Spark knock - too rich or too lean?

Discussion in 'Troubleshooting & Diagnosis' started by Tapp, Jun 20, 2009.

  1. Tapp

    Tapp Member Gold Member

    34
    0
    Dec 1, 2007
    Dry Fork VA
    I know this question has been asked a thousand times here but I still cannot find a good answer to fix my problem. I have a 70 Z with the original setup, (carb, pistons, and cam) Have set the timing to 34 degrees full advance @ 2500 rpm, have a mixture of about 50%-100 / 50%-94 octaine and have the carb adjusted to the highest idle I can detect....still a spark knock when I get down on it. The knock is not right off the bat, only after the RPM's get up to around 4000. I recently replaced the plugs with AC/Delco R43 and gapped are .035". Can't remember if I had it before then as I am just breaking the newly rebuilt motor in and haven't reved it too high. Also the temperature stays under 180@<2000, 190@2500 & begins to work its way up to 210@>3000rpms. Any suggestions???
    Thanks as always for your help! You guys always come thru for me!
    tapp
     
  2. heavy duty

    heavy duty Veteran Member

    529
    0
    Sep 29, 2005
    Cactus Forest Arizona
    Explain spark knock.If you have a rod knock than you have bigger problems than timming or the carb.
    Detonation or "ping" as most call it is very bad for your engine. Its like hitting the pistons with a hammer. Try retarding the timing a little.
    Dont disconnect the vacuum advance! At light throttle, high manifold vacuum advances the spark a considerable amount. As the throttle is opened further for acceleration or to climb a hill, etc., the manifold vacuum drops off and relaxes so that the distributor assumes a more retarded position.
    Disable the vacuum advance and you can expect awful gas milage and spark plug fouling because the engine is forced to operate with the spark retarded under cruising conditions. This reduces the thermal effiency of the engine.
    "Real racers" use centrifugal advance-only distributors because they dont drive their cars on the street.

    Detonation or "Ping" is spontaneous combustion of fuel in the chamber instead of the desired controlled "slow" and even burning.
    The most common cause of detonation is over-advanced ignition timing, but there are other causes which should not be over-looked.
    It can also be caused by full-throttle acceleration with too-high a compression ratio, high operating temperatures, too-hot spark plugs,
    and lean fuel mixtures can all be contributing factors to cause destructive detonation. Results include hammered out bearings,
    broken rods and crankshafts, sharp-edged holes in piston crowns, and broken ground electrodes on spark plugs.
    It also breaks ring lands and cracks piston skirts. Spark must be retarded. If you hear detonation or so called "Ping" and the engine
    is not being run close to 38 degrees total advance, try increasing the size of the main jet. Or, look for exposed plug threads or a sharp
    edge on a valve or somewhere else in the combustion chamber. Excessive carbon build-up can also be the culprit.
     
    Last edited: Jun 20, 2009
  3. GoldenOne7710

    GoldenOne7710 Veteran Member Lifetime Gold Member

    4,184
    158
    Jun 23, 2004
    Athens, GA
    Try backing off your timing and have it 'all in' at 3000-3500 RPM and see what happens. 2500 RPM is a bit too quick in my opinion.
     
  4. Tapp

    Tapp Member Gold Member

    34
    0
    Dec 1, 2007
    Dry Fork VA
    Thanks for the suggestions guys. I was going to switch out the spark plugs today but on second thought, if it is pre-ignition, which is very much what it sounds like, spark plugs should not have anything to do with it if my thinking is correct. If it were the plugs wouldn't something have to tell it to fire? Seems it would either have to come from the spontanious combustion bit or over advanced timing. Unless there is someting weird going on with the stock distributor I would think it could be ruled out. Is there nothing in the carb that could cause a lean condition once I pour the coal to it?
    Thanks again,
    Tapp
     
  5. markw

    markw Veteran Member

    816
    0
    Jun 6, 2009
    Illinois
    Check the rear float level. Could be leaning out when vac secondaries open.
     
  6. Tapp

    Tapp Member Gold Member

    34
    0
    Dec 1, 2007
    Dry Fork VA
    Thanks Mark. I did recently "adjust" the front and rear float levels, both to be level with the bottom to the hole. Where should the rear one be or is that going to make a difference if it is leaning out? One other thing that was kind of strange, when I loosened up the locking screw for the rear float adjustment, gas would spew out of the fitting. It did not when adjusting the front one. I replaced the o ring thinking it was not sealing but it didn't help.
    jt
     
  7. markw

    markw Veteran Member

    816
    0
    Jun 6, 2009
    Illinois
    I always get some fuel leakage when adjusting float level on both primary and secondary sides. Fuel at the bottom of the sight hole is good. You can make slight adjustments up or down (2-3 wrench flats max) but it makes very small changes. Not enough to fix your problem. Try disconnecting the vacuum secondary rod to eliminate the secondaries. Are you sure your timing is ok? Valve springs good? Valve seals ok? Oil on plugs? You mentioned newly rebuilt motor. Is cam going flat? Are you sure it's spark knock and not mechanical?
     
  8. Tapp

    Tapp Member Gold Member

    34
    0
    Dec 1, 2007
    Dry Fork VA
    I'm fairly certain the cam is good and the knock is not mechanical. With the car running great with the exception of the pinging at the higher rpms (only on hard acceleration) it makes me wonder about the point of could the mixture be leaning out on heavy acceleration. What all could cause that?
    tapp
     
  9. markw

    markw Veteran Member

    816
    0
    Jun 6, 2009
    Illinois
    If you're sure the timing is right then it's probably the carb. You might want to rebuild it. Could be some plugged passages or a power valve that's stuck shut. Did it sit a long time while you rebuilt the motor? Maybe something crawled inside and died. Make sure the fuel pressure isn't dropping at high rpm. How do the plugs look?
     
  10. Tapp

    Tapp Member Gold Member

    34
    0
    Dec 1, 2007
    Dry Fork VA
    Plugs look a little brown, not light tan. They are pretty new < 100 miles and only have color on one side right now. Not all the way around. I had sent the carb off to Holley to be rebuilt while the engine was out. I think today I am going to keep backing off the timing 4-5 degrees until either the chatter goes away or the car won't run. I'll also try and disconnect the vacumnn secondary as well....I'll get back to you.
    Thanks again!
    jt
     

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