View Full Version : High Speed Miss Problem
09-20-2003, 10:50 AM
I've got a high speed miss that generally occurs around 7000. It might start around 6800, or until 7000 or so. The combination is a 421 sbc with afr 227 heads, cam motion solid roller cam, holley hp1000 carb, and a holley strip dominator intake. The ignition consists of a 7al-2 box, pro power hvc coil, crank trigger, 8361 distributor locked out, old style multi step retard, msd 8.5 super conductor wires, and ngk -6 plugs gapped at .030. I had this same problem with my old motor last year, and thought that I had it fixed. As soon as I got this motor running and took it to the track, it started popping again, just like with the old motor. I have been through the ignition system with a fine tooth comb and don't see anything wrong. I replaced the wires, closed the gap on the plugs, checked the cap and rotor, checked rotor phase, installed a noise filter on the heavy wires going to the 7al-2, and have even checked my valvesprings, thinking that it may be floating the valves. I don't really think that the problem is fuel-related, though I won't rule it out yet. I tried removing the chip from the rev limiter to see if that was it, but it made no difference. Any ideas or suggestions would be appreciated. Also, what does the crank trigger sensor ohm when good? I will check this to make sure it is up to snuff. Thanks again, and if I've left anything out, let me know. Chris
09-21-2003, 04:36 PM
The MSD Crank Trigger Pickup, PN 8276, resistance across the two leads is 65-85 ohms. To focus in on the problem, I would bypass or remove the old style Multi-Step Retard unit that you are running and also install a sheilded magnetic pickup cable, PN 8862. Then run another test and see if the problem is still there.
09-22-2003, 01:35 PM
Something else I failed to mention in the first post is that this missing problem gets worse when the nitrous is activated, leading me to believe that the problem is ignition-related. I'm running the shielded cable. Another question: how would you go about testing the coil to see if it's breaking down? I have the #8251 coil. Thanks, Chris
09-22-2003, 10:12 PM
Check the tower of the coil to see if you are getting any carbon tracking on the tower. As for fully testing the coil, the best way is send it in an let MSD perform a open gap coil test on a load bench to see if it is breaking down. In the field, the only test you can do is to physically check for arcing or carbon tracking on the coil. If you are running a heavy shot of nitrous, then you will need to run a smaller gap than 0.030.
10-20-2003, 02:19 PM
Just wanted to let you guys know that I got this problem fixed. It ended up being a problem inside my tach. I have a Autometer #6811 pro comp tach and tried a friends Ultimate playback tach and the problem went away with the different tach. I'm going to send the tach in for repair. I thought I'd post this just in case anyone is fighting a similar problem.
10-20-2003, 02:35 PM
The tach going to the tach output should have no effect on this. The tach output is separate from the output drivers of the ignition box and can completely fail and the output portion of the box will still operate. If that fixed the problem that is great, but it is odd that it did fix. If you have any other problems let us know.
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