    If I'm not mistaken Porsche calls out their cam specs at 1.0 MM of valve lash. Check me on this, and if correct did you do that?
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  8. #8
    Senior Member Einar Irgens's Avatar
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    Ed,

    I'm sorry, but I don't understand what you mean.

    Einar

  9. #9
    An air fuel ratio meter takes a lot of guesswork out of jetting the carbs. My guess is install larger idle jets or check yours for blockage and use a good fuel filter. If your Venturi are too small it will choke the motor. I use PMO 46mm carbs on my 2.7 RS spec motor, no flat spots and great performance on the top end. PMO recommended them.
    Last edited by 66S; 07-29-2023 at 10:27 AM.

  10. #10
    Senior Member Einar Irgens's Avatar
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    Ok, I see. I will try larger idle jets.

    These are the sizes I have used:

    34mm main venturis
    60 Idles
    150 mains
    180 air corrections.
    F3 emulsion tubes

    Any reason to believe Zeniths and 2.7 rs spec engine could be a mismatch?
    Last edited by Einar Irgens; 07-30-2023 at 01:41 PM.

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