Engine Engine Misfiring Bad!

Discussion in 'Fox 5.0 Mustang Tech' started by GoldenEagle91, Jun 9, 2014.

  1. Hi Guys,

    To give you some background as to the problem I am currently having let me start with some history. Approximately 2 months ago I installed long tube headers, new plugs, (motorcraft AGSF52C) and wires. The engine was misfiring for a while and it would typically occur if I was just cruising along with the rpms anywhere between 1200 and 1900 rpms. It never really missed as far as I could tell if I was on the throttle a tad harder. I decided to change the fuel filters in my carburetor because I had assumed that they were clogged and that was why I was misfiring for so long.

    So I go to remove the old filters from the carb and come to find out that there were no springs in there at all so whether or not it was really filtering fuel is up for debate. While we're at I got my engine timed to 10 degrees BTDC with a timing light. The mechanic who was working with me on this stuff noticed that my ignition coil looked quite worn out/not supplying required voltage and asked what the spark plugs were gapped at. I had them gapped at .050 before but he suggested going to .035 because of the poor condition of the coil so we did. I go to drive down the road and the car is missing worse than it was before in that same RPM range I mentioned above.

    I was wondering if you guys would suggest regapping the spark plugs to a larger gap because what I am thinking right now is that the gap as it currently sits is not large enough to effectively burn all of the AF mixture in the cylinder. One last thing is that it does not miss hardly at all if I am accelerating at a bit of a quicker rate but if I am slowly making my way through the gears it starts to miss like crazy.

    Any help is much appreciated.

  2. What are you using for ignition, distributor and coil?
  3. .035 is good for a NA car. I suspect you either have plug wires cooked by headers, or a lousy coil.
  4. I am currently using what appears to be either a standard OE distributor, and a coil that looks like it was pulled out of a junk yard. There is a crane cams ignition box under the hood though. I have been rebuilding an engine and these are not the components that I will be using. For my rebuild I have an MSD distributor and coil.

    I do not think that a wire got cooked by the headers as none of them sit close to it at all. It may be an absolutely lousy coil. The previous owner has the coil mounted on its side. From what I understand, doing that burns up the liquid in the coil and turns it into a pile of poop.
  5. Does the distributor have a vacuum advance? If not, pull the distributor cap off and try twisting the distributor rotor. If it is the correct distributor, it should move a fair amount in one direction and the pop back the to the original position.

    If it doesn't, you have a EFI distributor that has no built in spark advance. That's a sure sign that someone didn't understand what they were doing. No spark advance and the engine runs with sub par performance. Some of the newer, expensive MSD boxes (the 6AL-2) can do spark advance, but the older ones don't.
  6. The distributor does have a vacuum advance on it and it is hooked up to the carburetor. From other things I have read, the vacuum advance on distributors doesn't always work. Is that correct?
  7. The vacuum advance works correctly if it is properly connected to vacuum and there are no defective parts in the vacuum advance mechanism. It's primary purpose is to add additional advance when the engine is in cruse mode to improve fuel economy.

    If you had the original EFI setup still intact and working, the computer would self diagnose many of the problems that you are seeing.

    The ignition box is the next suspect.

    Alternative method:

    21. While you are at the electrical part, you'll need a Duraspark or similar ignition system. The 85 Mustang GT 5 speed has a suitable Duraspark distributor with a steel gear compatible with the roller camshaft. The EFI ignition depends on the EFI sensors to advance the spark. Rip out the TPS and MAP/Baro sensors and the computer will have no idea of the proper ignition timing for best performance. Running a fixed timing setting is only for test purposes or for a race track only car. Don't try it on the street: the results will not be nearly as good as a properly setup Duraspark or equal. Crane makes a really nice distributor for non-EFI applications. . See http://www.cranecams.com/index.php?show=browseParts&lvl=4&prt=127 for more information. Cost is about $400, which makes the 85 Mustang reman units look really appealing.

    Duraspark II ignition diagram:

    Diagram courtesy of /www.billwrigley.com
    See http://webpages.charter.net/1bad6t/duraspark.html for more help.
    Note the ballast resistor shown in the diagram: you’ll need that too
  8. Well cripes, I guess I don't know whether or not the vacuum advance is functioning or not. Do you think putting in the new coil would solve the problems or at least help some? The car was never actually EFI. I just need to get by for the next month or so and then the engine that I have been rebuilding will be going in with all new MSD ignition components.
  9. A defective vacuum advance would only affect your gas mileage. It won't cause the engine to misfire.

    What year, make and model car are we talking about?
  10. We are talking about a 1979 Mustang Ghia. 5 Liter with a 670 holley street avenger