Finally got around to pulling the plugs out tonight and they are brown on the end. Does that mean not enough fuel?

I have a 78 spitfire and it will start first pull everytime. Runs good at first. Saturday and sunday both I rode them to a friends house about 5 miles from my house. Both times it died within the last 3/4 of a mile to his house. It will then start after about 5 pulls and sometimes it will go a half ...