What would a rev box or cdi box wutever do for a 400ex would it be good for it?

 

For a stock one no. They just let you rev higher and may give you a hotter spark.

 

Yes it would...even on stock. I had a White Brother's rev box (an increase of 1200 RPM's) on my stock EX and it helped stretch my 2nd gear out when climbing at the gravel pits I go to. There was a small but noticeable difference. The biggest difference would be seen on a quad that is modified more ex. full exhaust, 440 big bore kit, different cam, etc. I now have a White Brother's E-Series full system on my EX and it is great. If I were you I would buy one (White Brothers). White Brother's is a reputable company that does much testing on their products. Brand new on ebay "Buy It Now" for around $50. When I got mine they were $90-100 new. The prices have come down considerably.

What are you trying to do anyway? More top end, more acceleration or just more power?

 

sounds like it helps 2nd gear out alot thats wut i really wanted? ne thing i wouild have to mess with if i got it?

 

Nope. Simply unplug the old stock cdi box (located in the front of the quad under the headlights in a rubber "boot" holder mounted side-ways). There are two clusters of wires (that only plug into the cdi box one way). Unplug these wires and plug them into the new rev box. Reverse the installation process. IT IS THAT EASY. A couple of things I thought that I would mention: Make sure that your bike is turned off when you remove the old cdi box. And the other thing is that if you miss a shift, as I did soon after installation, you will notice that the bike REALLY DOES REV THOSE EXTRA RPM'S. Don't worry though, you will get used to it. I have had mine on for over a year with no problems.

 

The other down side to this is that it makes it hard to start in cold weather. The box comes from the xr250. It was made for a kick start model. The 400ex is electric start. The kick start requires fewer rpm's to start compared to the ex. Once the quad is started or warmed up the box works very good. this is the only downside to the box. WB doesn't make the box, all that they do is buy it and repackeage it in there box with there name on it. Almost all of the cheaper ones comes from the same supplier. Some of the more costly ones have been fixed for the starting problem. You have to ask them which one it is, they might not evenknow or will not tell.

 

Wow,, This is an old story,, If you do a search on here you will find lots of good info on this subject.

You actually only gain around 1000 more RPM's,, But it also raises the spark curve up about another 100 RPM's. basically it takes 100 more rpm's before the ignition fires a spark..
A real good fix for this, I know, I used it for 4 years with no problem , is to replace the stock battery with this one..
Yuasa # YT12A-BS

This is a stock battery from a suzuki street bike.. It is the same dimensions, so it will fit stock location, but it has 175 cca vs 125 cca stock..

Do a search, You will find lots of info on this site about this...

 

thanks trx i will look into that nd i wil def look into purchasing a cdi box.