I purchased my first ATV last summer, it's a 2001 400EX. The guy I bought it from added modifications such as White Bros. exhaust, K&N air filter, and he jetted the carburetor. I've ridden it about 8 times and have had to replace the spark plug 3 times! I think it must be running too rich and fouling the plug. Do I need to adjust the carburetor and if so, how do I do that? I do not have an owners manual and I've not done any mechanical work on ATV's so I'm new to this. Thanks.