hello, I have a 2008 honda 420fm that I got after someone else took it apart and I was told 4th gear was out of it. I inspected all of the parts and replaced 4th gear and couple of other gears that appeared to be worn where the shift collar splined to them and i replaced one shift collar that appeared to be worn and the shift drum, because it came with one of my gear kits, and all 3 shift forks because they were worn. I was told that a young boy had ridden it alot and that he liked to keep his foot under the shift lever and was constantly putting pressure on it. it worked fine when I got it all back together for about an hour and a half or two hours and called the owner to come get it and had him to ride it before he took it and he said it felt normal to him. he took it home and changed the tires on it and he said he rode it down his driveway and it did fine and then he said his son rode it down the driveway and came back and said it was still not fixed and he brought it back to me and now when you put it in 4th gear it goes into gear but if you hold the brake and give it some throttle it sounds like the shift collar and gear are slipping until it idles back down and then it is normal. all of the other gears appear to be working as they should. I have inspected the shift linkage and can not see anything that appears to be wrong with it or worn out in it. does anybody have any idea of what could be causing this problem. I am not real familiar with these but I was wondering if the gear shift cam could possible be worn enough to cause this but not be easily noticable. thanks in advance.