I'm working on a friend's 69 K5. It has a 350/350/205. It sat for about a year while he was deployed to Afghanistan. He parked it at another friend's house. While he was gone, his friend's son decided to "help daddy work on the truck" and filled the fuel tank using the garden hose. It then sat that way for six months. We replaced the entire fuel system, including the tank, lines, pump and carb. We finally got it running. It idles fairly well, but when you accelerate there is a loud popping noise from the carb. I thought it might be a timing issue, but after triple checking the timing and plugs, I could find no problem. I started pulling one plug wire at a time to see if I could isolate the cyllender. It turned out to be #4. I pulled the valve cover and had my wife crank the engine. The #4 exaust valve did not appear to be opening. It only moves about 1/8 of an inch. I think it might just be a collapsed lifter. I am going to switch the intake and exaust lifters to see if it makes a difference. If It doesn't, it may have a flat cam lobe. What do you think the chances are of it just being the collapsed lifter? Or, am I totally off track here? Thanks.