I am trying to track down an idle, possible miss, and a vacuum issue I am having. Car has been running great until yesterday when I went for a drive and it would not idle for more than 30 seconds when in drive before it would die. It started back up every time, but had to put it in neutral whenever I was stopping at a light. I changed the plugs since they are the only thing that I have never changed. After changing plugs I wanted to see if I had a miss. Well the engine idle changed and the engine pretty much died when I pulled 5, 7 or 8 but would just idle really bad when I pulled 1-4. When I pulled #6 it does not affect it at all. So I swapped wires, no change. Went back to another plug, no change. I know for a fact that I have spark at the plug on #6 but when I pull the wire the engine is still running pretty smooth. Another thing I noticed is that I am pulling about 14 hg of vacuum as compared to the 18+ I had last time I checked and the gauge is not stable on 14 it bounces around like crazy (but it has always done that). I sprayed carb cleaner around looking for a vacuum leak, but didn't find anything. I took it for a drive and it pulls like crazy but I haven't had the car running long enough to notice if its not pulling like it should and it still dies after about 30 seconds if I am stopped and in drive.
So finally here is my questions. First can the #6 actually be firing, and for some reason it doesn't affect the engine the same if I pull the wire or will you always be able to tell if you aren't firing on all 8? Or do I have another issue going on that I am not seeing? Engine is a 302 with Edelbrock Performer Cam, RPM intake, and a 1406 carb. Idle is at 1000RPM in park and about 600 in drive which I had to up from about 800 in park because now it just dies at anything less than 1000RRM when I put it in gear. One thing I have changed since the last time I drove it was the thermostat it had for some reason a 160 and the needle would never leave the line on the gauge so I put in a 195 and know the motor is running about 185 degrees. Could this have messed up the Carb tune? I realize this is very long, but any help you can give would be great.

 

If I am reading this correctly, number 6 is dead. You need to do a compression check to confirm this. Once confirmed, do a cylinder leak down test and listen for where the air is escaping; if you hear it through the oil filler hole, the problem is cylinder/rings/piston, if you hear it through the tail pipe, the problem is an exhaust valve. If you hear it through the carburetor, the problem is an intake valve.

 

since you changed the plugs, they should be all new. pull #6, you should be able to see if it's firing or not.
Further make sure to swap spark leads. lets say exchange the cables that you use for #2 and #6 cylinder. Like for like. If the issue moves to #2 you're lucky and it's the lead

 

Thanks for the help so far. I pulled the #6 and #5 plugs to compare. The #6 is clean compared to the #5 so I don't think that it is getting fuel. I don't know if this means anything or not but I pulled the plugs right after I shut the car down and the #6 was dry. Should it be wet if its not firing and getting fuel? I also pulled the valve cover a turned the engine with a socket on the crankshaft bolt and from what I could see the valves springs are being compressed the way they should and I didn't see anything obvious. I don't have a compression gauge, so that will have to wait until I can track one down. What would an engine with a burnt valve act like? Any other advice or suggestions would be great, thanks.

 

If you have the spark plug loose will it hiss as you crank or turn it?