Outer Limits 4x4 Board

I've narrowed a running rough problem down to the 3rd cylinder. When I pull the lead off the spark plug while its running, there is not much noticeable difference. If I pull the lead off any of the other plugs, you can hear a definate difference. I've tested the spark plug and the lead and replaced the plugs, distributer cap and points not long ago. What could be the problem with the cylinder and how can I fix it?

Compression test it. If there is no comp or very low, it could be a valve, either bent/not seating properly or cracked. If there is comp do a wet test and see if it improves the pressure (wet meaning put a few squirts of oil in the plug hole and re-test). if the comp does improve, that will determine that it is a ring problem.

Defective wires are very common. They get bent sometimes too tight in their packaging, or before you buy them, and that can damage their insides.

Swap out that wire with another one, and see if there's any difference.

I had a 1.3L with the 3rd cyl. compression low. I sold it, so i didn't end up fixing it...

I tried a different lead but still had the same problem. Would it be worth trying any of the oil additives to clean the valves or seal leaks, etc?

shoot a butt load of carby cleaner down there and maybe knock off some carbon deposits on the valves. Maybe the valves aren't seating right from too much crap. It's worked well for me before on a V8 Jaguar engine.

Oil additives can help increase compression, but it's only a temporary fix.

Did you test the compression yet?? From memory, i think they're supposed to have 140psi, and the 3rd cylinder i had was 80psi.

I haven't tested the compression yet. The tester I have is too short to get down into the hole. I'll have a go at giving it a good blast.
The problem isn't really noticeable at high speeds. Does that still sound like a valve problem?

It's always hard to notice problems like that at high speed, especially in the loudest 4x4 ever created (sierra's).