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By slumlord444 - 8 Years Ago
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Been fighting this for a while. Engine only has a few hundred miles on it with fresh rebuild. Miss didn't seem to be there when I first got it running but engine didn't seem as strong as it should on the high end. Mummert heads and custom Isky cam. E carbs and original T bird distributor. Ignititon breaks down around 4,000 RPM. Had Pertronix I in it with the Pertronix coil. Switched out to a Pertronix III with the rev limiter and the new matching coil. and same miss. The external resistror has been removed but it was in place with the Pertronix I. Checked the grounds and ok. Still missing. Pulled the plugs. Two fouled and a couple seemed to have a leak around the porcilin. Installed new different brand plugs and new Pertroncx plug wires. Same miss. Prety much shuts down, not just droping a cylinder. Had my retired mechanic friend take a ride today and he agrees it is ignition. I have and issues with the distributor in the past and have had bushings put in it twice since I bought the car in '65. Mehanic buddy suggested runing a jumper wire from the postive battery post to the positive coil termina to make sure its getting full voltage. Haven't tried that yet. Need to dig out my spare non tach drive distributor with points and see if the distributor is the issue. Any ideas on where else to look. Engine starts, idles, and runs strong up t around 4,000 RPM.
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By Gene Purser - 8 Years Ago
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Is your total timing too high?
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By slumlord444 - 8 Years Ago
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Don't think so. Was at 10 with the old Pertronix. Had problem then. Cranked it up to 14 with new Pertronix. No change. Too much advance would make it ping but shouldn't cause ignition breakdown.
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By Ted - 8 Years Ago
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You don’t mention your spark plug gap. If more than 0.035”, then try reducing it. If low ignition voltage is suspected, then reduce the gap to 0.025” and if that helps or cures the problem, that will point you in looking in more detail at the coil and/or Pertronix unit. You might also want to insure you have adequate grounds both inside the distributor and going from the engine to the body/frame.
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By slumlord444 - 8 Years Ago
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Plug gap is .035. Checked grounding. Same problem with old and old Pertronix and coil.
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By Ted - 8 Years Ago
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With the Pertronix III, you should be using a 0.6 ohm coil. Your misfire at high rpms still sounds like an ignition issue but don’t rule out a carb or carbs that’s simply too rich in that upper rpm band. On the flip side of this, it’s also possible that the misfire is the result of one or more secondary barrels not getting the required amount of fuel. Faulty O-Ring seals around the secondary fuel tubes can prevent fuel from being pulled into the secondaries. You can pack a small amount of white grease around those O-Rings for the short term to see if it makes a difference.
Double check that the coil is getting full voltage. If you are running an electric choke, insure it is getting voltage from somewhere besides the coil connection as that will create a voltage drop by itself. The coil voltage needs to be coming directly from the ignition switch with no other accessories being tapped into it. You don’t mention your compression ratio but as the compression ratio goes up, so does need for higher voltage at the spark plugs; reducing the spark plug gap reduces the voltage requirement at the spark plugs.
Too much total ignition timing will also make for a high rpm misfire. When performing a total timing check at 3000-3500 rpms, make sure the vacuum advance is disconnected. The aluminum heads are typically happy with about 35° of total mechanical ignition advance. You don’t mention if you are running the vacuum advance or not but if you are, it’s important that it be hooked up to the carburetor and not to a direct vacuum source. Assuming you have the vacuum advance hooked up, you might try disconnecting it to see if the misfire goes away.
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By slumlord444 - 8 Years Ago
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Centrifical advance only. Worked fine before rebuilding the engine. Will run jumper from battery to coil to make sure it is getting 12 volts. Flat top Ford pistions with the aluminum heads. Had the block surfaced just enough to make sure block was flat and straight. Should be around 9-91/2 to one which shouldn't be a problem. Coil is new Pertronix to match the PetronixIII. Sure sounds like ignition rather than fuel but wouldn't be the first time I was wrong. Mechanic buddy is prety sharp and he agrees it's ignition. I am still leaning towards distributor bushings because I have had issues with it before. At some time I will dig out my spare distriburor and try it. Have other things taking up my time at the moment that are more pressing. Thanks Ted.
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By pegleg - 8 Years Ago
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If you have another known good coil, try it just for the heck of it.
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By slumlord444 - 8 Years Ago
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Tried an older good coil and the new Pertronix and no change.
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By Sandbird - 8 Years Ago
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Does your engine miss and cutout if you rev engine to 4000 rpm while sitting still in neutral?
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By slumlord444 - 8 Years Ago
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Haven't tried that. Not sure I want to but back in the late '60's when I was bracket racing it with a milder cam and stock heads I was launching it a 5500 rpms at the Alton Dragway.
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By miker - 8 Years Ago
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Yeah, we're getting too old to change parts that often. And launches are better with modern tires.
Seems like you've checked and mentioned everything except the cap and rotor. I've had those fail in ways that aren't obvious, particularly the spring loaded center contact to the rotor. I've also had the odd case where the rotor is right on the edge of the plug it's going to fire with the advance all in, leading to a miss as the voltage requirement goes up. Now that I think about it, that was when I was replacing the points with a control module. A race car buddy had me drill a hole in an old cap so I could see where the rotor was pointing with the advance wired open. Am I making sense?
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By GREENBIRD56 - 8 Years Ago
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I have seen - but wasn't the operator of - a cap that had a hole cut in it. And a timing light rigged to the plug wire at the hole - so it acted as a strobe light to view the rotor position at that wire. When the engine was running you could see the advance system move the firing point as the rpm rose. I would suppose that centered in the travel is what would be the goal?
There was a time when you could find "made for the purpose" clear plastic caps that allowed this view without cutting a hole.
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