The first goal mouth action in this Lincolnshire Junior Cup second round clash came on five minutes when a cross into the box by Jordan Smith was not gathered cleanly by home keeper Tom Maplethorpe.
However the danger was cleared by the home defence.
United took the lead on 25 minutes when interplay on the left between Michael Howard and Luke Rayner-Mistry found Jordan Smith just inside the area and he drilled his shot past Maplethorpe into the bottom corner.
On 35 minutes Jordan Smith was just wide of the post with another shot from distance.
Fulbeck had their first attempt on goal after 40 minutes when a shot from 30 yards was well over the bar.
The next goal mouth action came when a Josh Bartlett free kick was headed just over the bar from Jamie Skinner.
The half ended with United holding a one goal lead. Although United they had a greater share of the play Fulbeck stuck to their task of trying to close down the visitors and were very robust in their play.
Within four minutes of the restart United increased their lead with a brilliant individual goal.
The ball was played out to Jordan Smith on the right side of the penalty area and, as it dropped over his shoulder, he volleyed it first time across the keeper and the ball nestled into the bottom corner to double.
Rayner-Mistry then put in a cross which was only half cleared by the home defence and the ball fell to Jordan Smith whose cross shot was only just wide of the far post.
On 60 minutes Fulbeck got themselves back into the game when, from a corner, the ball was not cleared by the United defence and Chris Bates knocked the ball home from three yards.
United went back onto the attack and on 65 minutes Howard was played through, however Maplethorpe was quickly off his line to block the shot.
At this stage United were in total control of the match with their quick passing play and less and less was being seen of the Fulbeck attack.
On 70 minutes United scored number three when Chamberlain was released on the right and his miss-hit shot fell invitingly for Michael Howard to knock the ball home from close range. Another United half chance came when Rayner-Mistry was just wide with a header from a Chamberlain cross.
With the game drawing to its end United increased their lead when inter-play between Broom and Chamberlain set up Andy Reeson, who made no mistake with a sweetly struck shot from 20 yards.
Manager Mark Hallam said: “We came up against a good side that made it hard for us to break them down but we showed great patience and our superior quality showed in the end.
“To make four changes and still perform at that level shows what great depth we have in our squad and credit to the players for showing such a great mentality.”
United: Ben Smith, Josh Bartlett, Ross Muirhead, Andy Appleby (Sean Chamberlain), Michael Howard, Jamie Skinner, Arron Scutt (Mark Hallam), Adam Broom, Luke Rayner-Mistry, Jordan Smith and Andy Reeson. Unused Sub: Jason Smith.
Man of Match: Arron Scutt
This Saturday United have a difficult away fixture at CGB Humbertherm in the Challenge Cup. Kick-off is at 2.30pm.