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High speed misfire

Discussion in 'Performance' started by kzooman83, Dec 11, 2014.

  1. ImperialCrown

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    If this has a mechanical choke, does it run better with the choke on or off at higher speed when warm? This may help with the mixture question.
    Spark plugs tend to carbon-foul easier when cold (first 5 minutes) and the tips can tell a story. Carbon-fouling can cause a misfire under load.
    How did you determine what type and what spark plugs are you running?
     
  2. dana44

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    The heads are ported so a choke isn't needed, at least not on any of the heads I port. Being able to move the rpm the miss occurred with the power valve and the fact the change in carb proved the problem was with the carb itself.
     
  3. kzooman83

    kzooman83 Active Member

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    Thanks for the replies again. I actually do have a wideband O2 gauge in my spare parts stash that I inherited from a friend a few years ago (I get more spare parts that way), but I don't yet have a sensor for the thing. I have dual exhaust with no crossover at the moment; where should I install the sensor if/when I get one?

    The carb has electric choke and it is full open within a couple minutes of starting the car. I think I am running champion spark plugs, IIRC. They are stock replacement plugs from autozone. I have been through a few sets of plugs since this problem first started, though; I have literally been chasing this misfire for years.

    Dana, I have actually not ported these heads yet; I have been working on and off porting a set of old 318 heads so I can figure out what I'm doing before I screw up the heads on my big block.
     
  4. dana44

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    My bad, I thought you had a set of template ported 906 heads on her.
    Couple things here. Stock plugs might be a little bit off given the engine is 9:1, so stock sparkplug for the highest compression 400, 1971, so as not to be too hot a plug, but that should be more of a wear problem, not a misfire issue, and isn't in my book given the problem has gone away mostly with the new carb.

    Sensors usually just hook up to the existing O2 sensor, and you do need to get that H pipe in there to help balance the two sides. So location should be about 18-24 inches past the dump (so you have room to install a bung into the exhaust pipe without really fighting everything, can't remember if you had header or not, and if so, about three inches forward of the collector end). I bet you are also savvy enough to wire one on each side with switches to switch from one side to the other in order to compare banks, too, right?

    You have narrowed down every possibility very well, and this is the only missing thing to know if it is a lean or a rich problem when the problem occurs. With the old carb it is not jetting, it is not the power valve, and if it is the secondary itself, it is the lack of fuel at the vacuum opening, or air bleeds/fuel bleeds plugged in the body, but a fuel/air ratio needs to be known to correct the problem that is occuring. Not enough, same problem and lean problems occur, too much, waste of fuel and lack of performance. You can set up the sensor and gauge for the cost of th O2 sensors and three switches, or a dyno tune shop for several hundred dollars.
     
  5. ImperialCrown

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    Are the plugs being replaced because of rapid wear or fouling or some other reason?
    Do they appear to burn rich or lean or normal?
    At higher engine speeds with open throttle, I would suspect lean.
     
  6. dana44

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    I suspect lean, too, but one can't tell how much without knowing fuel/air ratio in order to know how much to correct the issue, or where to correct the error, be it primary jets or secondary vacuum plate.
     
  7. kzooman83

    kzooman83 Active Member

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    I usually replace the plugs preemptively every few years or so. I have seen them look rich, lean, and normal as I have tried different things with the carburetion and ignition timing. I haven't pulled them out in a year or two, so I would have to pull them to see how they are looking now.

    I guess my next upgrade will be installing wideband o2 sensors and an exhaust crossover.

    My goal with this carb is to replace the main body and then swap it back on as part of a package including the comp cam I have in storage and a set of TTI headers to open up the breathing.
     

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