im in the process of pulling out my motor to start my build its a 1999 400ex motor all stock with a white bros slip on. i want to build this motor but keep it a 400 im not going to bore it out as of now im not sure how well this will work so im looking for imput. im not trying to beat raptors or anything like that i just want to boos my power for one bad trail quad. i was thinking a cam, hi comp piston (stock bore 11:1), full exhaust, port and polish better clutch maybe... im not sure what else anything you guys could tell me would be great ideas, brands things like that
building a 400ex motor
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16 Posts
Here are some options if you haven't already...
- Jet Kit
- Advance Key (Curtis Sparks)
- Maybe a carburetor.... Might not even be worth the money to do it though. - Don't know the difference it wold make, never ran slip on, but maybe a full exhaust
- CDI Box.... They have their ups and downs...
I'm no expert, just some stuff I thought of that maybe will help ya out. Hope the build goes well for ya!!!
- Jet Kit
- Advance Key (Curtis Sparks)
- Maybe a carburetor.... Might not even be worth the money to do it though. - Don't know the difference it wold make, never ran slip on, but maybe a full exhaust
- CDI Box.... They have their ups and downs...
I'm no expert, just some stuff I thought of that maybe will help ya out. Hope the build goes well for ya!!!
Joined
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4,410 Posts
Here's my two cents worth.
If you are going to put in a hi-comp piston - bore it out 20 thousandths. A motor that old should have enough wear on the cylinder to warrant a bore job and .020 is not a "BIG BORE", it's only going to increase the displacement from 397.378 to 397.565cc's (as close as I can calculate it). You gain about 1/4 cc.
Put in a high lift cam of your choice, do the head work (waste of money unless you are racing), install whatever exhaust system you like, modify the intake (air output has to be complimented with air input) and have the carb jetting checked (it may or may not need rejetting). The bore job and hi-comp piston will boost low end grunt. The cam will boost top end power at the expense of low end power, but the exhaust will get you back some of the low end lost by camming. Changes to the intake will enhance high end performance and with a less restricted intake you might have to jet the carb up a few sizes to utilize the higher rpms available.
Now for better acceleration, change the sprockets to (if you are not putting on a new chain) 14t on the front leave the rear at 38t, or if you are putting on a new chain leave the front at 15t and change the rear to 41t.
Clutch change is only necessary if your clutch is giving you problems or if the terrain you ride requires specific clutch usage. My clutch is only used for starting and stopping and for feeding power when negotiating hills and certain turns. For most of my riding, I shift gears (up or down) without using the clutch. My ride is an 04 and it still has the stock clutch in it. I raced every weekend until July of 2007 when a injury side-lined me. May of 2010 I started training again and I hope to get back to racing this year.
You owe me 98 cents now
If you are going to put in a hi-comp piston - bore it out 20 thousandths. A motor that old should have enough wear on the cylinder to warrant a bore job and .020 is not a "BIG BORE", it's only going to increase the displacement from 397.378 to 397.565cc's (as close as I can calculate it). You gain about 1/4 cc.
Put in a high lift cam of your choice, do the head work (waste of money unless you are racing), install whatever exhaust system you like, modify the intake (air output has to be complimented with air input) and have the carb jetting checked (it may or may not need rejetting). The bore job and hi-comp piston will boost low end grunt. The cam will boost top end power at the expense of low end power, but the exhaust will get you back some of the low end lost by camming. Changes to the intake will enhance high end performance and with a less restricted intake you might have to jet the carb up a few sizes to utilize the higher rpms available.
Now for better acceleration, change the sprockets to (if you are not putting on a new chain) 14t on the front leave the rear at 38t, or if you are putting on a new chain leave the front at 15t and change the rear to 41t.
Clutch change is only necessary if your clutch is giving you problems or if the terrain you ride requires specific clutch usage. My clutch is only used for starting and stopping and for feeding power when negotiating hills and certain turns. For most of my riding, I shift gears (up or down) without using the clutch. My ride is an 04 and it still has the stock clutch in it. I raced every weekend until July of 2007 when a injury side-lined me. May of 2010 I started training again and I hope to get back to racing this year.
You owe me 98 cents now
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