What is the main cause of carbon build up on pistons?

 

The engine breather set up.

 

also owners not revving these up often enough..old school blow the carbon out still applies..owners that spend most of their time shifting at 3500 and not going past 4000 rpms ever will make these prone to more carbon buildup

 

The combustion process is not complete so it does not burn the fuel completely thereby leaving unburned fuel on the piston. Harley's love revs and will carbon up a lot less if rpms are kept around the torque peak (3000-3500).
Harley could run a leaner mixture but engine heat would be an issue.

 

carbon in oil. oil no combust. cakes up on piston

 

Besides all of that, carbon on pistons is normal, and probably even useful, up to a point.

 

Absolutely not, carbon on the pistons causes hot spots which then causes preignition and in these Harley motors can easily be avoided when going to a nice breather system, I've seen a picture of a v-twin with 8k miles and the piston looked terrible then a picture with 27k miles and it was a world of difference pictures attached, the crap system is the main culprit of carbon build up

 

Carbon build up is a normal occurance, but oil being injested into the engine via the breathers can increase it dramatically, and when it builds on the valves, performance will decrease as well, as the air flow is compromised.
You can plumb the breathers externally.
Scott

 

Also running the engine cold all the time. You need to get out there and get the engine up to its normal operating temperature.

 

okay my two cents. also runing the engine for short run intervals. when the min run time is one hour run time on the highway. at temp it clears all that out. also interstate operation at speed...