Here's a look back at all of the running gags given to Bulk and Skull throughout the years.
In MMPR S1, Bulk and Skull's gag was simplistic. They were the school bullies and bumbling idiots. Often times the humor was over the top but they were still a fun group.
After being saved by the Power Rangers in the season's first episode, Bulk and Skull spent the rest of season two trying to figure out their true identities. Sometimes they were close (only for Billy to switch the video tape). In the episode When is a Ranger not a Ranger, Bulk and Skull actually do discover the ranger's identity, only to have their memories erased when they tried to save the rangers.
Season three saw a popular change for Bulk and Skull. They became members of the Angel Grove Jr. Police Department. Most of the time they were nothing more than their stereotypical idiotic selves. In all seriousness, one has to wonder how the heck do these two not end up getting fired after all their constant blundering. However, there was one cool moment in Catch That Cab when they actually did catch a car thief.
Along with In Space, Zeo is the season where Bulk and Skull really shined. They had perhaps more development in this season than any other. Initially they were still members of the Jr. Police Department, but quit out of loyalty to Lt. Stone after he got fired. Then they joined Lt. Stone to form their own detective agency. This season, Bulk and Skull had some pretty cool moments, including my all-time favorite Bulk and Skull moment in Instrument of Destruction. There, Skull bravely showed off his musical talents in which Bulk later approved.
In Turbo the writers simply ran out of ideas and didn't know what to do with Bulk and Skull. They were initially turned into Chimpanzees because the actors who played them were experimenting with a Bulk and Skull spin-off show. That went nowhere, perhaps due to Turbo's declining ratings. Even in the second half of the season when they were turned back to normal and the writing had improved, it seemed like the writers still didn't know what to do with them. Their running gag for the second half of the season was getting hired and fired from various part time jobs, which became too repetitive.
Bulk and Skull spent In Space with Professor Phenomenus searching for alien life. Phenomenous added more comedic relief, but Bulk and Skull got a lot less screen time compared to the first five seasons. This is mostly due to the fact that most of In Space's story took place away from earth. However, Bulk and Skull had perhaps their most popular moment during the season's finale, Countdown to Destruction, in which they mimicked the "I am Spartacus" moment.
In Lost Galaxy, Bulk and Professor Phenomenus boarded Terra Venture and accidentally left Skull behind. From what I have heard, this was due to contract issues with Skull's actor. Bulk and Professor Phenomenus ran a coffee shop on Terra Venture called Comet Cafe. However, the two were rarely seen throughout the series.
Bulk and Skull showed up for the 10th Anniversary episode, Forever Red, in Wild Force. Here Bulk had mysteriously gotten back to earth from Mirinoi (bad writing) and ran a juice shop named Bulkmeier. Their appearance was just a little wink to longtime fans of the show.
In Samurai, Bulk returned to teach Spike, Skull's son, the way of the samurai. Here the writing just completely fell apart. Bulk and Spike have next to no interaction at all with the Samurai cast, and feel like nothing more than mere add-ons for fanservice.