[Jason Pittenger]

I have iridium spark plugs and my aftermarket ignition controller in. On a whim, I decided to try a larger plug gap. Based on the graph found at the link below, a 1.4mm gap can fire a 17:1 AFR. I didn't use a standard gapping tool, as those damage the iridium tip.

I bumped the rear plugs up to that gap (they were at 0.9mm) to see what happens. I only did one cylinder because I want to compare results. With true dual pipes, I should be able to hear misfire when one side gets quieter.

Denso Iridium Spark Plugs Turbo DSM

Iridium: 8 times stronger than platinum, 6 times harder, and with a melting point 1200 degrees higher. New laser welding techniques make it possible to make a spark plug with an iridium electrode. The result is a plug that requires less voltage to spark, burns more of the fuel in the cylinder...

machv.com

 

[Jason Pittenger]

I set the rear plugs to 0.045" (1.15mm) and kept the front at 0.035" (0.9mm). I found during decel, the front cylinder pops more than the rear. It took me awhile to get my head around this, but the pops during decel are because the mixture is too lean to burn in the cylinder, causing unburned fuel to collect and burn in the exhaust. Since the rear cylinder is still managing to ignite in the cylinder some of the time during decel, there's less fuel to ignite in the exhaust.

I'm also having a lean misfire coming off idle. I actually had it die while pulling out into traffic! I can reliably recreate it by trying to accelerate in 4th gear with RPM at 1500. (Not recommended for lugging reasons!). I noticed each time that it was the front cylinder that was misfiring.

In both cases, it seems like the wider gap is helping with lean burns. I don't know if copper plugs and/or the stock ignition system can handle gaps wider than the recommended 0.035" (0.9mm), but it seems to help with Iridium plugs and my aftermarket ignition controller on stock coils. It is band-aiding a lean condition that I need to solve, so I'm going to leave the front gap alone until I'm certain the lean condition is gone.

 

[Jason Pittenger]

The misfire was due to a configuration error in my ignition controller. I am now running 0.045" gap on all 4 spark plugs.