Spark Plug Gap

I'm running a 557 MBR with a 1471 M5 on alcohol. I'm having an issue with the plug gap in one cylinder. I am using 6061-10 plugs and gap them at .014". The first test run of the season, #4 cylinder went dead around half way through the pass and when I check the plug it was .018" plus. The rest were at .014". I thought I either missed one or it was a bad plug. Put a new set in for the weekend and on Friday it ran fine. I didn't think to check the gap since it ran good. I put the same plugs back in for the second run and about half way through, #4 went dead again. Checked the plug gaps and sure enough, #4 is .018" plus and the rest are still a snug .014". Just eyeballing the center of the plug, it doesn't seem to be sinking. What could be the cause?

 

Mike is right, .018" won't hurt it. Put on a brand new plug wire and check rotor phasing.

 

All of the plug wires have 5 runs on them. I'll check under the cap. I used to gap them at .015", but tried to tighten it up a little to see if it solved anything. It is in a pulling truck, so my timing is the same all the way through the run. You can see the cylinder miss intermittently from about 6000-7000 rpm and then above 7000 its just blowing raw fuel. The rest of the ignition is clean, just that one cylinder when the pressure starts to build.

 

Change the plug wire!

 

Same as Mike says, I have had 2 sets of new wires and one would not fire all the way through a run. Changed it and all was OK!

 

Not saying it isn't the wire, I can change it.

How does the gap increase?

 

Wires do weird things. I use Moroso now after talking with Jonathan young tuner on here. He is a dealer for lot of parts and is a straight shooter. Give him a call. He's seen a lot of things.

 

If the cylinder is detonating it can start to push up the crimp on the plug which holds the porcelain in place. If it pushes up, the center electrode goes up with it, away from the ground strap which increases the plug gap. Way back in the day, I have seen porcelains shot right out of the plug case after a few runs on the same plugs with some detonation. That would explain the increased gap but why would it stop firing? If it stopped firing and then hydrauliced it could jack up the porcelain and produce the same effect. So you are back to why does it stop firing....fat cylinder and it washes out? Does the plug show hot or cold?
Jeff Johnsen

 

Inspect the boot inside and out for black tracks.

 

The first time they were new plugs, #4 showed a half burn while the rest were full, not quite up any threads. When it ran good over the weekend on new plugs, they all showed the same, maybe starting to creep up that first thread. Since I used the plugs again, I can't tell the heat the second time.

 

Mike I have read your article and pulled this from it

That is pretty much where it is. They aren't quite burning up the threads yet. I ran it with this tune up all of last year. Since they run 12-15 seconds, I have 18 gpm going to it. I'll take a closer look and see if I can see the porcelain moving at all.

 

I realize it's off topic, but mine doesn't get real sporty until at least the 2nd thread.