plug fouling difference front and rear Harley EFI & Harley Carb Discuss plug fouling difference front and rear in the Harley Tech & Harley How-to forums; I have a 113 cu. inch S&S with a shorty G and today I noticed that the plug on the rear cylinder is much cleaner (whiter)than the plug for the ...

[wkd]

I have a 113 cu. inch S&S with a shorty G and today I noticed that the plug on the rear cylinder is much cleaner (whiter)than the plug for the front cylinder which is somewhat black except for the very tip which is tan. The ignition is I.S.T. so I presume this is not a timing problem. I believe the I.S.T. does not allow any timing modifications. Ignition was factory installed. Any idea what causes this? (Left plug in image is for front cylinder, right one is for rear)
Best regards
Adrian

Does It Idle Clean Or Does It Smell Rich? Sounds Like Its Too Fat.an Overrich Condition Will Wash The Porcelin Clean.choke Is Ok?plug Reading After Running Sustained Rpm?

[hdnt1550]

The rear cylinder always runs hotter then the front. This could be your problem. My Twincam does the same thing.

[frisco-rigid]

Hey guys

With much respect to Pancho,, this is the second time on this site I've heard a reference to an overly rich mixture causing a spark plug porceline to be "washed" clean .. I must say I've never seen or even heard of this sort of thing before.. An overly rich mix can wash down the cylinder walls but plugs....????
You know, why aren't both plugs washed clean.?

Any plugs I've seen that ran in a very rich mix were both very dark and sooty unless of course, there is another prob.. Also, sometimes the idle mix can be very rich or lean but the actual "running" jets are correct and give a correct read..

As Pancho suggest you should do a high-speed run with shut-down to see if the Main jet is proper.. Then a moderate slow-speed to read the intermediate jet.. Fast first, it tends to clean the plugs then slow for a semi-clean plug read.. Do you know how..??

You must remember, if you read the plugs after just pulling in from normal driving and idling the snap-shot you'll see is not a good representation of the jetting.. Now, if the idle "snap-shot" is uneven it can simply be the way you engine idles or an intake leak or it needs a spacer.. I'll explain..
My 93" shovel used to have a severe diff with the Super E tween the front and back till I put in a 1" spacer, it virtually totally cleared up.. I do not know why.. Now I run Miks only and jeeze what a great diff..
Also, my deep-breathing 80" has a very even color til it gets an intake leak and literally every time it does, the cylinder that reads white will show a leak when the intake flanges are sprayed with carb cleaner..

I would hope that your speed runs will show a much more even read..

I thought I'd come and add this,, the rear running hotter is Not a constant it is mostly anecdotal and relative to individual sickles.. For exanple my 120" front is often hotter than the rear, why,, heck I dunno.. The shovel and the 80" are mostly even.. It's the beast baby..

Hey Frisco,
What I refer to with the term(washing) is that the threads will be sooty and the porcelin will be just off white with usally a few black specs.The overrich condition keeps the combustion temp low and wont let the plugs color just as a too lean condition will melt them down along with serious engine damage.We can use the old drag racing method, lean it out until it pops in high gear then go up on the main.I don't know if the plugs are soaking wet when he takes them out or if the IST ignition is not functioning correctly.This ignition adjusts itself and remembers its setting via a knock sensor.They offer a pc link to pull up ignition curves but it is rather pricey.More importantly I will be riding cross country this summer and was wondering how much of route 66 is left in arizona out your way?

[frisco-rigid]

Hi Pancho
I thought of a better way to word this so I came in and edited a bit............

With my memory I had to go bach to my books to remember what the plug condition I think you are describing is since it's been soo long since I've seen it..
I think what you are describing is high-speed glazing from super heat from hard acceleration and amplified by a lean condition, however, I do not think it is what WKD is experiencing..
I also googled it looking for something new or something I had just never heard of or learned about.. In every example of lean mix the plugs were clean white, blistered, melted, etc.. And in every example of rich mix the plugs were always very dark, sooty, usually dry but sometimes wet..
If you have some other info that is just completely eluding me about this I'd be thrilled to see it..

As far as Route 66 goes,, I can't tell you much.. I've visited a couple towns like Seligman, there's a good old time Cafe there, but I believe most of I-40 runs parallel to it or IS it.. I bet 4Cams knows more about it..
Route 66/I-40 is close to 200 miles from me Via I-10 to I-17 to Flagstaff..
When you come out I would sure try to meet.. BUT remember if you get close to May 15 or Sept. 18, Ruidoso is a hop and a skip from I-40 in NM and that's where I'll be hoppin and skippin baby.. Keep it in mind..

[Wrightturn]

Have never seen a too rich plug that wasn`t dark. Not on gas anyway. Very limited experience on alcohol or nitro. Rules are different.

If the plugs vary from one cylinder to the other most likely an air leak. I run compression tests to rule out other issues.

I have not worked with IST. Would like to know how reliable and how well it works. Since adding sensors to a bike that doesn`t have them is contrary to my politics I may be limited to old bikes. I am OK with that.

FRISCO,
I stand corrected brother! I looked up various sites to see if I could find a description of what I was trying to relate.The closest I could find was to hot of a plug causing preignition which are the black specs on the porcelin.Some of these sites broke down the readings into thrids on the porcelin as well as the threads,electrode and ground strap which shows the initial timing and total advance.Additionally a particular view of one plug showed the electrode sooty about half way and the porcelin clean, this was described as a rich idle combined with to lean at WOT.
WKD,
I read your post again and belive that the IST ignition will retard the rear cylinder timing a few degrees since it normally runs hotter than the front,my 95'' TC at 10:2-1 with -2 degrees on the rear cylinder shows m ore color on the rear plug and has no intake leaks as long as it runs good I think you're ok.Frisco I'll be pulling into Vegas on July 22nd or 23rd and I'll probably be thristy.

[frisco-rigid]

Ooooh Man, I love Vegas..!!

If it looks like I can get to My Favorite City bout the time you're there, which doesn't look so good right now, I will let you know..
I am perpetually thirsty when in Vegas..!!