hello
I have fxef fat bob model 1985
my problem is after a couple of weeks rear sparkplug gets carbon and the motor runs bad,I change new one and after this comeback and I must replace another one
what is the main reason for that?

 

What carburator, and plugs? Do you get good spark to the rear? What ignition are you running? Did you do a compression test?

 

98hotrodfatboy asked you some good questions that need to be answered for us to give solid suggestions...

Carbon builds up on a spark plug due to a rich fuel mixture, weak spark, or oil being burned .....

Rich fuel mixture can be due to stuck choke (or enricher on CV40 carb), leaks within the carb fuel passages, or a misadjusted float bowl.

Weak spark can be due to weak coil output, high resistance in plug wires, or bad plug. (We can rule out bad plug as you have changed it already)

Oil being burned can be due to bad valve seal, worn rings, or excessive blowby into intake.

Since you have been only experiencing the condition on one of two plugs, I would start with weak spark, then suspect oil burning in the rear cylinder...

How old are your plug wires...?
Have you ever changed out the coil?

Once the spark is verified to be good... then a compression test would be next on my list. If you get weaker compression numbers on the rear cylinder, I'd do a leak down test to see the cause of the weaker compression.

If the compression numbers are all good... then time to pull and rebuild/clean the carb... looking for issues that could point to overfueling... but it's hard to have a leaky, misadjusted carb that only affects one cylinder...

The above is just a general plan of attack, that can be adjusted by your answers to the few questions asked...

It would also be helpful to know:
Mileage
Any engine mods
How long you've owned the bike
If it ran well before the plug issue popped up... or did you just buy it and it's been this way since you've taken ownership...

Looking forward to your answers and any other observations you may offer...

 

A picture of the plug might help ??