I recently had a stock 400EX dropped in my lap, and want to make it a super quad. I have a Scrambler 500HO that gets most of my attention, so can any body give me any suggestions?

 

First thing I'd do is drop a 14 tooth front sproket in.

 

The 14 tooth front sprocket is a good start but after a few more mods it might do more harm than good. I know not all 400ex's are the same. My friend bought one the same day I bought mine. Both were brand new and mine would beat his in a drag on pavement. Then we switched quads and he beat me on my quad. There is only a 10 pound weight difference and I weigh more than him. When I got mine the first thing I did was a 14 tooth front sprocket. Its good for taking off fast but you lose a few miles/hr on top end. After jetting, filter and a pipe, mine ran pretty good. I got the idea to put my stock sprocket back on it and ran like a bat out of hell. I asked around and the reason why was because when I did those mods it gave me lots of torque in my lower rpm range. With a lower gear ratio with the 14T sprocket, I was reving right through all of my usable power. I was not staying in my power curve long enough to really put the power to the pavement. Then I put a 16T front sprocket on it and a 36T rear, and still it helped more than I thought. On my quad a higher gear ratio is the only way to go, but yours may be different, no two are the same. For now the 14T should help but later on it may be worthless. Thats my opinion from personal experience. I hope I helped.

 

If you don't want to spend a ton of cash and want some good results a simple Slip-on and a Jet Kit make a pretty big difference (despite what you may here).

-Josh

 

many people like the 14t front sprocket. i personally don't. they are nice for a track but i am not on a track too much. my friend has one and i just smoke him 3rd gear on up. he bought it and took it off the same day he bought it. even in the drag race he did not beat me off the line. for instant throttle in trails or on tracks i can see but that is not what i wanted. i guess how much power you want to gain. if you want to gain a lot you will have to do some engine work. if you just want to spend a little i would: 1. adjust the throttle cable. tighten it up till the carb opens fully. 2. you could buy a pipe and k and n airfilter. 3. if you don't want to do that you can leave the stock pipe on, drill holes in the airbox to get more air and rejet. i personally have the stock pipe on with holes in my airbox and i run a 150-152 mainjet. this seems to be a good setup. just remember, with holes in your airbox the filter will get dirty rather quick if you ride in dusty conditions.

 

The best site for EX owners is:EXRiders.com

go to the reviews section and the forum for great information, make sure to scroll down the forum and check out freq. asked questions for the most talked about mods and info.

I say:
1. Get a service manuel.
2. Take off airbox lid and rejet to 155 main,42 pilot,move needle clip down one or two notches.
3. Grind the welds down on the inside of the header pipes(inside the front) with a dremel tool/with grinding wheel bit till smooth with the rest of the pipe.
4. Get aftermarket airfilter,K&N is number 1 choice, UNI is number 2(cheaper though) AND get a outerwears AIRBOX cover.
5. Adjust throttle (for some reason 400EX's come with only 75% throttle available, make it 100% by pulling the rubber boot off the throttle body and loosen the twist screw thingy,twist the throttle cable out/away from the throttle body and retighten the screw.)
6. If you want drill some holes in the first baffle plate in the stock muffler(remove spark arrester to see it).
I've tried this and it is very hard metal to drill through! I suggest using a small drill bit first to drill a pilot hole then use a one inch hole buroghing bit(don't know what its called but it looks like a small metal top with two sides cut out of it, you know the things you spin as a child? a top? ya know you spin it and it stands on its top? well anyway good luck and have fun with your new 400EX.