At first when I tried the NGK Iridiums, my bike sucked in Gas Mileage. From 45 to 35 mpg. It started Quicker, and bike felt the same, but I took em out anyway. Got to reading a little more about different spark plug gaps, and decided to try a different gaps with them. I should add, I have a wire type gapper. Went from the factory .40 gap to .35, and its noticably stronger in the 2500-3500 rpm range with the NGKs now. Even riding 2 Up it's noticable. The Exhaust Note from the Screamin Eagle slip ons even sounds different, and better. With stock plugs, if I don't let my bike warm up fully before I drive off, it doesn't pull as hard, and the exhaust note doesn't sound as good. But when she's nice and toasty, the SE exhaust has a hamonically trumpet like sound. Like multiple notes that musically go together. Bumps raise on my arms it sounds so good. But now, the NGK Iridiums @ .35 gap instead of stock .40, it's sounds harmonic all the time, and pulls a little stronger than before. Even without warming up! Haven't​ checked my Mileage yet, but I'll be posting it later.

 

You bike runs a Delphi ECM that uses the spark plugs to sense for detonation.. The problem is that the Iridiums make the ECM think that there is detonation when there is not and it retards the ignition timing.. That is the reason for your gas millage loss... You'd be better off with stock plugs..

 

I put NGK iridiums in my twin cam 88 and they fouled up instantly with carbon coming out of the exhaust. The bike hesitated too. It is EFI. Do you think the problem is related to the ECM? Lots of guys have no problems running iridium from what I've read.

 

Go back to stock plugs first.. Probably not the ECM. The problem is that that tune of the motor will make the issue better or worse.

 

 

Hi Max,
I put the old OEM plugs back in and everything worked fine again. I have a FI2000 Cobra tuner installed. Should I try the Iridiums again and lean the fuel mixture? Could that help?

 

 

I just thought they would produce a better spark but I guess I'm wrong. I'l stay with the tried and true. What plugs do you recommend other than the OEM ones? I don't have a HD dealership nearby. Thanks for your help!

 

 

Such foolish thinking. Use HD stock plugs--or a direct locally available equivalent. Aftermarket (space age) plugs will not do a damn thing for performance, starting, longevity or anything else. Lots of goofy ideas regularly witnessed on this site. Lots of aftermarket items for HD definitely improve performance. Plugs are not one of these.