For the strangest reason, my 400ex doesn't work sometimes. I'll be driving along and then all of a sudden, there is no power at all! It can only rev at low RPMs and if I give it gas, it sounds like it's either being flooded or something's just not happening. A few times, I've pulled the clutch in and revved it and right afterwards it ran fine. Sometimes this does not work. I do not understand what could be happening? I'd like to mention that I run 87 octane.

My chain is also very loose and 3 teeth on my front sprocket are worn down (about half way). I am planning on buying a new chain (is it 520X94?) as well as a 14T front sprocket (a dealer told me that a chain will cost around $90 Canadian, but some websites show lower prices (much lower)). I ride around a lot in 5th but for some things, I would really like to have that extra torque.

 

A good cleaning of the carb, will probably get rid of the problem. Sounds to me, like the main jet is sucking up particles, which are floating around in the carb float bowl.
Try draining the contents of the float bowl, into a clean paper towel, and see if you can spot any solids.

 

How do I clean the float bowl?

 

Itll have to come off the bike, and be bench cleaned.
Heres a how site,carb clean, But basicly you need to remove the 4 screws on the bottom of the carb, and the float bowl will come right off. If theres particles and crud in there, then remove the jets and clean them too.