Greetings from Chattanooga, my fellow Harley enthusiasts!
I'm a newby to this forum and, at 61 years old, answered the call of the wild and bought my first Harley Davidson, a 1995 FLSTN. I had a choice between putting the cash we saved into needed upgrades to our 53 year-old house or a Harley. My priorities won out. My wife's did not...
Now to the issue at hand. The rear cylinder's spark plug keeps failing. I've only racked up about 100 miles, short rides(10-12 miles) around the neighborhood and a few 60mph sprints on local county roads. Yesterday I had to put a second new spark plug in the rear jug after it quit firing. The electrodes and ceramic center of the original HD 5R6A plug were coated with black soot, so I figured I had run the manual choke too long causing it to foul out. When I looked at the replacement plug (Autolite 4265) that only had about 50 miles on it there wasn't any black soot as seen previously on the original HD plug. However, as before, the old girl started right up with a new plug. At this point, I'm hesitant to go anywhere without carrying a new spark plug and socket wrench in my pockets. I'm also worried I'll ruin the spark plug threads in the aluminum head with frequent spark plug replacement(even tho I apply Never-Seize to the threads and don't over-torque the spark-plug when installing).
I'd like to know if rear cylinder spark plug failure is a known issue caused by a rich running carb, a bad coil, bad spark plug wires, or combination thereof. Or perhaps I need to install premium plugs...
The engine has an Andrews cam and hi-performance ignition(prev owner couln't provide any other details as to exactly which cam or ign. syst.). It pulls strong when accelerating, but has a slight miss at light throttle and at steady speeds. It idles OK at stops as long as it's hitting on both cylinders.
I have basic engine repair and maintenance experience, a decent set of automotive repair tools, and a rudimentary understanding of ignition systems(don't grab the end of a spark plug wire when the engine is turning over) if this helps anyone who is willing to put into words any advice on how to proceed from here.
Thanks kindly, and Semper Fi!

 

Thanks for all the input, guys...
After doing some research on other forums, I checked the resistance in the rear jug's ignition wire and found it fluctuated wildly when flexed. I also found a hot spot where it rubbed against the forward cylinder's wire. So I ordered a new set of wires from Amazon; Sumax ThunderVolt 50 by Taylor 8.2mm High Performance Wire
Will update this thread after I install and evaluate them.

 

Glad you figured it out. It's tough to get a lot of technical replies during working hours. You probably would have received more later in the evening. Let us know if the new plug wires fix the problem. Congrats on the new bike and welcome to the forum.

 

Thanks Thumper09,
I thought there'd be some older retirees like myself during the day... guess not.
All the same I appreciate your heads-up and will let y'all know the results in a week or so.

 

Keep an eye on the corrosion that hides side the boot. Also, my rear cyl is starting to use oil, causing the plug to gunk up. I have a lot of miles on my 1995 Dyna also. I change plugs about every 15 to 20,000 miles.
Wires do rub on each other, causing wear also. Cheap to have a couple plugs already gapped and ready to go. I've had HD plugs fail for no reason at all. Left me stranded on I-5 once. Went and bought NGK plugs. I've been using them for 10+ years. But, it is getting "hesitant" to start lately. Maybe it's time for some more plugs!

NGK BPR5ES is what I use.
According to the owner's manual (the same book is for your FL)~
The gap is .038" to .043"

Replace plugs every 10,000 miles (gotta do that!)

Torque to 18-22 ft. lbs.

 

Thanks Norty Flatz, I appreciate the tips!
The plug wires currently installed have the letter "H" on them, probably 20 year-old OEM wires. I'm hoping the new hi-perf plug wires mentioned earlier will solve the problem. Maybe I'll install a set of NGK's to go with them.

I spent some time on the I-5 parking lot in So Cal many years ago('79,'80,'81). I imagine it's gotten a lot worse since then. Have no desire to go back there to find out. Helluva place to break down...

 

How did the front plug look? Was it indicating a good mixture from the carb? Changing the plug wires would be a good start.

 

Mine gets a bit sooty as well, but I check tem regularly and my carb runs rich - winter project. I carry an extra set of plugs on the bike in case I ever need them. Welcome, and hope the wires work.

 

I use only three brands of spark plugs in my HD's.
1 Bosch
2 NGK
3 Champion
in these sequence..
Never had any problems with spark plugs whatsoever...(knock on wood)

 

http://www.harley-performance.com/ha...park-plug.html

Success !