Went to start up my 2003 Big Bear 400 FWD the other day and noticed oil under the bike. Closer inspection found the rubber boot on the rear arm assembly came loose from the rear crankcase housing from what might have been a stick coming in contact with the holding band that keeps it in place and the oil appeared to be coming from the dislodged boot. Is this differential oil coming from there? Wasn't sure if the differential oil was in that assembly as well as the differential itself. Any info on this will be appreciated.

 

You mean the rubber boot that has a metal O ring clamp and the bolt that tighten's it looks to be a hex type bolt? If so, this is what happened to me. Boot was off,
O ring clam was on metal housing. I couldn't get in there to tighten the hex bolt, but was able to put on the rubber hose.

Regarding oil leak. I too am having an oil leak. The oil was leaking from behind the yolk in the transmission where it connects to the engine. A seal rotted and was causing oil to leak out. That was leaking on the rear engine passenger side of ATV in the back. $400 fix ay the yamaha dealer which it is under going now. Someone else posted in my thread about having same problem and how mud caused the seal to rot. I guess this is why Yamaha redeisgned the shaft to a one peice in 12 models. If it is caused by mud I would think that it is from mud that dries up over time. I plan on hitting that area with a hose and making sure there is no mud in there after I am done riding, if I am riding in muddy sections of my land. I also plan on packing some red grease in there to help protect it, and also might use this chain lube that is like a wax base for additional protection. Had i known about this earlier, I would have made sure it was always cleaned out. I wouldn't spray any type of degreaser product in there to clean it out as that area needs to be greased as there is grease fittings int he yolk area there which is extremely hard to access and pump greases into.

I still think Yamaha has a very nice design and they improve designs as common problems occur over time. I was impressed on how they kep the brake lines nice and secure and not having gaps like polaris has where foreign objects could take out the lines in the rear. Not that I think polaris isn't any good, it's just to me I look for build quality first and maintenace costs. And Yamaha's belt is one of the best out there compared to other brands like Kawaski where they recommend tightening the belt every 90 days or something like that.

Anyway, to get back to your problem, if it is that hose i mentioned, taking the rear wheel off would give better access to that clamp to tighten it back up.

 

Broke everything down after getting a Moose carb kit in. Had to get all of my service in before the hunting season. Looks like the float malfunctioned and just flooded everything with gas. The gas ran throughout the engine and any area it could get into. It was streaming gas from the bowl drain tube while I was in the woods and I clamped it close to get me back into camp and was going to get a kit to put in the carb later. Bad idea. Didn't think gas could get in the places it got. Flushed the engine oil to make sure my oil wasn't contaminated with gas because the entire crankcase was flooded. Finished all my service and kit installation around 11:00 last night with the help of a good friend and fired her up. So far so good. The only smoke I'm getting is from where the oil came into contact with the exhaust. Don't see oil dripping anywhere right now. Ethanol fuel is our enemy. Sorry if anyone thinks different out there but it has been a serious problem and cost the sportsmen in this area tons of money. This job would have cost me at least double had I paid someone to do it and I really think could have been avoided had I been using conventional fuel.