'02 YZ85 chronic plug fouling...help

I'd start by getting a factory manual for the bike, they are worth their weight in gold many times. Next pull the carb and see what brass is in the carb and where the needle is set. This jetting might have worked for the previous rider because they used a different mix or fuel type. Also pick up the plug specified in the shop manual even if it is a resistor plug, it's the correct plug. When you put the carb back together be sure and clean it well and check the float adjustment. Now you will know what you are starting out with. While you have that spark plug out getting swapped for the correct one run a compression test on the top end. If you don't have the tools or ability to do this get it done at a local shop with a good rep. These little bikes go through top ends a lot faster than most people think and low compression could be causing your plug fouling as well..

search for (yz plug) this topic comes up many times.
But surley get the owners/repair manual. this should always be the first thing you buy for your bike!
it does prevent lots of head pain

hope this helps

My sons 03 yz85 was jetted rich and the FLOAT setting was incorrect.
used to foul plugs all the time until I adjusted the float.


also I went to a small front sprocket for woods riding.
This keeps the revs up and speeds down which helped a lot.
and its easy to change back when the speeds come up