2009 Rocket 3 - 4th Gear Problem

When I select 4th gear, it is as if does not go in all the way. It goes in, but the moment I release the clutch and apply power, teeth grind on each other.
I realized than when I select 4th on a downhill, leave the clutch and apply almost no power, it will stay in gear. But the moment I apply the slightest power it jumps out of gear.
If I gear down from 5th to 4th and let it run against compression it will stay in gear, but again, the moment I apply power it slips out.
I already replaced the dent spring and it is still the same. All other gears are working fine.
What I do now is to just go from 3rd to 5th , but this is not ideal.
Any help would be much appreciated.

The teeth on the forth gear are junk . I have an 09 touring with the same problem. How many miles on the bike?

only 19,000 km, that is about 11,875 miles.

Mine has about 58,000 thousand mile. If you do a search you will see there were quite a few early tourings that had 4th gear failures .

The one of Martin-Brighton brought me to this site but looking at his video it seems like his problem was more that it did not want to go into gear. In his videos his gears look fine and I did not see a definite solution besides that it might be warn out selector. I checked on Fowlers web site where I buy my spares, I can get new gears there but does not seem like new selectors are available.

Your transmission needs some attention. Won’t try to scare you but the circlips that hold certain gears in the proper position on the input and output shafts have been known to come out of their respective grooves. This allows one gear to be out of position when the transmission is trying to be in a different gear. The result is two gears try to engage at the same time. Although one gear is just getting hit on the corner or edge of the teeth. Your picture looks like you live outside the US. In New York state there is a shop the specializes in motorcycle racing transmissions called Robinson Industries. I sent both by shafts and gears (as an assembly) to them for a tune up. They break it down, back cut teeth on the dogs, they have an improved 4th gear design (solid gear not a gear with 4 spokes) and the widen the grooves for the circlips and re-assemble it with stronger circlips. They have a website you can check out. They solved my problem and I can truly say the transmission shifts smoother and quieter than stock. I removed the engine, opened it up and sent off the gear shafts, and out it back together myself. Total Robinson costs were around $750USD 4 years ago. If you need someone else to do the labor to open it up, let me just say it will cost you an arm, a leg, your firstborn, and you will probably have to pimp out your wife for the rest. Sorry

Check this forum post

Feedback: Got Rocket 4th gear problem fixed:
I had it stripped by Triumph in Cape Town and the dogs on the 4th gear were gone. The matching gear also damaged and the selector fork shows slight wear. What I find interesting is that from the pics below you will see only every 2nd dog is damaged.

The bike only had 19,000 km on the clock when they stripped it.

I imported all my new parts from Fowlers who rendered excellence service and now she is running again.
First time since I bought her that I can enjoy her.

I do have one issue after the operation. There is now a bad vibration when I go over 160 km/h. It does not go away, I pushed it to 180 and the vibration got worse. That means it is probably not wheel balancing. The vibration is in the frame, not in the handles.