i was wondering if anyone knew how to wake up my 400ex engine. i have the basics, a pipe, filter and jet kit. would a 1 tooth taller rear sprocket give it A LOT more bottom end, and would it be worth sacrificing top end. i dont really ride full throttle. mostly in the woods, and sand pits. i know at times it feels a bit boggy under these circumstances. is a taller sprocket a good idea? also what else is out there to make my engine wake up???

 

You won't notice one tooth out back very well. If you are going to buy a new rear sprocket, go for at least a two or three tooth change. You could also go one smaller on the front sprocket instead. SInce you have the obligatory exhaust and air filter, you might be looking at engine mods if a gearing change doesn't do it for you.

 

If you aren't to worried about top end go ahead and go down a tooth on the front sprocket. You will feel a difference. If you want to get serious and can afford to spend a couple hundred bucks I would go with the 440 kit and a cam. I included an ebay link below, its a pretty sweet package for a good price.
440 kit

 

Or if your not to big on the idea of re-sleeving and want to save a few buck you could go with something like the 426 kit. Check the link below.
426 Kit

 

how are those big bore kits on reliability? and if i went with the 426 would i have to use race fuel? would it matter which sprocket i changed front, or rear? will one make more of a difference? will either of the big bore kits require race fuel, or a higher octane fuel? what if i blow my engine? will i be SOL?? or can you bore it out even more if that happens?

 

I've known some people that have had these big bore kits in there EX's and never had any reliability issues with them. Both of the kits that I listed were running 11:1 compression so you should be fine running pump gas on both of them. Usually you don't need to get into race gas until about a 12:1 compression ratio. As far as the sprockets go, changing the front is generally cheaper and easier to do. I went down one tooth on my 450R and it does feel stronger throughout the RPM range. Yes you can bore, if not you can always just re-sleeve the cylinder. Personally if I was you I would dish out the extra cash and go with the 440 kit.

 

my 400ex is broke down every month with the stock engine. cant emajine mine with a engine kit.