James (22), whose parents Linda and Eddie live at Springfield Farm, Carluke, beat his existing personal best of 76.71m by almost 4m, much to the delight of the man himself and friends and family in the 500-strong crowd.
"I knew it was a good throw when I heard my dad let out a roar at the other end of the stadium!" James, the current UK Champion, told Gazette Sport.
"And a mate of mine, the hammer thrower Mark Dry, shouted 'Jimbo!' at the top of his voice as well.
"When I looked at the electronic board and saw how far the javelin had gone I experienced a combination of relief and a great feeling.
"I just couldn't believe it. It came as a complete shock and I was feeling a bit emotional."
For more information on this article, see this week's Carluke and Lanark Gazette which is in the shops now.