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i got a 400 ex that im in the process of building. i have decided to go with the 426 bore kit. i do alot of hill climbing and looking for the perfect setup. which cam stage would do me best 1 2 or 3? and what would i expect from each? of course im going to get a full pipe and a fan for the oil cooler. which would be better on the carb. jet the stock or go with a 450r?. im confussed on the compression choices i have seen and dont seem to know which is best. i would like to run pump gas like 03 octane but with my build dont know if thats possible. would a bigger cdi box be needed or help?would like the bike to run cooler as when we do ride its hard and long in the mountains and some mud. im also replaceing the timeing chain with the better one cant name it off the top of my head but i know what it is. is there anything else that would improve my proformance that im missing like port and polishing my head? valves? again im open to all suggestions and would like to compare them with other threads i read. i want a good reliable bike withe the power in all ranges with it of course being reliable in the trails. also im been searching for a free downloadable manual for my bike but have had no luck. any suggestions would be greatly appreachated thanks
 

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If you wanna go all out get bigger valves port and polish head,lower like 11.1 compression for the piston.Get the 450r carb everyone says it's much better.A cdi box will help.But if you go doing all of this it will no longer be reliable.
 

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also im been searching for a free downloadable manual for my bike but have had no luck.
You can find the repair manual for the 400EX at maxmekker.com
 

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It is very hard to make a self built quad reliable. I dont care how good of an engine builder you are. When you put aftermarket parts on they are not factory tested that way instantly reducing reliability. If your looking for the most reliable set up you would go stock on the motor and just add exhaust intake and other mods not in the motor. Just my 2 cents take it for what its worth.
Also not trying to look like a hipocrite i have built my motor but i dont take long rides because built motors take time and lots and lots of mantainance. Rarely will you go for a long ride with a built motor beating on it the whole time and not come back with some sort of problem just the way it all works.
 

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These guys got a good point. I would go with the 450r carb, aftermarket exhaust, and filter. A huge difference can be made in a quad by moding the the suspension and gearing. You could go with some 450r shocks and drop a tooth on the front sprocket. A smaller front sprocket will make the quad a little slower but you will gain some torque, besides it only costs like 20 bucks so if ya dont like it your not out much $. Everyone says there is no relacement for displacement but I think suspension and gearing are just as important as displacement. Plus you keep the motor reliable.
Almost forgot get a good clutch kit. A aftermarket cdi will give you 1K more rpm thats about it. You can also run a aftermarket fan to keep the motor cool but Iam not to shure how that would work out with the mud on a ex.
Click on the link I posted earlier and scroll down to the bottom. There is a usefull cdi trick/info.
 

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Click on the link I posted earlier and scroll down to the bottom. There is a usefull cdi trick/info.
That is some good info. I went ahead and copied it and posted here.

IGNITION HOP UP TRICK!

Many aftermarket CDI boxes sold for the 400EX are actually stock Honda XR250R boxes. The unmodified XR250 box will give you about 800 more rpm on top and a faster ignition curve. You can purchase this directly from your local Honda dealer and save money.

Order: CDI for Honda XR 250 1992-2004 Part #30410-KCE-671
 
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