I've had the same exact problem. Everytime it'd rain hard, or even a really heavy dew would make it sound like a Volkswagon. I had two problems... the distributor was getting condensation inside the cap, interrupting the electronics. I also had the 6 wires in the harness that plugs into the module come loose, and they were getting wet too. If I could get the engine running long enough to dry it out under the hood, it was ok. Otherwise the car had to sit for a day or two. Drove me nuts till it was found.
yeah i think that was the problem. i lost my mind when i thought running a fender well cold air intake without wheel well protectors was a good idea. the filter was like a wet spoung. i know, im such an amature
just to reiterate what the others said, The first 289 in my '69 had the exact same issues. The MSD distributor was getting wet as it was some how dry rotted when I got the car. Got a hole new distributor for kicks and it worked fine. I know it was the cap because the buddy of mine I gave the old one to just replaced the cap and all was good.
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