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Ignition question

Discussion in 'The Hokey Ass Message Board' started by patrick2965, Aug 5, 2013.

  1. I made a 900 mile round trip in my SBC powered A. In the course of the trip, it developed a cylinder miss. After getting home, tested header exhaust port temperatures and found low on only #1. Swapped plug wire and examined spark plug appearance. No change in engine beat after refiring to listen and measure temps.
    Distributor cap is garden variety black plastic with aluminum ......dadgummit....terminals to send spark. Moroso fat plug wires. 9.25 compression. R45TS plugs with .040 gap. GM point distributor with Crane electronic conversion. MSD coil. Crane whatever box that mimics and MSD to a point. 2200 lb car with stick and 3.50 gear.
    This is the question.
    Am I killing the plug wires or distributor cap with plug gap? The ignition box and coil are going to provide what they do based on resistance as spark plugs also do.
    Plug gap based on what I thought was correct made me change from .035 gap to .040. The engine beat got stronger based on that experiment and a few thousand miles.
    I wonder if the cam is going flat at #1, but the sound of the mechanical aspect doesn't sound that way. I would think a significant clatter would develop if that was the story, could be wrong, haven't pulled the valve cover to measure valve/rocker movement differences if there.
    Exhaust temp heat with engine running at header port is 260 area except #1. It is many degrees low, 140.
    I don't think I want to know the answer, but it needs to be fixed.
    My cheapest best guess is distributor cap, could be wrong.
    Joined: Jun 28, 2011
    Posts: 388

    from Dinuba

    Sure, cheap stuff first. Whoever said don't be a parts changer never ran a shop. You gotta show that you are concerned about their pocketbooks.
    Vacuum gauge? It's a wonderful diagnostic tool. Looking for steady vacuum. Flicking neede is a BAD thing...
    I like your diagnostic approach. Yes, cool cyl is the miss. But why?
    Did you actually check for spark on cyl 1?
    Inductive timing light is poor man's o'scope. Place clamp over each wire as it runs. Does light flash on each one?
    Crank evenly? Compression check number 1 cyl and compare good cyl.
    SBC flat cam often backfires through carb. Does it do that?
  3. No carb backfire yet if cam is going flat. I don't have a good inductive timing light, therefore the use of another plug wire.
    I don't have a Timmy to hold the plug to ground. Haven't checked compression yet, but no oil signs on spark plug.
    You're on same track
    I question myself on plug gap because in compression some spark plugs will not do their job, as turbo, supercharger, high compression motor folks know very well..
    And in line with this, I question myself of killing a distributor cap and plug wires with plug gap.
    Resistance to ground is why the beat control in the distributor, engine compression, timing, plug gap, plug wire, distributor cap material and contacts, have so much meaning to if an ignition system works or not. Blower/turbo folks know this better than I do.
  4. barobert
    Joined: Apr 14, 2010
    Posts: 104


    Look once to the pictures of each spark, that can give you a indicator when the fire in one cylinder is good.

  5. cigarcaptain
    Joined: Jun 11, 2009
    Posts: 43


    Possible broken valve spring. Not always a noise associated when this happens. Good luck, hope its a simple fix.
  6. Commish
    Joined: Jan 9, 2010
    Posts: 379

    from NW Ok

    First thing you need to do is check compression, if good then move on to the other things. Cam going flat on intake lobe will give a sharp unmistakeable backfire through the carb, exhaust going flat will often give you an unusual sound at the end of the tailpipe, but no backfire. I don't think your plug gap would be hurting the cap or wires.
  7. Dooley
    Joined: May 29, 2002
    Posts: 2,750

    from Buffalo NY

    had this happen.....just developed a miss and thought i wiped the cam out, turns out a spring broke....sometimes is is hard to see....good luck
  8. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    from Texas

    A broken valve spring might not show up at cranking compression if it's a dual valve spring. You need to get the rocker cover off and check out the spring and measure the lift on number 1 compared to the adjacent cylinder. A common problem is also vacuum leaks off of fittings in the number one runner that will kill the cylinder at idle.

  9. I have put thought into pulling the rocker cover off to look at valve lift and spring condition. Compression is a thought too.
    I did find an odd looking spot inside the distributor cap, so I 'm getting a new brass terminal cap after work today. I hope that does the trick.
    Thank to all for ideas, I will be using the advice.
  10. cooger
    Joined: Nov 5, 2008
    Posts: 233


    Far as the plug gap, you shouldn't have any problem there. Most conversions say around .40-.45. I've used a Pertronic's at .40 with a mild sbc with no problem, good running. Now I use MSD pro billet with .40, outstanding.
  11. Here's how it shook out.
    I put in an order for a brass terminal distributor cap at the O'Reilly leprechaun connection I use with my job. Installed after work and started the engine. I swear it sounds stronger than before I took the trip.
    Appearance inside the removed cap had some pits by terminals not associated to miss at #1. I don't think aluminum terminal distributor caps like to carry the resistance of a larger spark plug gap and compression related increased resistance. I could be wrong.
    Hittin' on all eight, I'm happy. Now I have to clean all the dirt from that drive to where I grew up and shitty I-57.
  12. henry's57bbwagon
    Joined: Sep 12, 2008
    Posts: 677


    Good to hear about such a simple repair, once it narrowed down. My buddy had a high rpm miss and it was the coil wire shorting through a steel sleeve to ground, only under load.
  13. Good deal !

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