Is there anybody that can tell me what can I do to increase my mid to top end power?
That is my main idea behind the other questions that I write before.


2000 400ex: Dyno jet kit,K&N filter,modify lid,completeBig Gun Exhaust.

 

The mods which will help your mid to top end are the following: Cam change; cams are available which can use the stock valves, valve springs, and rockers, or you can spend all the money and change all of those parts if you go with a full-race high-rev cam. The high compression piston will help your overall power (with or without an increase in bore). A bore increase will add additional power if you want to spend the additional money. A larger carb is supposed to really help top end. That is actually my next mod.

From what I can tell about your bike, I would start with the piston and cam, see how you like it, and go from there.

 

if your bike is not pulling in 4th and 5th why don't you try changing the rear sprocket 1-2 teeth larger if you keep your 16t front or 1-2 teeth smaller if you use your 15t front. Up your compression is also a cheap add on. Have your head milled, but keep a eye on valve to piston clearance, use clay on the piston crown, install the head and rotate the motor so both the exhaust valve and intake valve completely open and close. remove head and measure the thickness of the clay at its thinnest area, it should be no less than .030. I would guess that 11-1 compression ratio would be as high as you could go on 93 octane, but check with someone who is building them for the max you can go. Also make sure the valve springs are still in excellant shape, have them tested or just replace them with a slightly heavier spring, weak springs will cause valve float much earlier than your redline on the motor. Many racers back off all rocker arms to releive valve spring pressure when the motor will not be run for a length of time to save them from getting weak ( but this is on push rod car motors that makes this very easy to do}, the point is though you could have a weak valve spring and not know that it is hurting top end power. There are many things you can do but these are cheap and effective. good luck

 

Sylder,

Are you running pump gas? What is your altitude?

 

What about installing the thinner head gasket to increase compression? Although I’m not sure what the exact new compression would be. That would be a lot cheaper and safer than milling the head. I know they have one for the XR400 and I figured it should work just the same on the 400EX.

 

86atc250r,

I run 92 octane pump gas with octane boost. I use the 104+ brand in the black bottle. It claims 5 to 7 points increase. I ride anywhere from 200ft elevation (Glamis), to 1200ft (near Phoenix), up to 7000ft near Flagstaff. I probably don't need to run the boost up at the higher altitudes, but I do anyway.

 

The stock XR head gasket is more thin than the EX, the stock EX compression is 9.1:1, the XR is 9.3:1

 

Actually I was referring to a head gasket that is even thinner than the stock XR gasket. I think I read about it on Off-Road.com but I can’t remember. They were talking about easy mods for the XR that would work on the EX.

Does anybody know anything about this mod?