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Thread: Idle Misfire and Misfire at 5500 RPMs

  1. #1

    Idle Misfire and Misfire at 5500 RPMs

    I have an 87 Shelby Z with the following mods:

    Ported head
    3 Bar Map
    +40's (Using DSM injectors)
    Shel Game Cal
    Grand National turbo (essentially a S70)
    3" Exhaust
    8.5mm Aurora Wires
    New Cap
    New Rotor
    New plugs (a range colder) gapped at .030
    Walbro 255
    New rubber fuel lines under the car and under the hood
    New fuel filter
    Vacuum Block
    52mm Throttle Body (intake neck ported to match)
    TBI Cam and Followers and 3* of advance using offset cam key
    PT Lifters
    Ported Exhaust Manifold
    Ignition timing set at 13-14*

    The problem I am having is that I have a fairly inconsistent misfire at idle as well as a specific misfire at 5500 RPMs. The misfire at 5500 happens in both first and second gear at 5psi and 9-10psi. I have verified cam timing using the timing light trick, although when it starts to misfire at idle, the "standing image" of the cam gear seems to rotate towards the firewall so its no longer in line with where it should be. It goes back after the misfiring ends. My wideband is not functioning at the moment, but when it was, I was getting 11.3:1 at WOT at 5psi.

    So far I've tried resetting cam timing about 3 times, ignition timing has been at 12* and 14*, tried a new coil, gapped the spark plugs from .035 to .030. Plugs display no speckling and haven't had so many miles on them as to turn them tan. Nor are they black with soot/oil.

  2. #2
    Hybrid booster
    Join Date
    Jul 2006
    Notre-Dame de L'Ile Perrot, QC

    Re: Idle Misfire and Misfire at 5500 RPMs

    Your intermediate shaft may be worn allowing the distributor to move excessively.
    Remove the cap, grab the rotor and see how much play there is (back and forth) Normal shouldn't be much more than 1/16"

    A friend of mine chased a similar problem for quite a while. His rotor would move more than 1/4'' *whoa*
    We removed his intermediate shaft and it was worn beyond belief, so was the gear on his oil pump. Replaced both and it's as good as new.

  3. #3

    Re: Idle Misfire and Misfire at 5500 RPMs

    Alright, I'll check that when I get home today. AFAIK it doesn't have much play, but I hadn't been looking for play either.

  4. #4

    Re: Idle Misfire and Misfire at 5500 RPMs

    I checked, it has 1/16-1/8" of play

  5. #5

    Re: Idle Misfire and Misfire at 5500 RPMs

    Old thread, but I'm still having an issue with this. I've tested up to 3rd gear. It starts to break up around 5000 rpms. Plug gap is set at .025. It idles better with the plug gap closed. Still somewhat stumbly but less pronounced. I have also added an evac setup that is pulling 10" Hg at idle and maybe 1-2" Hg at WOT. Turbo spools and the car pulls hard from 4000 to 5000 when its starts to misfire.

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