Looking for some advice as to what might be causing half the spark plug to have this black residue. Bike is a 78 shovelhead with S&S Super E. The bike always starts on no more than two kicks when cold. Idles fine. Runs good on highway. It's a 78 so not anywhere near what my 2020 Electra Glide can do on the open road. Don't like to ride faster than 70 due to vibration but that could be caused by many issues. Can ride all day 70 or slower. No misfires, coughing under load. Recently did a compression check, cold, both cylinders were at 120. Any ideas?

 

Unlike todays 6 speed road bikes - the shovel was designed before those hi ways were even built - and at some point pushing it that fast will drain your bank account - we limit the early machines to back roads and 55 if you want it to last - if you have a **** ton of money and dont care have at it

the valve seal is not up to the task reason your getting that as well as engine speed - 90 to 110 is wide open and they stock gearing wont go faster

so your at 70 your using 3/4 of total engine speed usage

 

Thanks for the feedback. I really enjoy the riding experience of my shovel, limited speed and vibration, it's all part of the ride!. I typically keep it a cruising speed, 55-60 mph. I let the bike tell me how fast she wants to go.
Besides burning oil and fouling the spark plug is there any concern to the motor riding the bike this way? I usually only take her out for short rides on the weekend, 20-40 miles at a time. I would prefer to wait until next winter if possible to tear into the motor, but if the recommendation is to address immediately I'm open to that.

 

If it’s not fouling to where it runs bad, I wouldn’t worry too much about it.
I would suggest just running a standard plug and not waste your money on a platinum or whatever that plug is. Put a couple Champions in it and ride it till you want to get into it.
I run RN12 in mine and they’re a little hotter but I’m stroked and running a bigger cam…

 

My 84' has had this issue for a longgg time on the rear plug. Runs great. Good compression. I pull the plug periodically to clean. I run a cap full of Marvel Mystery in the fuel every other fill up. I have to add some oil every 1k miles or so. Is this the best approach? No. But I know the bike and it hasn't missed a beat in this condition.

When the top end needs a refresh I will address it correctly (most likely a valve guide/seal).

My 84' FXSB does up to about 70 *comfortably*. It will do more for short bursts.... run higher than 70 for miles and **** will be cracking/breaking/falling off. If I need more grunt I have a TC95" FXDX to get my fix.

This is the rear plug after almost a full season of riding before I started introducing Marvel Mystery. The plug does stay cleaner with the Marvel..... it's a Mystery!

 

I will definitely give the Marvel Mystery a try.

 

WE add marvel to the oil 1/2 quart few hundred miles before every oil change

we use sea foam and marvel in the gas as sea foam keeps gas from turning for at least a year sitting in the tank 5 gallons gas 3 oz of marvel every tank full when coming back with the machine after a ride i add it with a full tank

 

My 78 and 20.

 

my mate whos a mc mech has never seen a plug like that and hes even interested in the cause