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Discussion Starter · #1 ·
Hey, My name is Arnold. New to this forum. I bought a 2001 400ex this past summer. It's beginning to smoke an awful lot, going through 1/2 quart every time I take it out, so its going to need the top end freshened up. What is a good build that will offer me a decent performance gain, while still maintaining good reliability? I'm new to building engines, so any advice and help will be appreciated.
 

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Welcome to the forum

Since no one has offered their opinion, yet, here's mine......

You should have a decent performance gain by just correcting the engine's deficiencies. If you want more than that, you can safely bore to 416cc (87mm) without the need of having a "heavy" sleeve installed, or get a 436cc cylinder kit.

Increasing the bore will give the engine a boost in low end torque, but causes a reduction of high end RPM's. You can get back the top end with the installation of a high lift cam. I recommend a HotCams stage 2 cam to get back the lost top end RPM of the larger heavier piston - if you choose to bore larger than 86mm

If you only bore to 86mm (407cc), you can use the stock cam and gaskets. If you bore larger than 86mm, you need a special head gasket. If you install a HotCam, you will eliminate the auto-decompressor.

Finally, for a modest increase in both low and high end performance, install a reduced restriction exhaust.

Just boring the engine and installing a stage 2 cam will not require re-jetting the carb. Installing an aftermarket exhaust may require bumping the main jet up to a larger size, but if you make changes to the intake (larger carb, running with the air box lid off, changing the air filter to a less restrictive variety) WILL require re-jetting. If you decide on a bigger carb (only recommended if you enlarge the ports and use a stage 3 cam) (usually reserved for engines being modded for MAXIMUM performance), the jets will be smaller than stock due to the reduced velocity of airflow through the carb. If you use the stock carb, change the exhaust and modify the intake; the carb will need major re-jetting to all 3 of the jetted circuits (idle, mid-range and full throttle).

My final recommendation - dismantle the top end of the engine, check to see if the bore is worn to the point of require boring, perform only the work required to renew the engine. Be sure to check the condition of the lower rod bearing - refreshing the top end on a worn rod bearing can cause the bearing to fail leading to other damage; including, but not limited to; piston/cylinder damage, bent valves, broken valve guides and broken engine cases! It is possible that the rings are simply worn out due to infrequent oil changes or incorrect rated oil having been used. Most motor sport engines that run the same oil in the engine and transmission require JASO rated oil or API oil rated SH or lower.

Nice thing about the 400EX is the motor can be rebuilt without having to remove it from the frame. Bad thing about an 2001 model is: the swing arm bearings are probably bad, the axle bearings are probably bad, the suspension (ball joints, tie rod ends, a-arm bushings, steering shaft bearing, shocks and rear shock linkage) probably all need servicing or rebuilding also.

Happy wrenching
 

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Discussion Starter · #3 ·
Thanks a lot for taking your time to respond. Is there a site you order from that you've had luck with? I'm not worried about spending extra cash on good quality parts.
Also, did I get a decent deal? I paid $1300. It has a FMF power core slip on already. It also has full flight A-arms, nerf bars, and he put wheel spacers on. He said he got the "right wheel spacers". They bolt right on to the axle, and then the wheels bolt to the spacers. Or does it really matter which spacers? The swing arm bushings seem fine, and I replaced the rear axle bearings and seals over the summer. One thing is the heim joints, I think is what they are called, on the A-arms are squeaky as hell. I'm constantly putting grease on them to quiet them down. Is that normal, or a sign they are worn? Or how do I tell? Thanks in advance.
 

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I own and operate an independent motorsports repair shop. Been doing it for about 40 years.

You can get your parts from whatever source you trust.

Did you get a decent deal? What do you think? If you are satisfied with it, then you got a decent deal.

I'm not a fan of wheel spacers. They make the track wider and increase the load on already overloaded axle bearings. The swing arm is carried on needle bearings. Those bearings and all the suspension linkage need to be washed out and regreased each year. No one does it, so just presume they are due for replacement.

Full Flight a-arms DO use Heim joints. Up-side, they are replaceable and durable - down-side, they need constant lubrication.

Anything else?
 
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