My family bought a 400ex me and my brother in 08. I believe its a 2002. Motor is stock. Stock air filter and no go fast parts on it all. I bought a clymer shop manual for it first thing. I adjusted the valves..... which were much easier to do than on the 3 wheelers mentioned in my previous threads, took apart carb and cleaned it and reassembled it, replaced spark plug, changed oil and filter with bel ray thumper and adjusted clutch cable. Dumped out old gas, refilled with premium and took it out summer of 09 for the first ride. There are no mechanical problems at all. It was ridden hard for just the few times it was taken out. Well its winter now and we've been taking it out here on the ice on Lake St. Clair. My brother rides it mostly and hes drives it like grandma but thats good at least theres no wear and tear on it. Today I rode it and for the first time got to see what this machine is made of. When I was in last gear and getting to about half throttle and keeping it there, the rpms seem to be high enough that going into the throttle any more would blow up the engine. I gave her more throttle on a straight away and noticed that there was no difference in speed. It needs something done to it. Chances are, a pipe and air filter would fix that problem along with a jet kit. But what can I do on the cheap. I have heard of drilling holes into the air box. Okay where should I drill the holes and what air filter should I get. And what jet do I need. THats all I'm going to do. Probably a UNI filter and holes. But what jet should I get. Elevation is less that 2000 feet, its about 600-1000 feet above sea level. Any input on this is appreciated. If I do get a pipe in the future what is the most common one put on these?

 

The only way you will get any noticeable improvement in performance is to open up the intake and exhaust at the same time...this allows the engine to 'breathe'. This is done by installing a less restrictive air filter, like UNI or K&N, and a freer flowing exhaust, of which there are many choices. My own preference runs to HMF or Supertrapp, as I have had extensive experience with both of those lines of exhaust systems.
You will also have to rejet your carb to run a richer main jet, or else you WILL burn up the engine. 1 or 2 sizes larger will probably suffice.

 

I went and orderd the uni air filter and a jet two sizes bigger than 148. I also ordered a carb rebuild kit since I will be working on the carb. Now do you guys recommend drilling holes in the air box or leaving lid off?

 

i always here that taking the lid off is better. It is also not perminent. Once you drill holes theres no turining back. There are also airbox lids that have a filter on the top of it. You may want to look at those.