In another match, played simultaneously in Kailali, visiting team Morang FC was denied victory after Far Western FC leveled the score late in the match to steal points at its home.
The win propelled Jhapa two places above in the standings at third with equal six points with second-placed Manang Marshyangdi Club, but is separated by goal difference.
Lumbini took the lead in the 43rd minute through a penaly before Karna had restored parity by leveling the score in the first half injury time and struck two more in the second half.
Lumbini was awarded a penalty after Sumin Mainali had brought down Manoj Gurung inside the area.
Pukar Ghatraj stepped up and broke the deadlock by converting a penalty. But Lumbini's rejuvenation at home did not last long as Karna leveled the score before sailing into the half capitalizing on Dinesh Karki's assist at close range.
Karna was back in action and score sheet in the 72nd minute when he won a penalty on a solo effort. He was brought down inside the area by Bimal Basnet and Karna coolly converted a penalty to put Jhapa in the driving seat.
Man-of-the-match Karna completed his treble of goals six minutes before time tapping in from a close range after Dilen Loktan had sent him through with an inviting assist.
Jhapa's new signing Ahmed Abbas Iddi made his debut replacing Yogesh Gurung in the 59th minute.
"We were playing at their home and they put us under pressure early in the match. As the match wore on, we felt comfortable and sailed away with three points," said Jhapa's coach Bhagirath Ale.
"Today's win was similar with our win against Police. We withstood the pressure and stunned them with a win."
Bhagirath's counterpart Yam Gurung highlighted Jhapa's equalizer as the turning point of the match.
"They were the better side after the equalizer. They shooting and finishing was up to mark and our defense was not rugged in the latter half. We will work on our weaknesses in our upcoming fixtures."
The stalemate put Morang and Far West at joint sixth place with one point each and equal goal difference. Their draw pushed Nepal Police Club at the bottom of the table with no points.
Visitors Morang raced into the lead in the 55th minute, before Far West's African recruit Cedric Aba had leveled the score in the 78th minute.
"We were hopeful about winning the match but our players did not play well. We were hampered as our three vital players Ananta Tamang, Dinesh Rajbanshi and Sunil Bal did not play," said Far West coach Kiran Shrestha.
Morang's coach Paras Chaudhary said that his team could not win the match due to injury concerns.
"We have some injuries in our team. I am satisfied with the draw."
Related story
Kane scores a hattrick as Spurs beat West Brom
