Iridium Plugs -
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post #1 of 9 (permalink) Old 09-21-2011, 06:54 AM Thread Starter
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Iridium Plugs

I put a set of NGK iridiums in my HD "thing" and have the following observations. These are carbs, not EFI.

Had to decrease the idle speed screws by 1/2 turn.

Had to increase the pilot jets from 20.0 to 22.5 (Mikuni 45's)

Had to retard the timing 2 degrees.

Had to go from an 8 to 9 heat range on the plugs to get any color.

Had to raise the needles one notch.

This all tells me that the Iridiums are throwing a vastly better spark causing a more complete burn of the mixture. Starts and idles much better too.

I also "index" the plugs though I doubt that had much to do with the difference.

So if you're on the fence get them. They are better. By far.

I got mine. Boy do I ever got mine.
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post #2 of 9 (permalink) Old 09-21-2011, 07:55 AM
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Thanks I was rocking on the fence with this but you've pushed me into them.
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post #3 of 9 (permalink) Old 09-21-2011, 02:52 PM
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I have put Iridium plugs in all my bikes with a noticible change for the better in every one.
Good upgrade.

oldnfast(ish) Racing towards old age
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post #4 of 9 (permalink) Old 09-21-2011, 03:10 PM
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Fitted Iridiums to several bikes I have had including my 1090, never noticed any difference! Not that expensive so thought may as well just fit the best.

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post #5 of 9 (permalink) Old 09-21-2011, 10:18 PM
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Did you leave a "Resistor plug" and put in a non resistor p/n? That would make sense of what your saying. Most NGK plugs are resistor. It was created to prevent burning of the electrode during Idle prior to variable energy coil controlled ECU's. Since your HD thing is variable energy putting the wrong type plug would cause you to make this many changes to "FIX" your bike. Look at your COIL and see if it has a "Plasma" wire? this will tell me more about the ignition ECU. Tell us what ECU you have, I can proceed from there. P.S. I think I have the software to update your ignition ecu if your interested. what controller are you using on the ignition...
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post #6 of 9 (permalink) Old 09-22-2011, 06:57 AM
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Wrong there...the resistor plugs are designed to suppress radio noise generation. Nothing to do with electrode wear. Resistor plugs became standard on bikes long before EFI systems became standard, as did resistor spark plug caps for the same reason. Remember those funky metal shrouded plug caps from the early 70's??? Radio noise in the early CDI units of the day caused problems (not to mention the static you generated in that car radio next to you at the stop light).
If you install non-resistor plugs in many current bikes they will drive the electronics an example: Suzuki Hayabusa tachs get very erratic when non-resistor plugs are installed. Since the modern high performance bikes use stick coils, they can't include that resistance, so the plug becomes very important in RF noise suppression.

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post #7 of 9 (permalink) Old 09-22-2011, 07:11 AM Thread Starter
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I don't have an ECU on this bike. Carbs and an electronic ignition which can be programmed from Daytona Twin Tech. The coils are just regular old coils with a single wire input from the ign module. The set up is known as "single fire" which means each cyl has it's own coil. Not the old style wasted spark set setup using a "dual fire" coil.

I'm not familiar with what you mean by plasma wire etc. I don't think this is that technologically fancy.

Keep in mind that I do alot of screwing around and tuning with both the ign curves and carbs trying for the "grail". All I know is that without touching anything else the iridiums caused me to make the changes I did with the results I got. I have A/F ratio gauges, cyl head temps and egt gauges on this thing so I have a pretty good idea of what's going in and coming out and it's effect on performance.

For example- I can tweak the pilot airscrews while riding and watch the A/F go up or down and the egt's respond accordingly. It doesn't take much to get a change either. The cyl head temps are much slower to react so they're more of a relative reading than instantaneous indication of change.

I am also familiar enough with the beast to tell from the feel, sound and performance of the engine to tell if a change was positive or negative but not as precisely as a dyno would do. Which is good enough for me. The old ways still work just fine on this thing. Ya' gotta' be part of the bike and it part of you. It's an HD for God's sake. Fortunately. I don't think my mind could handle one more high strung, tempermental electronic nightmare. This ancient technology is a joy to fiddle with. Ain't that right Noel?

I got mine. Boy do I ever got mine.

Last edited by lee; 09-22-2011 at 07:17 AM.
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post #8 of 9 (permalink) Old 09-23-2011, 12:14 AM
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Yes. Esq'z me, your 100% correct about the noise. It was to suppress noise. One of the many problems back then was that the ign systems would put out constant power "joules" regardless of rpm. When engines are at low rpm they have a very low dynamic compression. This lower compression would be Less resistance on the gap allowing the spark to jump easier with extra energy to increase the plasma field around the arc. This increase energy would cause more local heat at the tip of the strap slowly erroding the strap away. The increase gap would then turn the spark plug wires into more of a RADIO than they already were. Now remember I'm speaking for HIGH mileage applications. This ofcourse was just one of the many reasons for the noise. Eventually SPARK ECU's had variable spark energy that helped with all of this.

DISCLOSURE***** I agree with you 100% Esq'z me...

Whew... ok... now lets jump ahead.

Take a Harley. Air Cooled, causes "VERY" limited compression ratio, not to mention their cams have a ton of overlap... Bigger cams usually more overlap. This causes very low compression at idle and low rpm. Because of this "most" IGN controllers have a power curve built into it lowering the spark energy to the coil at lower rpm's, especially IDLE otherwise radio and tach kaos....(not to mention another contribution to the many reasons Harleys are hard to start when the Battery is old and has lower CCA's.****DISCLOSURE*** only one of the many reasons.)

Whenever I change between resistor and non resistor plugs I have to retune bikes on the dyno at low rpm and idle.

"LEE" the plugs are better and yes I agree with you too.
P.S. some of the Spark ECU's have whats called a PLASMA wire going to the coils. This is the white wire on the Harleys. It monitors for non fire and alerts the ecu of KNOCK/Detonation.

I thought this was going to be 2 cents... Sorry it turned into a dime.
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post #9 of 9 (permalink) Old 09-23-2011, 08:34 PM Thread Starter
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No a dime. More like a quarter and good stuff at that. Thanks.

My overlap is 35 degrees. Idles like a dream.

I got mine. Boy do I ever got mine.
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