throttle freezing

Good morning, I am new to the forum. I'm 75 years old and love getting ATVs back to usable condition. I particularly like the 2006-2009 Arctic Cat 250 Utility. I currently own 6. The 2006 I am working on only has reverse and low gear. The "drive shaft" in the transmission is completely stripped where the high gear would mesh. Has anyone on here replaced the shaft. I'm told I have to split the case. I was hoping that someone on here could let me know how hard it will be. I already have it stripped down to where the shaft coming in from the clutch side is the only thing left. I was hoping I could just remove the driven clutch and drive it out from there. Directions say different. Any help would be greatly appreciated.

Здравствуйте, нужна помощь по ремонту багги Arctic Cat 700 EFI, пропала искра, датчик коленвала импульс есть, Самодиагностика показывает две ошибки AC99,AC96. все провода проверил на целостность, все датчики блоком определяются, не включается топливный насос. Провода от ECU до реле насоса и от реле до насоса исправны. Поставил подменный ECU, ничего не изменилось.   

Will the top end of a 400 fit a 366 the price is so much cheaper
 

I bought a 2008 arctic at 366 with 120hrs. When i red line it in neutral is smokes blue a lot. When I put it in high range it has problems getting over 35km/h. I bought it and had to clean the carb. It ran ok after i cleaned it. Not I have this problem. 

Okay so I bought this 1997 sazuki king quad from a guy and he had it for a few years. 
He had it registered and on the trails last year but he said it bogged and backfired when he would try to drive it. so it sat for a year. 
I bought it for 800 bucks and got it running that night and it ran great for a few days 
But I parked it the other night and went to start it again and it was really hard to fire up and when I did it was bogging and idling high then low again.
So I took the carb out and cleaned it (ended up doing it twice because it was still doing the same thing) 
Now today I have it running slightly better, but it is still like 20% 
I have been looking it up and trying to figure it out but I can't find the issue, has anyone else had this problem? I'll attach a video of it running and giving some throttle