Hey guys my name is Rich i was once a member a few years back and im not that great with computers,My monte is a 1986 305 carb car i purchased 4 years ago for 300$ with 68k miles on it.I love this car and you guys have already helped me with a transmission issue (TCC SOL) &4th gear sens. 2 yrs ago:

 

I hope someone can point me in a direction with this issue i have during heavy rain yesterday i started my car to move it in my driveway when all of a sudden the car felt like it was running on 7 cylnders .....and running rough the car starts right up and idles but under a load its really rough.....What couldve gotten wet other than the dist cap .....1986 carb car

 

I popped off the Dist cap n from the vibration of the "missfire or atlest (thats what it feels like)the rotors screws were very loose.I tightened them n the problem still Exists.

 

Your rotor button in the top of the distributor cap could be worn or wet and/or there could be a microscopic crack in the distributor letting in moisture. It does happen.
I have gotten 2 defective ones right out of the box.

 

P343 is correct, I had that issue on a vehicle before. inexpensive fix, wouldn't hurt to check spark plugs either.