| Last spring I had a problem that sounds similar to yours. My truck would idle fine, but after driving for about 15 minutes it would begin to misfire at high rpm. As you drove it longer the misfire would get progressively worse until it was misfiring as low as 2000 rpm's.
I took me a while to find the culprit. I had installed the Petronix ignitor more than a year before and never had any trouble with it. I didn't want to believe the ignitor was the source, but after putting the points back in and the truck running well I had to believe it.
I spoke to Pertronix about it and they helped me solve the trouble. The most common way that the UM-161 ignitor is installed is to put the red wire between the coil and ballast resistor so that it is at +12V. In reality the ignitor should be run at +24V. It will usually work at lower voltages, but sometimes variations in coil or ballast resistance can cause the voltage between them to drop below 12V which is what was happening to me.
After wiring the ignitor up to +24V (or closer to +28 really with the engine running) no more trouble. Most people wire their ignitor at +12V and have no trouble, but it should be run at +24 according to Pertronix. I spent quite a while talking to them on the phone, they were very helpful.
Anyway, not sure if that's your problem, but it sounds similar to the issue I had last year. |